EFFECT OF HABITAT QUALITY ON COMMUNICATION AND SIGNALLING IN
THE BLACK-CAPPED CHICKADEE

by

Thibault Grava
B.Sc., Universite Jean-Monnet, France, 2004
M.Sc., Universite Jean-Monnet, France, 2006

DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN
NATURAL RESOURCES AND ENVIRONMENTAL STUDIES

UNIVERSITY OF NORTHERN BRITISH COLUMBIA
April 2012

© Thibault Grava, 2012

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ABSTRACT

Vocal performance in songbirds describes the ability to repetitively perform
energetically-expensive acts (vocal vigour) and/or to consistently perform difficult motor
tasks (vocal skill). This concept is intrinsically correlated with the birds' condition - birds in
better condition tend to signal with higher performance than birds in poorer condition (e.g.
vocal performance is condition-dependent). Condition of birds, however, is also influenced
by the habitat quality in which they settle, and has been well studied in the black-capped
chickadee. My Ph.D thesis explores the links between habitat quality and vocal performance
in this species.
Previous studies have shown that habitat quality affects song output in this species
(reduced vocal vigour in poor-quality habitat) and it was speculated that reduced song output
may be linked to poorer condition of birds in these forests. To document the relationship
between immediate food availability and vocal performance, I conducted a supplemental
feeding experiment on male chickadees during the peak in dawn singing in early spring. My
results demonstrate that food availability is an important factor influencing song output, and
that song output reflects the condition of the birds at the time of singing. The difference in
song output across habitats, thus, appears to relate to difference in food availability at the
time of singing.
The ability to maintain internal song structure (vocal skill) has previously been shown
to be condition-dependent in chickadees, with dominant birds having better vocal skill than
subordinate birds. My supplemental feeding study demonstrated that this vocal skill is not
influenced by short-term resource availability to the signaller at the time of singing, but vocal

i

skill is affected by habitat quality - birds in young forest habitats (poor-quality) are less able
to maintain internal frequency ratios in their songs than birds in neighbouring mature forests
(high-quality).
The ability to maintain internal song ratios may rather reflect longer-term access to
resources that could affect neural development of song centers. The developmental stress
hypothesis states that birds suffering from a high stress during song learning are likely to
experience lower neural development, which in turn reduces their ability to learn and render
their songs. My results suggest that song structure in black-capped chickadees may reflect
the birds' resource access or environment during the song learning period. I explored the
condition of the juvenile birds settling in either habitat to test whether birds in young forest
show indications of higher stress or evidence of lower food access during the time of song
learning, which could help explain the difference in song structure observed across habitats. I
found that the birds that settle in young forest have lower corticosterone (CORT) levels in
tail feathers (associated with poor condition in other studies) prior to settlement than do birds
settling in mature forest. This period coincides with the memorization period, which is a
critical period for neural development. My results also demonstrate that juvenile males
entering their first breeding season, and which settled in young forest, suffer from a lower
development of the syrinx and of the RA (robust nucleus of the archistriatum, neural center
in the brain associated with song production). This difference in development could arise
from resource differences across habitat during the memorization period and could result in
the observed differences in song structure.
In the black-capped, structural features of the song are used both by males and
females to assess the signaller condition. Because habitat quality affects the ability to

ii

maintain internal song structure, and if this feature is used by males to assess the signaller's
condition, habitat quality could have a long-term effect on the relative perception of the
bird's status. Using playback experiments, I demonstrated that males show lower
responsiveness to song stimuli originating from young forest - this suggests that consistent
song structure is used by other males to assess a signaller condition and that songs
originating from dominant birds in young forest are not considered as equivalent threat to
similarly-ranked stimuli from mature forests.
My results demonstrate the long-term effect of habitat quality on the bird's ability to
produce the internal structure of their songs, which may have far reaching influences on the
ability to attract mates, extra-pair partners and secure territorial resources in this species.

iii

CO-AUTHORSHIP

For all chapters in this thesis, I was the primary investigator and led the design of
studies, collection of data and conducted all analyses. I wrote the initial drafts of all
manuscripts and was responsible for incorporating comments and feedback on previous
drafts into the final versions seen in this thesis. However, despite the use of first person
singular in writing within the thesis, I would like to acknowledge that this work was not
conducted in isolation. Angelique Grava assisted in all aspects of field work for this study,
and without her assistance, this work could not have been conducted; to acknowledge her
contribution, she is included in authorship on all publications that stem from my work. In
addition, the large scope of the analysis associated with chapter 5 of this thesis was reliant on
contributions and the expertise of two outside labs, as well as the field assistance of James
Bradley at UNBC. Marc Avey, Jillian Avis and Christopher Sturdy (University of Alberta)
conducted analysis of brain and syrinx physiology on our collected specimens, and Graham
Fairhurst and Garry Bortolotti (University of Saskatchewan) conducted the analysis of
feather CORT on our study specimens. All these collaborators are included in publication of
Chapter 5. Finally, my supervisor, Ken Otter, contributed to experimental design, data
analysis and writing on all studies included in this thesis, and is included in authorship on all
resulting manuscripts.
Publications stemming from this thesis (published, in press, submitted or in prep)
Grava T., Grava A. and Otter K.A. 2009. Supplemental feeding and dawn singing in blackcapped chickadees. Condor. 111:560-564.

iv

Grava T., Grava A. and Otter K.A. 2012. Vocal performance varies with habitat quality in
black-capped chickadees (Poecile atricapillus). Behaviour. 149: 35-50.
Grava T., Grava A. and Otter K.A. Habitat quality affects the perception of dominant males
based on their relative song consistency. In preparation for Animal Behaviour.
Grava T., Fairhurst G.D., Avey M.T., Grava A., Bradley J., Avis J.L., Bortolotti G.R, Sturdy
C.B. and Otter K.A. Effect of habitat quality on early condition and neural development of
song center in juvenile black-capped chickadee. In preparation for Flos (Public Library of
Science) Biology.

v

TABLE OF CONTENTS

ABSTRACT

i

CO-AUTHORSHIP

iv

TABLE OF CONTENTS

vi

LIST OF TABLES

ix

LIST OF FIGURES

x

ACKNOWLEDGEMENTS

xiii

1-

1

GENERAL INTRODUCTION

1.1 Generalities about performance in birdsong

1

1.2 Neurological and mechanical constraints on bird vocal skills

3

1.3 Relationships between body condition and vocal vigour in birds:

9

1.4 Condition and corticosterone levels in songbirds:

11

1.5 Effect of habitat on condition and behaviour

14

1.6 Black-capped chickadee - life history and singing behaviour:

15

1.7 The study of vocal performance in the black-capped chickadee and the effect of habitat quality:
20
1.8 Study site and caterpillar frass assessment

22

1.9 Outline of thesis:

25

2-

SUPPLEMENTAL FEEDING AND DAWN SINGING IN BLACK-CAPPED CHICKADEES

26

2.1 Introduction

27

2.2 Methods

29

2.2.1. Study species

29

2.2.2. Study site

29

2.2.3.Winter banding and dominance assessment

30

2.2.4. Supplemental feeding and control of other factors affecting dawn singing

31

2.2.5. Recording of the dawn chorus

32

2.2.6. Statistical analyses

34

2.3. Results

34

2.4. Discussion

39

vi

3-

VOCAL PERFORMANCE VARIES WITH HABITAT QUALITY IN BLACK-CAPPED CHICKADEES
41

(POECILE ATRICAPILLUS)

3.1. Introduction

42

3.2. Methods

44

3.2.1. Study species

44

3.2.2. Study sites

45

3.2.3. Winter banding and dominance assessment

46

3.2.4. Experimental design

47

3.2.5. Sound analysis

50

3.2.4. Statistical analysis

50

3.3. Results

51

3.3.1. Principal components analysis

51

3.3.2. Effect of habitat on the ability to maintain internal song structure

54

3.4. Discussion
4-

56

HABITAT QUALITY AFFECTS THE PERCEPTION OF DOMINANT MALES BASED ON THEIR

RELATIVE SONG STRUCTURE

60

4.1. Introduction

61

4.2. Methods

63

4.2.1. Study sites

63

4.2.2. Winter banding and dominance assessment

65

4.2.3. Playback design

66

4.2.4. Playback experiment

67

4.2.5. Statistical analyses

68

4.3. Results

70

4.3.1. Principal Component Analysis

70

4.3.2. Effect of habitat on playback response

71

4.4. DISCUSSION
5-

74

EFFECT OF HABITAT QUALITY ON EARLY CONDITION AND NEURAL DEVELOPMENT OF SONG

CENTER IN JUVENILE BLACK-CAPPED CHICKADEES

78

5.1. Introduction

80

5.2. Methods

84

5.2.1. Study species

84

vii

5.2.2. Song learning in black-capped chickadees

85

5.2.3. Study site

86

5.2.4. Feather collection

87

5.2.5. Ptilochronology

88

5.2.6. CORT analysis

89

5.2.7. Brain and syrinx analysis

90

5.2.8. Statistical analysis

92

5.3. Results

93

5.3.1. Condition during memorization phase

93

5.3.2. Condition during sensorimotor learning period

95

5.3.3. Brain and syrinx measurement after song crystallization

95

5.4. Discussion

100

6- GENERAL DISCUSSION

105

6.1. Synopsis

105

6.2. The effect of human activities on the production of sexual signal

108

6.3. Implications of my results on bird's life history and conservation

112

6. 4. Future research

116

REFERENCES

119

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LIST OF TABLES

Table 2-1 Relative proportion of total variation in dawn singing measures of each of the first
three factors in a Principal Component Analysis
Table 2-2 The contribution of dawn singing variables to PCI eigenvalues

36
37

Table 3-1 Contribution of the coefficient of variation of the seven frequency ratio variables
to PCI eigenvalues

53

Table 4-1 Contribution of the seven variables of song consistency to PCI eigenvalues and the
total percent of variation explained by each of the two first factors. Variables
having significant contribution to the PC (>0.33, Ho et al. 2006) are bolded

ix

69

LIST OF FIGURES

Figure 1-1 Schematic sagittal drawing of the songbird brain showing projections of major
nuclei in the song system. The motor pathway (green arrows) controls the
production of song. The green arrows indicate the descending projections from
HVC in the neostriatum to RA (Robust nucleus of the archistriatum), and from
there to the vocal nucleus (nXIIts), the respiratory nucleus (Ram), and the
laryngeal nucleus (Am) in the medulla. The red arrows indicate the anterior
forebrain pathway that is essential for song learning. It indirectly links HVC to
RA, via Area X in the parolfactory lobe, dorsolateral nucleus of the thalamus
(DLM), and lateral magnocellular nucleus of the anterior neostriatum (1MAN).
1MAN also projects to Area X. AM-nucleus ambiguus; DLM-medial portion of
the dorsolateral nucleus of the thalamus; IMAN-lateral portion of the
magnocellular nucleus of the anterior neostriatum; RA-robust nucleus of the
archistriatum; RAm-nucleus retroambigualis; X-Area X; nXIIts-tracheosyringeal
part of the hypoglossal nucleus. Picture and description modified from Brenowitz
etal. (1997)

6

Figure 1-2 Spectrograph^ representation of a fee-bee song of the black-capped chickadee
(A) and of one example of a typical pitch shifting during the dawn chorus
showing songs from the high-frequency range (the three first songs), of the lowfrequency range (the two next songs) and of the mid frequency range (the two
19

last songs).
Figure 1-3 (left side) Map of the JPRF young (red lines) and mature (blue lines) forest site.
Within each sites, two independent plots of 1600m2 (yellow squares) were

chosen as study sites, (right side) Map of Prince George, BC, showing the mature
(blue) and young (red) sites used in chapter 5.

24

Figure 2-1 Comparison of the PCI values (song output and start time) from fed and unfed
birds classified by rank. Increasing values of PC1 indicate higher song output and
earlier start times during dawn singing. Plots show mean value (dot), and
38

standard error (lines) of the data

x

Figure 3-1 Extraction of eight frequency variables used to describe the song consistency in
the fee bee song of the black-capped chickadee {Poecile atricapillus). Left:
spectrograph^ representation. Fee: first note of the fee bee song. Beel and bee2:
the two syllables composing the second note of the fee bee song. Right:
fundamental frequency monitoring. Ffee.start, Ffee.end: frequency variables
extracted from the fee note. Fbeel.start, Fbeel.max, Fbeelend: frequency
variables extracted from beel. Fbee2.start, Fbee2.max, Fbee2.end: frequency
variables extracted from bee2

52

Figure 3-2 Comparison of the PCI value (song consistency) from birds recorded in young
versus mature forest (General Linear Model: F(l, 15) = 5.42, N = 16, p = 0.03).
Increasing values of PCI indicate higher ability to maintain consistent frequency
ratios within songs across the chorus. Plots show mean value (square), and
standard error (lines) of the data

55

Figure 4-1 Dominant birds in mature forest respond more aggressively toward a dominant
intruder from mature than from an intruder from a young forest (Post hoc
Fisher's LSD test, df=26.2, N=8, p=0.005). There was no significant difference
in response to either stimuli class among focal males in young forests (Post hoc
Fisher's LSD test, df-26.2, N=10, p=0.70). Circles and squares stands for the
response to young forest and mature forest stimuli, respectively. Plots show mean
and bars show standard errors

73

Figure 5-1 CORT level in natal feathers are higher in birds that settled in mature forests
compared to birds settling in young forests (ANOVA, F(i>43)=4.48, N=48, p=0.03,
d=0.35). Dots represent mean and whiskers error bars

94

Figure 5-2 There is no significant difference in average growth bar widths (DGB) in the natal
feathers of birds that settled in either mature or young forests (ANOVA,
F(i,45)=0.35, N=48, p=0.55, d=0.004). Dots represent mean and whiskers error
bars

96

Figure 5-3 Syrinx weight of black-capped chickadee in their first spring is correlated with
CORT levels of the natal feathers (regression analysis, R2=0.1, N=20, p=0.03).97
Figure 5-4 (a) Birds from young forests have a lighter syrinx than birds in mature forest
(d=0.60). (b) HVC volume differ slightly across habitat (d=0.13) with birds in
xi

young forests tending to have a smaller HVC than birds in mature forests, (c) An
intermediate effect was found between RA volume in young vs. mature forests;
birds in young forests tend to have a bigger RA volume than birds in mature
forest (d=0.38). Dots represent mean and whiskers error bars

xii

98

ACKNOWLEDGEMENTS

This thesis was made possible thanks to the many people who supervised my work,
shared experiences and advice, and supported me scientifically or socially. It has been a real
pleasure to work at UNBC in Prince George: this small city has been very welcoming and the
University, a real whirlpool of scientific and social experiences.
Foremost amongst those to whom I owe thanks is my supervisor, Dr Ken Otter, who
has been extremely efficient, understanding and supportive throughout my PhD. His
scientific expertise has been crucial to the design and analysis of experiments presented in
this thesis.
This research has also been greatly supported by my committee members - Dezene
Huber, Brian Aukema, Glenda Prkachin and Hugues Massicotte. Their participation
significantly improved the overall quality of this thesis. I want to thank Hughes in particular
for our frequent and refreshing conversations en Frangais!
All the people who participated to the four field seasons at the JPRF and in Prince
George, were also critical in making this thesis possible. I cannot thank enough Angelique
Grava and Regis Didier, for their work at the JPRF, and James Bradley, for his immense
contribution to chickadee trapping and banding sessions around Prince George. Their
commitment and contribution to this work is limitless.
I want to thank Graham Fairhurst and Marc Avey who collaborated with me on this
thesis. Their expertise in feathers and brain physiology helped me greatly in the setting up of

xiii

the experiment that made chapter 5 possible. It has been a pleasure to work with both of
them.
Furthermore, in terms of maintaining sanity, this project would not have been
possible had I remained isolated in the birdcave or in the woods and for this I must thank
many others in Prince George who made my life more than enjoyable and truly memorable. I
had a lot of fun in this town and my range of experiences was made possible by much more
than what the thesis alone could have provided. Thanks to all of you, local dudes and NRES
students, I can say that my stay in Canada has been an unforgettable slice of life.
Finally I want to say a word about the one and only person who has transcended her
role as a partner to become a collaborator, advisor, field assistant and employer collectively!
My thesis would not have been possible without the emotional and scientific support of my
wife Angelique. Thank you so much for being with me through the thick and thin, good and
bad (and for waking up at 3am to record the dawn chorus three springs in a row!). I cherish
all our moments thus far and look forward to the good times we will experience together in
the future, and now as a small family!
Finally, I dedicate this thesis to loved ones who would have wanted to read these
words but are not able to be with me today. I think of my Dad, Jean-Louis Grava, and of
Alyssa Shaw - may you rest in peace. This work is for them, and for my son, Sasha, who I
hope will one day read this and feel inspired to discover the place he was conceived!

xiv

1-

GENERAL INTRODUCTION

1.1 Generalities about performance in birdsong
Acoustic signals in avian communication are generally termed "vocalizations" and
their structures are very diverse in the Order Passeriformes (the songbirds) (Catchpole and
Slater 2008). Individual vocalizations are typically defined as syllables (units of continuous
sound, the equivalent of notes in music) that have a fundamental frequency, which may or
may not vary in pitch over time or contain harmonic frequencies. Syllables may be grouped
consistently with other distinct syllables to form discrete vocalization types within species,
and these vocalization types may be used consistently in particular circumstances. One of
the most complex of these vocalizations in birds is distinguished as "song" and has been
described extensively by behavioural ecologist since the invention of sound spectrograms
after World War II (Marler 1957; Thorpe 1958; Marler 2004; Marler and Slabbekoom 2004;
Catchpole and Slater 2008). Songs are produced predominately by males, and appear to have
evolved through sexual selection and hold two main functions: mate attraction (intersexual
interactions) and rivals repelling (intrasexual interactions) (Collins 2004; Marler and
Slabbekoom 2004).
Bird song in Passerines is unlike other vocalizations in that it is partly innate and
partly learned (Marler 2004). Unlike the production of many other vocalizations (termed
"calls"), a male cannot produce his species-specific song if he does not go through a complex
learning process. This involves hearing species-specific songs produced by tutors, and
memorizing this structure in the neural centers of the brain devoted to song learning and
production (Marler 2004). However, there is also an innate component of song production in
1

birds, which results from the propensity of birds to preferentially attend to and memorize
song types of their own species (with the exceptions of birds learning other species' song
through mimicry), suggesting an underlying genetic influence on song learning (Marler
2004). Despite these underlying genetic templates, as well as reliance on learning from
tutors, individuals also show variability in the productions of songs that may reflect their
underlying physical condition.
There is immense diversity in the structure and complexity of songs across taxa, but
variability also occurs between individuals within the same population. Individuals do not
necessarily display with the exact same song output (units of vocalizations over a given
interval of time) and/or complexity of the syllable structure (e.g. modulation of frequency,
number or types of syllables produced during a vocalization). The inter-individual
differences in singing has been intensively studied through the concept of vocal performance
(Podos 2009; Byers et al. 2010) which states that singing, like any other actions, is limited by
factors that are inherent to the individuals.
Motor performance, itself, has been partitioned by Byers et al. (2010) into two
independent measures: "vigour", defined as "the ability to perform energetically expensive
acts repeatedly"; and "skill", which is "the ability to perform difficult motor tasks well".
These definitions apply to the vocal performance in many bird species because vocalizations
often represent a challenge in term of production, complexity and/or stereotypy (Podos et al.
2009). In the chorus of many songbird species, song output provides information about the
signaller's condition (Otter et al. 1997; Welling et al. 1997; Lucas et al. 1999; Poesel et al.
2004; Otter and Ratcliffe 2005; Murphy et al. 2008) and one might describe "vocal vigour"
in this context as the birds' ability to perform a chorus with a high song output or song rate.

2

The internal song structure might also provide information on the signaller's
condition in some passerine species (Lampe and Espmark 1994; Forstmeier et al. 2002; Pfaff
et al. 2010). For example, the ability to maintain a consistent structure from one song to
another correlates with other measures of male quality in black-capped chickadees (Christie
et al. 2004; Hoeschele et al. 2010), and may reflect selection on neurological links in song
learning and motor pathways (Byers et al. 2010; Podos et al. 2009). Females are known to
attend to these structural elements when assessing male quality (Searcy 1984; Forstmeier et
al. 2002; Reid et al. 2004; Byers 2006; Botero et al. 2009; Ritschard et al. 2010). Thus, the
ability to produce songs with a high level of consistency may be a measure of "vocal skill".
Song structure and production can, therefore, reveal information on individual
condition, and thus be used as a reliable signal in both intra- and intersexual selection. To
better understand the inter-individual diversity in vocal performance in bird song, it is
important to understand the mechanisms underlying song learning and production, and the
constraints associated with them.

1.2 Neurological and mechanical constraints on bird vocal skills
In songbirds, the learning, perception and production of song is controlled by a set of
interconnected brain nuclei known as the 'song system' (Fig 1.1) (Doupe and Kuhl 1999;
Brainard and Doupe 2002; Bolhuis and Gahr 2006; Bolhuis et al. 2010). The song system is
present in species that learn their song(s) by the imitation of species-specific sounds
(Brainard and Doupe 2002; Doupe and Kuhl 1999). The imitative learning of song occurs
during a sensitive period early in development (Bolhuis et al. 2010; Catchpole and Slater
2008), which is divided into two sub-periods - initially with a memorization learning period,

3

which is then followed by a sensorimotor learning period (Hultsch and Todt 2004; Bolhuis
and Gahr 2006).
During the memorization learning period, the birds hear and memorize the song of a
"tutor" - an adult conspecific. During this period, a model of the perceived song is encoded
into the bird's memory and will serve as a pattern to mimic in the subsequent phase. This
phase typically starts very early during the bird's development (often while the bird is still a
nestling) and can extend beyond the start of the sensorimotor learning period in some species
(Bolhuis and Gahr 2006).
During the sensorimotor learning period, birds attempt to match their singing
performance to the internally-memorized song they learned from tutors. This period involves
neural feedback loops with the song system in the brain. The auditory feedback of the
produced song appears to be compared to the internal template, and neural development in
the brain continues to be refined during this period until the produced song matches the
memorized song (Bolhuis and Gahr 2006). This complex neural system makes the link
between song learning and song production. This phase takes place until the birds enter their
first breeding season, when a peak of testosterone stops the development of the nuclei, a
phenomenon called brain crystallization (Hultsch and Todt 2004). In most species, which are
termed close-ended learners, this process happens once prior to the first breeding season, and
no changes in song development are seen in subsequent years. However, this is not
necessarily as static as once thought, as there are a number of species that appear to be openended learners. The Island Canary (Serinus canaria), for example, is able to develop new
songs after the first year, even as sexually mature adults (Nottebohm 2004). The plasticity is
made possible through the recruitments of new neurons, particularly in two nuclei of the

4

song system: the HVC (Higher Vocal Center) and area X (see fig 1.1). The recruitment of
new neurons, however, also happens in the song sparrow (Melospiza melodia) song system, a
close-ended learner, without the development of new song types. Thus, the distinction
between close-ended and open-ended learners is not as dichotomous as previously suspected,
and that these two groups differ more in the degree of plasticity rather than in the presence or
absence of plasticity (Benowitz et al. 2005).
The process of song learning in birds therefore involves the complex neural
development linking the auditory perception system to the nuclei of the brain used to store
the template, and then the neural linking of these centers to the syringeal muscles used in
song production. Thus, the vocal skill of birds (e.g. their ability to perform accurate
renditions of their songs) may be limited by neuro-muscular development during song
learning. The nutritional stress hypothesis (Nowicki et al., 2002) suggests that a bird's
condition during its early development stages can affect brain development, which could
result in a diminished ability to accurately acquire and/or produce their song. Nowicki et al.
(2000) demonstrated that a significant positive relationship existed between feather growth (a
food-limited trait) and song repertoire size in the great reed warbler (Acrocephalus
arundinaceus). Birds with limited food resources during their early developmental stage also
tend to display less stereotyped songs than birds that had unlimited food resources. Food
restriction, for example, seems to affect song learning ability of both nestling (swamp
sparrow, Melospiza georgiana, Nowicki et al. 2002; zebra finch, Taeniopygia guttata, Zann
and Cash 2008) and juvenile birds (european starlings, Sturnus vulgaris, Buchanan et al.
2003; Spencer et al. 2004). The effect of condition on song learning has been linked with an
effect on neural development. In zebra finches, birds who undergo developmental stress

5

Figure 1.1 Schematic sagittal drawing of the songbird brain showing projections of major
nuclei in the song system. The motor pathway (green arrows) controls the production of
song. The green arrows indicate the descending projections from HVC (Higher Vocal
Center) in the neostriatum to RA (Robust nucleus of the archistriatum), and from there to the
vocal nucleus (nXIIts), the respiratory nucleus (Ram), and the laryngeal nucleus (Am) in the
medulla. The red arrows indicate the anterior forebrain pathway that is essential for song
learning. It indirectly links HVC to RA, via Area X in the parolfactory lobe, dorsolateral
nucleus of the thalamus (DLM), and lateral magnocellular nucleus of the anterior
neostriatum (1MAN). 1MAN also projects to Area X. AM-nucleus ambiguus; DLM-medial
portion of the dorsolateral nucleus of the thalamus; IMAN-lateral portion of the
magnocellular nucleus of the anterior neostriatum; RA-robust nucleus of the archistriatum;
RAm-nucleus retroambigualis; X-Area X; nXIIts-tracheosyringeal part of the hypoglossal
nucleus. Picture and description modified from Brenowitz et al. (1997).

6

during the song learning period (through nutritional and hormonal stress, or parasitic
infections) have lower development of the HVC region, but RA volume or brain size was not
affected (Buchanan et al. 2004; Spencer et al. 2003; Spencer et al. 2005a).
The neuromuscular activity of song production involves a complex interaction
between three sub-systems: respiratory, craniomandibular muscle groups and the bird's vocal
organ - the syrinx (reviewed by Suthers et al. 1999). When singing, the energy responsible
for sound production is provided by the respiratory system, which forces air into the vocal
tract. In the vast majority of bird species, sound is produced during the expiration, whereas
inspiration happens between syllables. The pattern of expiration/inspiration gives the
rhythmic pattern of the song, or its temporal variation. The vocal organ transforming this
energy into sound is called the syrinx and consists of two separate sound sources. The
muscles on each side of the syrinx are separately innervated by the ipsilateral
tracheosyringeal branch of the hypoglossal nerve (NXIIts) (Suthers et al. 1999). This
configuration potentially allows for independent motor control of both sides of the syrinx and
consequently for the extreme complexity and virtuosity that can be heard in certain bird
species. Finally, the sound produced in the syrinx may be modified by the craniomandibular
muscle group in the suprasyringeal vocal tract, primarily through beak movement (Suthers et
al. 1999). Beak gape (i.e. opening) in particular has been demonstrated to correlate with
fundamental frequency in most of the notes that were analyzed in the swamp sparrows
(Melospiza georgiana) and white-throated sparrows (Zonotrichia albicollis) (Westneat et al.
1993). This correlation suggests that beak movement may play a role in filtering and
enhancing particular frequencies (Suthers et al. 1999).

7

While this system is highly evolved for allowing the large inter-species diversity seen
in bird song, there are mechanical limitations to the birds' vocal skills which are both species
and individually-specific (Byers et al. 2010). Studies of vocal mechanics during the last 25
years have illustrated that song production by male birds is intrinsically challenging,
involving the simultaneous control and coordination of breathing, syrinx modulations (the
left and right sides having separate innervations) and vocal tract modulations including those
of the trachea, mandible and oropharyngeal cavity (Nowicki, 1987; Podos and Nowicki,
2004; Suthers, 2004; Riede et al., 2006). A good example of those mechanical constraints
involves the production of trills (the rapid repetition of the same note in a short amount of
time - corresponding to the tremolo in music) in the swamp sparrow (Melospiza georgiana,
Podos 1996; 1997). Hand-reared birds that are trained to learn songs with a trill rate higher
than normal are not able to perform those songs despite evidence that they have actually
memorized and learned them (Podos 1996). These studies show that the constraints on vocal
skills are not necessarily based on neural limitations, but may also result from motor
constraints (Podos 1997).
The bird's vocal skills are, thus, limited by various constraints at different stages.
Brain development can be limited during the song learning period. If the bird's development
is perturbed during this learning period, the bird's ability to perform complex or consistent
songs (the bird's vocal skills) will be affected. Also, even if no problems occur during song
learning, the bird's vocal skills might be affected during the production stage, if the learned
song is beyond the bird's mechanical skills. At this stage, nothing can alleviate the
mechanical constraints that limit the production of the song.

8

1.3 Relationships between body condition and vocal vigour in birds
An important role of birdsong is for the signaller to give information about its own
body condition, to both attract females and/or to discourage intruding males. Thus, the song
has to "honestly" reflect the signaller's condition. This characteristic is known as conditiondependence (e.g. a condition-dependent signal) and can be true only if singing is
energetically costly (Zahavi 1975). Therefore, the singing behaviour of the signaller should
reflect its body condition. "Vocal vigour" (ability to produce songs at a high rate and sustain
this production over time) depends on the bird's ability to invest energy into singing
(Godfrey 2003). As such, the song output is limited by the signaller's "vigour" which reflects
its body condition at the time of singing.
Because song production involves a complexity of different subsystems (see above),
the muscular activity involved during song production increases the metabolic requirements
for the male (Suthers et al. 1999). An increase in oxygen consumption during song
production has been detected in three passerine species displaying songs differing in
complexity: zebra finches, Waterslager canaries (Serinus canaria) and European starlings
(Oberweger and Goller 2001). An increase in the metabolic rate also reflected song
production in the Carolina wrens (Thryothorus ludovicianus, Eberhardt 1994). While
studying the European robin (Erithacus rubecula), Godfrey and Bryant (2000) concluded
that behaviour, including vocalizations and activity levels, and energy expenditures, were
sensitive to manipulations of body condition. Because singing is a non-maintenance activity
that requires extra energetic investment, it will be undertaken only if the bird has enough
energy to sustain its other primary life-sustaining activities (Godfrey 2003). When a bird

9

attempts to sing, the amount of song produced will then reflect the amount of excess energy
the bird has available to invest into singing (its vocal vigour).
Because birds in better condition supposedly have more energy available than birds in
lower condition, their song production will therefore be higher. Studies looking at the
relationship between song output and condition showed that birds in higher condition have a
higher song output. For example, Lucas et al. (1999) showed that vocalization rate increased
with body mass and food availability in Carolina chickadees (Poecile carolinensis). Murphy
et al. (2008) demonstrated that song rate increases with body size and plumage quality in
eastern kingbirds (Tyrannus tyrannus). In the european robin (Erithacus rubecula), it has
been shown that males sing more at dawn when their food resources were constant than
when they were variable (Thomas 1999). In the black-capped chickadee {Poecile
atricapillus), dominants birds have increased song output at dawn (Otter et al. 1997).
Therefore, the vocal vigour or song output, which reflects the amount of energy the birds can
invest into singing, should be directly correlated to the condition of the birds at the time of
singing.
As a result, vocal vigour and vocal skill are two independent condition-dependent
signals entrenched in bird song. However, the temporal association between condition and
singing ability may not be the same - vocal skill may reflect condition at some time in the
past when song acquisition or neural development pathways were occurring, but the
immediate energy to invest in present singing may be reflected more in vocal vigour at the
time of signalling.

10

1.4 Condition and corticosterone levels in songbirds
In vertebrates, stress response is regulated by the hypothalamic-pituitary-adrenal
(HPA) axis. In response to a stressor, a cascade of hormonal secretion induces the synthesis
of glucocorticoids, such as corticosterone, from the adrenal cortex (Breuner et al. 2008).
Glucocorticoids mobilize energy reserves, which allow the individuals to respond to
stressors. However, long-term increases in both basal and stress-induced glucocorticoids can
have detrimental effects on immune function or reproductive success (Sapolsky et al. 2000).
In birds, many studies compared the corticosterone levels (CORT) in individuals and
their relationship to the individual condition (for review see Walker et al. 2005). The
hypothesis supposed that birds in higher condition would have lower CORT levels than birds
in lower condition; as a result, CORT levels were considered a proxy measure of relative
individual condition (the CORT-fitness hypothesis; e.g. Marra and Holberton 1998; Arlettaz
et al. 2007).
However, several problems occur with generalizing this hypothesis. First, many
different methods of measuring CORT exist, and cross-comparison of measurements using
different techniques are difficult. The intial technique used to measure CORT levels was
through blood samples. This technique typically involves catching the birds and collecting a
blood sample before the stress associated with handling causes CORT to increase (usually
less 3 min after the birds enter the mistnet). This measure serves as baseline CORT level
which is then compared to a second blood sample done after a period of induced stress (for
example keeping the birds 30 min in a bag in Schmidt et al. (2012)). This technique gives a
measure of baseline and induced CORT level at a very specific time, and allows researchers
to compare the inter-individuals acute stress-induced response. However, baseline CORT and

11

acute stress-induced CORT level do not give the same information. While the stress-induced
CORT levels give information on the ability of the bird to activate the HPA axis, the baseline
CORT levels provide information about the long-term activation of the HPA axis in the
background (Sapolsky et al. 2000).
Other techniques measure baseline CORT levels in feces or feathers. These two
measures give information about the CORT level over a longer-term than does blood
sampling. However, the time period reflected by CORT in feces vary across taxa, and is
associated with the rate at which food is processed, which can be relatively rapid in bird
species. CORT in feathers, though, provides an indication about the CORT level during the
time period in which the feather was grown, which reflects about three weeks for birds as
small as chickadees.
All these methods give measures of CORT that differ greatly in their significance and
time scale, thus comparisons between studies are best made using a common technique to
measure CORT. Even within techniques, inter-individual variation in CORT can change
associated with different phases of the organisms life history. Bonnier et al. (2009) compared
the baseline CORT levels in breeding females tress swallows (Tachycineta bicolor) and
found that females investing in reproduction (higher clutch mass) show lower baseline
CORT levels than females with lower clutch mass during the incubation stage, but the
inverse relationship occurs during the nestling stage. This study demonstrates the variability
in CORT level across individuals and the difficulty in using CORT levels as a measure of
individual condition unless the context is considered. As a result, to fully understand interindividual variation in CORT, we have to compare similar metrics of CORT (baseline vs

12

induced), preferably using the same technique, and compare individuals which are at the
same stage of their life history.
Using this approach, studies looking at the relationship between baseline CORT
levels and individual condition in nestling birds are beginning to reveal emerging patterns.
Using CORT injection in ovo in eggs of European starlings (Stumus vulgaris),Chin et al.
(2009) demonstrated that embryos with elevated baseline CORT ultimately grow as fledging
with more functionally mature pectoral muscles and larger wing areas (the two best
predictors of flight performance) than controls. Those results tend to show that elevated
baseline CORT level might be advantageous for nestling, allowing them to develop faster
and achieve greater size. In a study comparing baseline CORT in feather, Fairhurst et al.
(2012) showed a similar trend between Cory's shearwater chicks grown under different
nutritional conditions. Chicks from the nests of parents whose flight ability has been
impaired were of lower condition (smaller, lighter and reduced immune response) than
controls, and showed associated lower CORT level in their feather. Similarly, Kitayski et al.
(2005) found the same trend in tufted puffin chicks using baseline CORT level in blood; food
deprived chicks show lower CORT levels than control. These studies suggest that birds that
are in lower condition or have lower access to resources are less likely to invest in growth.
The mobilization of the HPA axis effectively starts mobilizing reserves and increasing the
metabolism. Thus, individuals with low food intake may be unable to sustain this metabolic
activity and might down-regulate the HPA axis (which would in turn lead to lower baseline
CORT levels in measures). As a result, low baseline CORT levels at the earliest
developmental stage could be predicted to be associated with lower growth or condition

13

(Kitayski et al. 2005; Chin et al. 2012; Fairhurst et al. 2012). This may, then, prove a metric
for studying the effects of habitat and early condition on song acquisition.

1.5 Effect of habitat on condition and behaviour
It appears that vocal performance in birds reflect the signaller's condition whether it
is at the time of singing or at the time of song learning. However, many parameters can
influence a bird's body condition, such as the quality of habitat in which the bird has settled.
Strong and Sherry (2000) used ptilochronology (measurement of daily growth bars on
feathers which gives an indication of food availability during the period of feather regrowth;
Grubb 1989) and biomass to show that ovenbirds (Seiurus aurocapillus) in habitat with low
access to resources were in poorer conditions than those where resource access was higher.
Grubb and Yosef (1994) also used ptilochronology to assess nutritional condition, and found
that loggerhead shrikes (Lanius ludovicianus) were affected by habitat quality. Similarly,
Carlson (1998) found that the condition of white-backed woodpeckers (Dendrocopos
leucotos) was a reflection of their territory quality. In the Paridae family, wintering
chickadees in poor-quality habitat carry higher levels of furcular fat than do birds in highquality habitat (van Oort and Otter, 2005); such high fat levels in other members of the
Paridae family occur among birds that experience decreased or inconsistent access to food
(Gosler, 1996; Gosler and Carruthers, 1999).
In turn, the effect of habitat quality on the bird's body-condition can have serious
consequences on the behaviour of those birds, particularly on condition-dependent
behaviour. The effect of habitat on avian behaviour has been extensively studied in the
black-capped chickadee in North-Central BC. In this species, birds nesting in poor-quality

14

habitat (young forest) have a lower reproductive success than birds nesting in high-quality
habitat (mature forest) (Fort and Otter 2004a). Territorial behaviour is also reduced in young
forest compared to mature forest in this species (Fort and Otter 2004b), which is typically the
result of these areas being perceived as lower in resources worth defending. Also, birds in the
young forests have higher hematocrit values (indicative of higher metabolic costs) during the
breeding season (van Oort et al. 2007). This suggests they might have to work harder,
despite still having lower breeding prospects, which in turn can affect their overall condition.
Finally, birds nesting in young forest have a lower song output during the dawn chorus than
the birds in mature forest (van Oort et al. 2006), which seems to indicate that the bird's vocal
vigour is affected by habitat quality.
It appears that habitat quality has a direct effect on the behaviour of the black-capped
chickadee in north-central BC through an effect on the body condition. Habitat quality
affects vocal vigour encoded in the fee-bee song through song output (van Oort et al. 2007).
However, whether this effect is due to ecological, mechanical or neural constraints and
whether habitat has an effect on song structure is not known. Because the black-capped
chickadee behaviour has been extensively studied over the past decade, this species presents
itself as a good model for the detailed study of the effect of habitat quality on the bird's vocal
performance.

1.6 Black-capped chickadee - life history and singing behaviour
Black-capped chickadee is a small resident generalist, common in deciduous and
mixed forests across northern North America. Birds overwinter in stable flocks characterized
by linear dominance hierarchies (Smith 1991). Flocks are composed of two to five mated

15

pairs that forage and travel together during most of the nonbreeding season. As the birds
maintain a strict hierarchy during these winter months, the rank of each individual can be
determined from interactions involving two birds (e.g. dyadic interaction) at winter feeding
stations (Smith 1991; Otter et al. 1997).
When the birds enter the breeding season, flocks break down and mated pairs start to
defend individual territories. Black-capped chickadees are primary cavity nesters, excavating
a new cavity each year; this nest construction begins soon after the pairs separate out of
flocks onto their breeding territories. During the period of excavation and during the early
period of egg laying, females are fertile and males begin a period of extensive singing at
dawn (the dawn chorus). Males sing from within their territory, typically close to the nest
site, at dawn, and choruses can typically last 20-40 min with males singing continuously at
rates of 12-20 songs/minute throughout this period. The chorus typically ends when the
female emerges from the nest cavity, at which point the pair often copulates. Chorus activity
typically stops when a male's mate begins incubation of the clutch (Otter et al. 1997; Mennill
and Otter 2007; Foote et al. 2010), resuming only if the clutch fails and the pairs renest (pers
obs). Nestlings fledge at day 16, typically early July in the study area. During the nestling
phase, a modified form of the song (faint fee-bees) is used by both males and females in
coordinating nesting activity (Smith 1991; Foote et al. 2010). While faint fee-bees are lower
in amplitude, and much more variable in frequency structure than the pure fee-bees, they are
noticeably similar in tonal structure, and may serve as an initial template for nestling song
learning (e.g. Baker et al. 2003). Following a fledgling period of several weeks, juveniles
disperse and little is known about the subsequent flock establishment, except that it can occur
during the early fall and individual birds can disperse as far as 10km from the nest before

16

settling into a flock (Smith 1991). During most of the year, chickadees consume a mixed diet
of seeds, berries, and invertebrates, but during the breeding season, they switch to a diet
predominantly composed of insects (Smith 1989).
Black-capped chickadee song consists of a very stereotyped two-note signal known
as the fee-bee. Very few examples of variation of this signal have been described in this
species; in fact, songs are remarkably invariant across most of their range consisting of the
northern half of the North American continent from the Atlantic to Pacific coast (Hailman
1989; Kroodsma et al. 1995; reviewed in Menniil and Otter 2007). In early breeding season,
breeding males sing long bouts of continuous 'fee-bee' songs close to the nest site at dawn
(Otter et al. 1997; Gammon 2004; Fig 1.2 A). These dawn song bouts often last over 45 min
with average singing rates of 12-15 songs per minute, with some males achieving maximum
rates of more than 20 songs per minute during the peak of the chorus. The dawn singing in
the black-capped chickadee appears to be directed towards both females and males (Otter
and Ratcliffe 1993; Menniil and Otter 2007). During the chorus, the signaller may shift the
song to a higher or lower frequency (a process known as "pitch shifting'; Fig 1.2 B), but the
frequency ratio between the fee and the bee note is stringently maintained by birds when they
transpose the overall pitch of the song (Weisman et al. 1990).
As with all passerines, the black-capped chickadees use their song to claim a
territory, and the fee-bee song is used also in this context during male diurnal interaction. In
this case, the song is typically used to resolve conflicts over territorial occupancy without
escalating direct fights. When interacting, males signals their willingness to escalate contests
by matching the opponents songs in frequency (frequency matching) and/or in time (song
overlapping) (Menniil and Ratcliffe 2004), a behaviour known as countersinging.

17

Among wild black-capped chickadees, juvenile birds start singing approximately at day
20 (Baker et al. 2003, confirmed by personal observations), which correspond to the date
they fledge from their parents. I feel confident in saying that this represents the starting point
of the sensorimotor learning in this species, and that memorization therefore occurs prior to
this date when the birds are still fed by their parents i.e. from hatching to fledging). During
this first phase occurring prior to feldging, the chickadees memorize the template of their
species-specific songs that they will try to reproduce in the next phase. Once the chickadees
disperse, the males enter the sensorimotor learning period. During this phase, the young
males practice singing songs they have previously memorized, attempting to match their
subsong attempts with their learned template songs (Shackleton and Ratcliffe 1993). This
sensorimotor phase lasts until the birds enter their first breeding season (Shackleton and
Ratcliffe 1993; Baker et al. 2003; Hultsch and Todt 2004).
During the entire learning period, the song system develops in parallel with the
acquisition of adult song (Bolhuis et al. 2010). As a songbird, the template for the tutor's
song in the black-capped chickadee is thought to be encoded in the neural substrate of NCM
(caudomedial nidopallium) and CMM (caudomedial mesopallium; Bolhuis et al. 2006).
During development, the anterior forebrain pathway is involved in the acquisition of song as
well as auditory-vocal feedback processing. The posterior pathway or song motor pathway
(SMP) controls the production of song and includes HVC (higher vocal center) and RA
(robust nucleus of the archistriatum), which innervate nXIIts (tracheosyringeal portion of the
hypoglossal nucleus) that control the syrinx ((for review see Mooney 2009; Fig 1.1). Normal
development of the song system is crucial during the sensitive period for normal
development of song (Nowicki et al. 2002; Christie et al. 2004; Grava et al. 2012).

18

bee

fee

0.2

0.0

0.8

0.6

0.4

1.2

1.0

Time (s)

B
#•

*•

m*

S

Si

St

A*

*•

0

8

10

\

»•

_r_
12

Time (s)

Figure 1.2 Spectrographic representation of a fee-bee song of the black-capped chickadee (A)
and of one example of a typical pitch shifting during the dawn chorus showing songs from
the high-frequency range (the three first songs), of the low-frequency range (the two next
songs) and of the mid frequency range (the two last songs).

19

1.7 The study of vocal performance in the black-capped chickadee and the effect of
habitat quality
Vocal communication underlies most aspects of chickadee ecology, from
overwintering survival through breeding behaviour. Yet, while the mechanisms of how
vocal interactions between birds convey particular kinds of information are becoming better
understood in the black-capped chickadee (Mennill and Otter 2007), most of this work
suggests that the context and/or reliability of signals can be significantly affected by habitat
alteration.
Two different signals within the fee-bee song in the black-capped chickadee give
reliable information about the signaller's condition. First, song output at dawn (or vocal
vigour) is known to reflect the male condition (Otter et al. 1997) and it is assumed that the
females can utilize this information to assess male quality (Otter and Ratcliffe 1993, 2005;
Mennill and Otter 2007). Female chickadees and other titmice appear to use this information
to choose potential partners for extra-pair copulation (Kempenaers et al. 1997; Mennill and
Otter 2007). However, the reliability of this signalling depends on high-quality males singing
at higher rates than lower-quality males, which may indicate their ability to acquire territories
that supply the male with resources needed to sustain such costly singing. Recent work has
found that variation in habitat quality may alter the relative signalling of dominant versus
subordinate males (van Oort et al. 2006) and may jeopardize the ability of females and males
to judge the relative status of different males in the chorus. However, the exact link between
habitat quality and song output is still missing. It is suggested that difference in condition at
the time of singing across habitat would be responsible for a lower song output in poor-

20

quality habitat, but no study has directly tested whether song output at dawn was affected by
condition.
Second, the internal structure of the songs can also give reliable information about the
signaller condition independently from the song output. Christie et al. (2004) found that the
ability to maintain a consistent within-song internal acoustic structure (which has been
defined as a vocal skill by Byers et al. 2010) during the chorus ("song consistency" hereafter)
is lower in subordinate males than in dominant males. The authors suggest that song
consistency does not reflect the condition of the birds at the time of singing (Christie et al.
2004) but at the time of song learning. Birds in poorer condition during song learning would
face a higher developmental stress, therefore affecting their ability to accurately learn their
species-specific song (see "developmental stress hypothesis" described previously). Female
black-capped chickadees also appear to attend to signals encoded into the structure of the feebee song that are related to a signaller's condition (Hoeschele et al. 2010). Therefore, song
consistency seems to both signal the condition of the signaller and is used by other
individuals as cues to the signaller's condition. However, the mechanisms that link song
consistency to condition in chickadees and how these are affected by habitat quality remain
unexplored. By comparison, vocal skills of a European parid, the great tit (Parus major), are
negatively affected by the presence of heavy metal pollutants at the time of song learning
(Gorissen et al. 2003). Though it has been assumed that song consistency in chickadees
reflects the early condition of the signaller, this has not been formally tested. Nor has the
influence of habitat quality on song consistency been addressed.

21

1.8 Study site and caterpillar frass assessment
My studies took place in the John Prince Research Forest (JPRF), north of Fort Saint
James, BC as well as in the surroundings of Prince George, BC (Fig 1.3). I studied two
populations of black-capped chickadee settling in two different habitats that differ mainly by
their age. I compared birds living in mature sub-boreal forest to birds living in very young
forest (harvested approximately 20 years ago and left to naturally regenerate).
In order to test whether young forest actually has more limited food resources for the
chickadees than the mature forest, I conducted a study to monitor the population of
caterpillars during the spring of 2008, 2009, and 2010. During the breeding season, blackcapped chickadees rely heavily on caterpillars that feed upon the trees' leaves. I used frassfall traps to collect caterpillar waste falling from the canopy on my study site. Frass-fall traps
consisted of a plastic bucket (diameter = 30cm) elevated at approximately lm off the ground
on a wooden stand. The open end of each bucket was covered with a mesh in a slightly
inverted funnel shape, upon which I placed paper napkin filters. These filters were the
collection surface for frass falling from the canopy. I positioned four traps in each focal
bird's territory. The traps were positioned around the bird nest, as it is typically at the center
of the territory. I set up four buckets five meters from each black-capped chickadee cavity,
one in each cardinal direction. Every three days, I collected the napkins and installed new
ones. Napkins were folded in upon themselves to ensure that no frass was lost during
removal, and these were immediately placed into paper bags for transport and storage.
Samples were then desiccated at 40°C in a drying oven for 48 hours. With a
magnifying lens, I sorted frass from plant debris, and weighed the frass with an analytical
scale to the nearest 0.0lmg. The four samples for each nest were then added and I averaged

22

the data across nests (N=36) for the total amount of frass in the habitat per period in
milligram/cm2/hour. Seven territories were monitored in 2008, 15 in 2009 and 14 in 2010.
After running an ANCOVA on caterpillar frass quantities with forest age (young and
mature) as the categorizing variable and year as a covariate, I found that more frass was
collected in mature forest than in young forest during the incubation period (F(ii26)=4.35,
N=36, p=0.047) but not later in the breeding season, although a strong trend tends to suggest
the difference in frass availability continues into the early nesting period (F(i.26)=2.65, N=36,
p=0.115). The incubation period occurs early in the season, and typically overlaps with the
dawn chorus period (the birds enter the incubation period towards the end of the dawn chorus
period). As a result, based on the quantities of the caterpillar frass collected in three
consecutive breeding season, my results suggest that the population of caterpillars is
effectively less important in young forest than in mature forest during the chorusing period.

t

23

"Tezzeron Rd"
young site

D.

"Cinnabar"
^mature site

•*5r

"ITP" youn£

site- - • • •

* J
. • tsr.
"Pinchi"
mature site

Figure 1-3 (left side) Map of the JPRF young (red lines) and mature (blue lines) forest site.
Within each sites, two independent plots of 1600m2 (yellow squares) were chosen as study
sites, (right side) Map of Prince George, BC, showing the mature (blue) and young (red) sites
used in chapter 5.

24

1.9 Outline of thesis
My thesis focuses on the effect of habitat quality on vocal performance in the blackcapped chickadee. I was particularly interested in the study of the interactions between the
constraints that limit vocal performance and the quality of the environment. My work
therefore concentrates on the role of habitat quality and food availability at different stages of
the birds' developmental history - during song learning and at the time of chorusing - on
condition and consequently on vocal skills and vigour in this species. In chapter 2,1 tested
whether food availability can affect song output, and thus whether the reduction in output in
poor-quality habitat could be equated to low resources. I secondarily tested whether shortterm supplemental provisioning of food during the chorus period has a greater relative
impact on vocal vigour among birds in young versus mature forests. In chapter 3,1
determined whether habitat alters the ability of males to produce more stereotyped songs and
whether song structure is affected by short-term feeding. The 4th chapter presents the results
of an experiment which explored the consequences of lowered-consistency in song structure
found in young-forest males, when these songs are broadcast to rivals during diurnal singing
interactions. In the 5th chapter, I studied the effect of habitat on the development of blackcapped chickadees during song learning. Using feathers collected from birds in both habitats,
I studied the food availability and the condition of juvenile birds at different stage of song
learning. This study was followed by physiological measurements made on euthanized birds
to compare the development of brain and syrinx in juvenile males newly settled in both
young and mature forest. I then discuss and compare my main findings in chapter 6.

25

2-

SUPPLEMENTAL FEEDING AND DAWN SINGING IN BLACKCAPPED CHICKADEES

Abstract- The dawn chorus is a collective behaviour involving a network of birds signalling
at the same time. Two hypotheses can explain the mechanisms of dawn singing. The
"condition-dependent hypothesis" states that the dawn chorus consists of a "broadcast
network" in which the signal is given in all directions but is not dependent on the singer's
neighbors' chorus. The song output is condition-dependent and can thus be limited by the
amount of food to which a signaller has access to in its territory. On the other hand, the
"social-dynamic hypothesis" states that during the chorus the birds are constantly interacting
with their neighbors. Under this scenario, the dawn chorus consists of an "interactive
network" and the song output is not dependent on the condition of the signaller but on the
interaction with the neighbors. To determine which scenario best explains the mechanisms of
the dawn chorus in the black-capped chickadee (Poecile atricapillus), I set up a
supplemental-feeding experiment in a dyadic fashion by providing 3.5 g of mealworms to
selected birds and compared their song output to that of unfed counterparts by controlling for
rank and habitat. I found that fed birds, both dominant and subordinate males, sang more
than unfed birds. I thus conclude that body condition is a critical variable contributing to
individual differences in song output in the black-capped chickadee and consequently the
functioning of the dawn-chorus network.

26

2.1 Introduction
Dawn singing in birds has often been viewed as a "broadcast network" (Burt and
Vehrencamp 2005); signallers broadcast songs omnidirectionally, and the songs can be
detected by multiple receivers simultaneously. In such a scenario, singing males need not be
interacting with each other directly, as in dyadic countersinging during territorial contests.
Rather, during the chorus, song output of males may honestly signal individual condition—
males display their quality to listening receivers by outsignalling rivals (Otter and Ratcliffe
2005). In support of this hypothesis, studies have found that song output at dawn often
closely approximates attributes of males' energetic state or access to resources (Welling et al.
1997; Lucas et al. 1999; Poesel et al. 2004; Murphy et al. 2008). In such a scenario, females
are able to assess individual differences in males' quality by assessing song output from a
large number of males simultaneously (Otter and Ratcliffe 2005), thus facilitating
comparison of her mate against potential extra-pair sires. This scenario also implies that song
output at dawn is resource-limited; for example, the more food a signaller has at its disposal,
the more energy it can invest in singing (Alatalo et al. 1990; Cuthill and Macdonald 1990;
Thomas 1999).
Burt and Vehrencamp (2005) and Foote et al. (2008), however, have used
microphone arrays to record multiple males chorusing simultaneously and found that malemale signalling may affect patterns of singing during the chorus. This result strongly
supports the social-dynamic hypothesis (Staicer et al. 1996), which proposes that the function
of dawn chorus is the interactive communication and adjustment of social relationships
among males. This finding implies that the dawn chorus functions as an "interactive
network" in which all signallers are interacting with their neighbors. These findings could

27

suggest that mechanisms other than simply the male's energetic state may control song
output during the dawn chorus. Previous findings that song output in chickadees is correlated
with social rank of males (Otter et al. 1997) could thus be interpreted in one of two ways:
either rank reflects differences in males' energetic state or resource access, which are also
signalled via differences in song output, or, alternatively, the correlation between social rank
and song output in the chorus could result from dominant males intimidating neighboring
subordinate males through interactive signalling during the chorus. The latter may not
necessarily imply a limitation of signalling in relation to immediate resource availability on
the part of subordinates but rather the suppression of song output by dominants. Under this
suppression hypothesis, chorus output is unlikely to increase with provision of moderate
amounts of supplemental food, as supplementation is unlikely to change the long-term rank
association between two neighboring birds. If the capacity of individuals to maintain song
output during the chorus (and thus engage in countersignalling) is resource-limited, however,
then food supplementation should increase song output by both dominant and subordinate
birds.
I supplementally fed a subset of male black-capped chickadees (Poecile atricapillus)
and recorded their dawn singing behaviour against that of unfed males of similar rank and
occupying similar nesting habitat, two factors known to influence song output at dawn (van
Oort et al. 2006; Otter et al. 2007). I therefore attempt to assess the importance of a
signaller's short-term resource access on song output during the dawn chorus.

28

2.2 Methods
2.2.1. Study species
The black-capped chickadee is a small resident generalist, common in deciduous and
mixed forest across northern North America. The birds overwinter in stable flocks
characterized by linear dominance hierarchies (Smith 1991). These flocks are composed of
two to five mated pairs that forage and travel together during most of the nonbreeding
season. As the birds maintain a strict hierarchy during these winter months, the rank of each
individual can be determined from dyadic interactions at winter feeding stations (Smith
1991; Otter et al. 1997). During most of the year, chickadees consume a mixed diet of seeds,
berries, and invertebrates, but during the breeding season, they switch to a diet
predominantly of insects (Smith 1991). In early spring, flocking is replaced by territoriality
as pairs begin excavating nests (Smith 1991). During this period, breeding males sing long
bouts of continuous "fee bee" songs close to the nest site at dawn (Otter et al. 1997; Gammon
2004). These dawn song bouts often last over 45 min with average singing rates of 12-15
songs min-1, with some males achieving maximum rates of more than 20 songs min-1 during
the peak of the chorus.

2.2.2. Study site
My study took place in the John Prince Research Forest, Fort St. James, British
Columbia, Canada (54° 40' N, 124° 24' W). I chose two plots of mature forest and two plots
of young forest as study sites. Each plot consisted of -16 ha of forest of homogenous age.
Both of the two mature-forest sites were characteristic of the mixed woods of the sub-boreal
spruce subzone in northern British Columbia (see Otter et al. 2007). Furthermore, both sites
were similar in age structure, neither having undergone commercial logging for more than 80
29

years. At both sites, trees were characterized by an average trunk diameter of 25 cm
(measured at breast height) and a canopy at approximately 25 m. At the two young-forest
sites, the trees had undergone nearly complete clearing in the past 30 years. Both sites had a
flora characteristic of a young, regenerating sub-boreal forest. These sites were characterized
by an average trunk diameter of 10 cm (measured at breast height) and a canopy at
approximately 8 m high.

2.2.3. Winter banding and dominance assessment
I attracted birds to temporary feeders filled with sunflower seeds using playback of
the chickadee's mobbing calls during February 2006 and 2008.1 captured male and female
chickadees on my study sites by using either mist nets or box (Potter) traps mounted on
platforms. I then marked birds with a numbered aluminum ring (Canadian Wildlife Service)
and three colored leg bands. Each bird was given a unique color combination allowing me to
recognize individuals in the field. I determined the bird's sex from body measurements
(Desrochers 1990). I confirmed the sex by observing behaviour during the breeding season. I
aged the birds by rectrix shape (Meigs et al. 1983), categorizing them as either second year
(i.e., entering their second calendar year and therefore approaching their first breeding
season) or after second year (i.e., entering their third or later calendar year and second or
later breeding season).
I determined the composition of flocks and their home ranges by following the birds
attracted to the feeder. I assessed dominance rank by observing dyadic interactions around
the feeders. I used three behaviours to identify the flock hierarchy: if (1) a focal bird
supplanted or chased away its opponent, (2) the focal bird gave a display that elicited a

30

submissive posture in an opponent, or (3) the opponent waited for the bird to leave before
approaching a feeder (Ficken et al. 1990, Otter et al. 1998), the focal bird was considered to
be dominant to its opponent. Thus, I determined the linear dominance matrix for each flock,
classifying birds as low, mid, or high ranking, according to their position within the flock.
Because a female chickadee's rank is correlated with the rank of her social mate (Smith
1991; Otter et al. 1999), I concentrated on determining relative rank of males within flocks.
In flocks consisting of three or more pairs, the male submissive to the alpha male but
dominant over the low-ranking male was considered mid ranking. I observed too few
interactions to determine the rank of six of the 26 birds; in these cases, I used age as a proxy
of rank, as rank is strongly associated with relative age (Smith 1991; Otter et al. 1999;
Ratcliffe et al. 2007).

2.2.4. Supplemental feeding and control of other factors affecting dawn singing
Both rank and habitat strongly influence dawn singing in the black-capped chickadee
(Otter et al. 1997; van Oort et al. 2006). Thus, to determine whether song output at dawn
accurately reflects variation in access to food, I controlled for the effect of rank and habitat
by selecting two males from similar habitat age and similar rank class for dyadic
comparisons. The two males in a dyad were selected, as much as possible, so as not to
include neighbors; typically, the males I paired resided in different forest patches. Only one
dyad was composed of neighbors, the situation resulting from a shift in territory boundaries
after the feeding had begun but prior to the day of recording.
I randomly selected one of the two males in each dyad to receive feeding; the other
served as the unfed control. Each fed bird received approximately 30 mealworms (~3.5 g)

31

every second day for six to ten days before I recorded its dawn singing (three to five feeding
sessions). As mealworms contain approx. 2.5 kcal g-1 of gross energy content (Martin et al.
1976), my supplementation provided up to 8.75 kcal of available food to the birds, close to
the average daily energy requirement (~10 kcal) predicted for the black-capped chickadee
(Smith 1991). I used a quiet playback of the chickadee's mobbing calls to attract only the fed
male to the feeder, which was placed close to the nest excavation on the individual's
territory. To be sure that only the correct male came to the feeder, I checked for 15 min after
the first feeding to ensure that no other chickadee (other than the male's mate) or Redbreasted Nuthatch (Sitta canadensis) ate the mealworms. Most of the time, the mealworms
were gone at the end of the 15 min; if not, I checked the feeder regularly during the day to be
sure the birds ate all the mealworms. Only males that I visually confirmed to be using the
feeders and taking the majority of mealworms provided were included in the study. To
control the effects of the playback on the song output of the fed birds, I used the same
duration of playback to attract the unfed dyadic control males to an empty feeder in their
territory for the same number of days as their fed counterparts.

2.2.5. Recording of the dawn chorus
I recorded both males in a dyad on the same day, following the final feeding session.
In this manner, I attempted to control for the effects of seasonality or abiotic conditions
(weather) on singing behaviour. All the dyads were recorded between 28 April and 12 May
2006 and between 5 May and 13 May 2008. Daily survey of the birds' movements permitted
me to find their nests and/or territory boundaries. As this species begins to sing close to the
nest, the survey provided good information on the potential locations of singing for each

32

male. Sometimes, however, the birds shifted territorial boundaries, particularly in young
forest where black-capped chickadees tend to be less territorial (Fort and Otter 2004; see
above note regarding one dyad being neighboring males). As it was important to ensure that
males recorded were the selected manipulated males, I eliminated from the study several
recordings in which the identity of the singers could not be conclusively deduced. This left
me with a sample comparison of 13 dyads (26 males) to use in analysis among the 26 dyads
that I attempted (eight of 12 dyads attempted in 2006; five of 14 dyads attempted in 2008).
The lower inclusion rate in 2008 reflects unusually late snow melt that year making both
recording and identification of chorusing males much more difficult.
I recorded dawn choruses with Marantz PMD430 audiocassette recorders or Marantz
PMD671 digital recorders in conjunction with one of the following directional microphones:
a Sennheiser ME 67 with a K6 power supply, a Sennheiser MKH 70 with an MZA14 power
supply, or an Audiotechnica ATB-815a. I began recording with the first song of the day and
recorded continuously until the bird stopped singing for five consecutive minutes (Otter et al.
1993, 1997), taking that pause as the "end" of the chorus. All recordings were made after
nests had been constructed but before incubation by the resident female, so my data reflect
song output during the mates' fertile period. The fertile period also reflects a time when
resources available to males in the population I studied are probably limited, as it occurs
several weeks prior to leaf-burst, when lepidopteran prey become abundant (Blondel et al.
1993; Hunter and Elkinton 2000).

33

2.2.6. Statistical analyses
I use five variables from recordings and observations made during the chorus to
quantify the dawn song's output and start time: onset time, chorus length, average song rate,
maximum song rate, and average shift rate. I entered the five dawn-song variables into a
principal component analysis, using STATISTICA (version 6.0, StatSoft, Inc.) to create a
composite variable representing chorus-song output. I then compared principal components
that contributed significantly to variation in fed and unfed males' chorusing behaviour by
using a general linear model with paired design (fed vs. unfed members of the dyad) with
rank of the dyad (dominant or subordinate) as a random classifying variable.

2.3. Results
There was no overall difference in song output of males between 2008 and 2006
(ANOVA, n = 26, P = 0.29), so I combined data from both years for analysis.
Principal component one (PCI) accounted for more than 56% of the total variation among
individuals (Table 2.1), and principal component two (PC2) accounted for an additional
20%. When I included five variables in the analysis, however, only PC1 accounted for more
variation than expected by chance according to the broken-stick method of factor
significance, so I used only PCI in subsequent tests (Jackson 1993, Legendre and Legendre
1998).
Length of the chorus, maximum song rate, average song rate, and average shift rate
all had a strong positive contribution to PCI. The chorus's start time had a smaller and
negative contribution (Table 2.2). As a result, increasing PCI values for an individual

34

indicate a high song output and earlier start of the chorus than for individuals with a lower
PCI score.
Across treatments, overall song output tended to be higher among dominant dyads
than among subordinate dyads (Frank = 4.39, P = 0.06). Supplemental feeding, though, had a
greater effect; within dyads, fed birds had a significantly higher PCI score than unfed birds
(Ffed vs. unfed

= 5.47, P = 0.032) (Fig. 2.1). Thus fed birds began the chorus earlier in the

morning and sang more frequently and for a longer period at dawn than their unfed
counterparts. However, there was no effect of rank on the difference in song output between
fed and unfed birds within a dyad (Frank x (fed vs. unfed)= 1.67, P = 0.23) (Fig. 2.1).

35

Table 2.1 Relative proportion of total variation in dawn singing measures of each of the first
three factors in a Principal Component Analysis.

Percentage variation explained
Total

Cumulative

______

__

PC2 20.2

77.2

PC3 12.9

90.1

36

Table 2.2 The contribution of dawn singing variables to PCI eigenvalues.
Variable

PCI

Length of the chorus

0.78

Maximum song rate (min-1)

0.92

Average song rate (min-1)

0.89

Start time

-0.60

Frequency shift rate

0.46

37

Unfed

• Fed

(0 P

DOM

SUB
Dominance Rank

Figure 2.1 Comparison of the PCI values (song output and start time) from fed and unfed
birds classified by rank. Increasing values of PCI indicate higher song output and earlier
start times during dawn singing. Plots show mean value (dot), and standard error (lines) of
the data.

38

2.4. Discussion
My results demonstrate the influence of food availability on song output at dawn—
birds provided supplemental food have a higher song output at dawn than their unfed
counterparts of similar rank. Furthermore, the increase in song output among fed birds was
similar whether they were dominant or subordinate. These results allow me to better interpret
the disparity in song output between dominant and subordinate birds during the dawn chorus.
If dominant males suppress the song output of their subordinate neighbors, for
example, by interactively matching and thus intimidating them during the chorus, one would
expect a pattern of higher song output among dominant males, as previously shown (Otter et
al. 1997). Under this scenario, the higher song output is akin to a badge of status; differences
in song output are maintained by social reinforcement. If the disparity in song output was due
simply to suppression of subordinates by dominant males, then I might expect only
dominants to increase song output with supplementation. The supplementation I provided
constitutes nearly half the birds' daily energy requirement every other day (providing the
male ate all the mealworms itself), but such short-term supplementation is unlikely to be
sufficient to reverse the very stable, long-term hierarchies of chickadee flocks. However, if
the ability of birds both to maintain song output and to interact dynamically at dawn is
resource limited, rather than socially regulated, then I would expect the fed birds in both rank
classes to increase song output. My study suggests that the difference between dominant and
subordinate males in dawn song output comes from, at least proximately, a difference in food
availability, which could in turn translate into differences in body condition of dominant and
subordinate males during the spring. Dominant males might have access to territories in
which food is naturally more available than in territories left to subordinates, which would

39

allow them to invest more energy in dawn singing. This scenario was supported by van Oort
et al. (2007), who found that dominant males' body mass and hematocrit level during the
breeding season are higher than those of subordinate males, suggesting that dominant male
chickadees are in better condition than subordinates. My findings do not exclude, however,
the dawn chorus from being an "interactive" rather than a "broadcast" network. On the
contrary, the more energy a bird can acquire, the more it might be able to invest in both high
song output and increased countersignalling during the chorus. Whether supplementation
increases an individual male's ability to engage in dynamic countersignalling during the
chorus, however, remains to be tested directly.
My results also suggest a proximate mechanism for the differences in song output
seen between birds settling in different habitat types (van Oort et al. 2006). Decreased song
output during the dawn chorus of birds in young forests may be, in part, a result of food
limitation in these habitats. Several other lines of evidence also suggest that young forests
provide fewer resources than mature forests (Fort and Otter 2004; van Oort et al. 2006; Otter
et al. 2007; van Oort et al. 2007). The resulting effect of this food limitation on signalling
could reduce the ability of females in poor-quality habitat to accurately assess males for
alternative mating strategies (Ramsay et al. 2000; van Oort et al. 2006). Continued studies in
supplementation will allow me to determine whether the effect of feeding differentially
affects the ability of birds to engage in dawn-chorus signalling in different habitat types.

40

3-

VOCAL PERFORMANCE VARIES WITH HABITAT QUALITY IN
BLACK-CAPPED CHICKADEES (POECILE ATRICAPILLUS)

Abstract- In vocal learners, such as songbirds, the ability to maintain an internal acoustic
structure between songs during a chorus seems to be positively correlated with the singer's
condition and may, therefore, represent a reliable signal. For instance, some internal ratios in
the black-capped chickadee (Poecile atricapillus) fee-bee song seem to be more stable in the
song of dominant males than in the song of subordinate males. Habitat quality is also known
to affect the behaviour of this species. Birds settling in young forest have a lower song output
and lower reproductive success than birds occupying mature forests, and it is suggested that
those differences stem in differential food availability across habitat. Because evidence is
accumulating that song performance can be altered by food limitation during the song
learning period, I explored whether habitat quality affects song structure in the same manner
that has been shown for social rank. The ability to maintain the internal structure of song
among birds occupying young forests was consistently lower than birds occupying mature
forests. My results demonstrate that the same difference in song structure exists between
male differing in social rank and between males that settle in different habitat types. My
results suggest that inter-individual differences in song structure originate from differences in
food availability during the song learning period.

41

3.1. Introduction
Motor performance in bird song has been intensively studied in the past few years,
and has been recently partitioned by Byers et al. (2010) into two components: "vigour",
defined as the "the ability to perform energetically expensive acts repeatedly"; and "skill",
which is "the ability to perform difficult motor tasks well". These definitions apply to the
vocal performance in many bird species because they often represent a challenge in term of
production, complexity and/or stereotypy (Podos et al., 2009). In the chorus of many
songbird species, song output provides information about the signallers' condition (Welling
et al. 1997; Lucas et al. 1999; Poesel et al. 2004; Otter and Ratcliffe 2005; Murphy et al.
2008) and I might describe "vocal vigour" in this context as the birds' ability to perform a
chorus with a high song output or song rate. The internal song structure, however, might also
provide information on the signaller's condition in some passerine species (Lampe and
Espmark 1994; Forstmeier et al. 2002; Pfaff et al. 2007). For example, the ability to maintain
a consistent structure from one song to another correlates with other measures of male
quality in chickadees (Christie et al. 2004; Hoeschele et al. 2010), and may reflect selection
on neurological links in song learning and motor pathways (Podos et al. 2009; Byers et al.
2010). Females are known to attend to these structural elements when assessing male quality
(Hoeschele et al. 2010), and thus, the ability to produce songs with a high level of
consistency may be a measure of "vocal skill".
Most studies that have addressed "vocal skill" in birds have focused on either song
complexity or repertoire size (Podos et al. 2009). In some species, however, it may be the
ability to maintain highly-stereotyped internal structure of the songs within and between
bouts of singing that constitutes vocal skill. Such may be the case with black-capped

42

chickadees (Poecile atricapillus), whose relatively simple fee bee whistled song maintains
very high stereotypy in internal frequency ratios and amplitudes (Mennill and Otter 2007).
During the chorus, individual males can alter the frequency at which song is presented, yet
they simultaneously maintain the relative internal frequency structure within and between
notes (a process known as pitch shifting, which is equivalent to transposing the song up and
down in pitch so as to maintain the same 'tune'). Songs at each frequency are presented with
eventual variety - a male will sing a number of songs at a particular pitch, shift to a new
frequency and sing a number of songs at this pitch prior to shifting again.
The ability to maintain these internal frequency ratios correlates with the male's
social rank within social winter flocks, which is a measure of relative resource holding
potential and lifetime reproductive success in chickadees (Ratcliffe et al. 2007). Dominant
males are better at maintaining both the internal song structure and the relative amplitude
between the fee and the bee note than subordinate males (Christie et al. 2004; Hoeschele et
al. 2010). Thus, the ability to maintain frequency ratios between songs (vocal skill) in the
black-capped chickadee also appears to offer some information on the signaller's condition,
similar to what shown previously for song output during the chorus (Otter et al. 1997; Grava
et al. 2009 - chapter 2). It is suggested that the inter-individual differences observed in vocal
skill in the black-capped chickadee can be explained through the "developmental stress
hypothesis" (Nowicki et al. 2002) - subordinate males may sing with a lower consistency
because they suffer from higher stress during song learning, which affects their neural
development and consequently their song learning ability.
Song output and condition, however, are also affected by habitat in chickadees. Birds
that settle in young forests in northern BC, Canada, experience lower reproductive success

43

and a general reduction in condition-dependent behaviour than birds that settle in
neighboring mature forests (Otter et al. 2007b). In particular, birds in young forest had lower
song output during the dawn chorus than birds in mature forest (van Oort et al. 2006). This
habitat effect on the birds' behaviour were suggested to be related to food limitation in young
forests during the non-breeding season (August to March; van Oort and Otter 2005) as well
as during the breeding season from April to July (Otter et al. 2007a; van Oort et al. 2007;
Grava et al. 2009). It is currently unknown, though, whether these habitat differences also
affect the ability of males to maintain the consistency of frequency ratios across songs during
extended bouts of singing. If poor-quality habitat reduces male condition, in particular
during song learning, they may be less able to accurately learn their songs. This could result
in a diminished ability to maintain this internal consistency of songs. To test this, I
compared the songs produced by males of similar social ranks across habitats of differing
ages, which have previously been shown to affect other metrics of individual condition (e.g.
reproductive success - Otter et al. 2007b).

3.2. Methods
3.2.1. Study species
The black-capped chickadee is a common, resident songbird of northern North
America, occupying mixed deciduous/coniferous forests. The birds over-winter in stable
flocks composed of two to five mated pairs which forage and travel together. Flocks have a
linear dominance hierarchy - males are dominant over females, and within sexes adults are
typically dominant over juveniles (Smith 1991). The rank of each bird can be determined by
observing dyadic interactions at winter feeding stations (Smith 1991; Otter et al. 1997). In

44

early spring, flocks begin to disband and pairs establish and defend individual breeding
territories (Smith 1991). During this period, breeding males sing long bouts of continuous
'fee bee1 songs at dawn (Otter et al. 1997; Foote et al. 2008). These dawn song-bouts often
last over 45 min with average singing rates of 12-15 songs per minute, with some males
achieving maximum rates of >20 songs/min during the peak of the chorus (personal data
from 2008, 2009 and 2010).

3.2.2. Study sites
The study took place in the John Prince Research Forest, Fort St James, British
Columbia, Canada (54°40' N, 124°24' W). Four plots of mature forest and four plots of
young forest were chosen as study sites. Each plot consisted of approximately 1600m2 (16ha)
of forest of homogenous age separated by at least 500m and held between three and seven
pairs of chickadees.
Details of the sites are provided in chapters 1 and 2. The four mature sites were
characteristic of the mixed woodlands of the sub-boreal spruce sub-zone in Northern British
Columbia. Further, all the sites were similar in age structure, with no commercial logging
for more than 80 years. Dominant deciduous species in these sites are trembling aspen
(Populus tremuloides), paper birch (Betula papyrifera), and black cottonwood (Populus
balsamifera ssp. trichocarpa). Subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus
contorta) and hybrid spruce (Picea glauca x P. engelmannii) form the predominant conifer
species in these mixed forests, but tend to be in lower relative abundance than deciduous
species. All these sites were characterized by an average trunk diameter of 25cm and a
canopy at approximately 25m.

45

The four young sites were forests that have undergone near-complete clearing in the
past thirty years. All those sites had characteristic flora of a young, regenerating sub-boreal
forest. Dominant deciduous tree species and conifer species were similar to those described
above for the mature sites, but early successional species, such as lodgepole pine, dominated
the coniferous component. These sites were characterized by an average trunk diameter of
approximately 10cm and a canopy at approximately 8m.
These forest classes match those previously reported to affect reproductive success
and song output levels in this species (see Otter et al. 2007), but are located approximately
200km from those previous sites. Further, my study sites are at approximately 100m higher
elevation (850m above sea level) than study sites in Prince George, BC, and are subject to
slightly longer winter conditions and greater snow coverage. Monitoring of breeding of birds
in the two habitat classes at the John Prince Research Forest since 2006 suggest that the
disparity in reproductive success between birds breeding in mature vs young forests is even
greater than reported in Otter et al. (2007).

3.2.3. Winter banding and dominance assessment
Birds were attracted to temporary feeders filled with sunflower seeds using playback
of chickadee mobbing calls during February 2006, 2008 and 2009. The feeders were used for
short periods (several days for banding, and only several hours for dominance assessments)
throughout the fall and winter, to alleviate any potential effects of the feeding on the song
learning. The feeders were used equally, and were typically placed simultaneously, in both
habitats. I captured male and female chickadees on my study sites using mist nets and
marked birds with numbered aluminum bands (Canadian Wildlife Service) and three colored

46

leg bands. Each bird was given a unique color combination allowing me to recognize
individuals in the field. Sex was determined by body measurements (Desrochers 1990) and
behaviour during the breeding season. Ages were determined by rectrix shape (Meigs et al.
1983), which several years of banding with known-aged birds in this area have confirmed to
be consistent for aging birds in either habitat type. Birds were classified as either second-year
(SY; i.e. entering their second calendar year and therefore approaching their first breeding
season) or after-second-year (ASY; i.e. entering their third or later calendar year and second
or later breeding season).
The composition of flocks and relative dominance ranks of birds were determined as
outlined in chapter 2. Briefly, three behaviours were used to identify the flock hierarchy the focal bird was considered dominant to its opponent if: (1) the focal bird supplanted or
chased its opponent, (2) the focal bird gave a display that elicited a submissive posture by the
opponent, or (3) the opponent waited for the bird to leave before approaching a feeder (Otter
et al., 1997). A linear dominance matrix was determined for each flock; birds were classified
as low-, mid-, or high-ranking, depending on their position within the flock. In flocks
consisting of three pairs, the male submissive to the alpha male but dominant over the lowranking male was considered mid-ranking. I observed too few dyadic interactions to
determine the rank of six of the 26 birds; in these cases, I used age as a proxy of rank, as rank
is strongly correlated with relative age (Smith 1991; Otter et al. 1999; Ratcliffe et al. 2007).

3.2.4. Experimental design
Some of the birds in my studied population have been associated with a supplemental
feeding experiment taking place in my study sites during 2006, 2008 and 2009. This study

47

compared the song output of fed birds versus unfed birds in a dyadic comparison, while
controlling for the factor influencing song output at dawn (see chapter 2). To determine
whether short-term food supplementation at the time of singing affected the ability of
individual males to maintain consistent frequency ratios in their songs, I compared the ability
to maintain internal song structure of fed birds versus unfed birds following the pairingprotocol used in chapter 2.1 found no effect of supplemental feeding on the ability of fed
versus non-fed males to maintain the internal song structure (Repeated-measures ANOVA:
F(i,i8) = 0.1, iV= 19, p = 0.76). I therefore included some of the fed birds in the present
experiment. Out of the 22 birds that received short-term food supplementation, 15 entered
the analysis of song structure across habitats - nine were recorded as birds in young forest,
and six as birds in mature forest. Wherever possible, I paired fed birds together in crosshabitat comparisons, or used the fed birds from the young forest sites in comparisons. In this
manner, any residual influence of short-term feeding would be controlled, or would tend to
make findings of habitat affects more conservative.
I recorded the dawn choruses of 32 males during 2006 (eight birds recorded), 2008
(16 birds recorded) and 2009 (eight birds recorded). Four birds in young forest and one in
mature forest were recorded in more than one year of the study, and so appeared in more than
one dyad. However, in all these cases, their dyadic counterpart differed between the two
years.
I paired one bird recorded in young forest with one recorded in mature forest. I paired
the birds by rank (dominant:dominant dyads or subordinate:subordinate dyads), which also
controls for age in chickadees, due to the close relationship between relative rank and
relative age (adult/juvenile) of males chickadee (Otter et al. 1999). I also controlled for

48

seasonality by minimizing the time separating recording of two paired males (difference
between two recordings in the same dyads was 4.3 days +/- 2.7). In fact, when more than two
birds could potentially by paired up, the date of recording was critical as a final choice - the
two birds that were recorded the closest in time were chosen to create a dyad. Those
criterions allowed me to reduce the possibilities to only one dyad that were chosen for the
analysis. As it was important to ensure that males recorded were the selected males (i.e.
dom/sub), several choruses where the identity of the singers could not be conclusively
deduced were eliminated from the studies. I also eliminated the recordings that were not of
sufficient recording quality to allow acoustical analysis. A total of 16 dyads were recorded
(four of six dyads attempted in 2006; eight of 14 dyads attempted in 2008; four of eight
dyads in 2009). Of the four dyads recorded in 2006, one was composed with subordinate
males and three were of dominant males. Of the eight dyads recorded in 2008, two were
composed of subordinate males and six of dominant. Of the four dyads recorded in 2009, two
were composed of subordinate and two of dominant males.
Dyads were recorded between the 28th of April and the 12th of May in 2006, between
the sixth and the 16th of May in 2008 and between the third and the 15th of May in 2009.
Chorus singing for each male was recorded from the first song of the morning until the
terminal song of the chorus (see Grava et al. (2009) for details on recording equipment and
methods). The chorus was deemed to have ended when the male ceased all singing for
greater than five consecutive minutes - as chorus song rates are typically 15 songs per
minute in my study site, this represented a marked cessation of signalling.

49

3.2.5. Sound analysis
I randomly chose nine songs distributed across the chorus for each bird (Christie et
al., 2004). To avoid the effect of pitch shift on the ability to maintain an internal song
structure, I avoided measuring songs within five songs before or after a pitch shift event. I
also selected songs distributed evenly throughout the recordings to have songs representative
of the chickadee's frequency range. I used the sound analysis software Seewave (Sueur et al.,
2008) to monitor the fundamental frequency of every songs. This technique allows for a
more precise data extraction than is possible on raw spectrograms. I then extracted eight
frequency variables from the fundamental frequency - the frequency at the start of the fee
note (Ffee.start) and at the end of the fee note (Ffee.end); the frequency at the start, end, and
maximum, of both syllables composing the bee note (respectively, Fbeel.start and
Fbee2.start, Fbeel.end and Fbee2.end, Fbeel.max and Fbee2.max) (see Fig. 3.1 for more
details).
I used those frequencies to calculate seven frequency ratios for each song (see Table
2.1). I calculated these ratios for each of the nine songs of every bird entering the analysis.
The coefficient of variation for each of those variables was then calculated to represent the
intra-individual variation within notes. This analysis created seven coefficients of variation
for each bird corresponding to the seven ratios calculated in each of the nine songs.

3.2.4. Statistical analysis
The seven coefficients of variation used to quantify the ability to maintain an internal
song structure were entered into a principal component analysis (statistica version 6.0,
StatSoft, Inc.), to create a multivariate measure of vocal skill. I then used a general linear

50

model to compare principal components that contributed significantly to variation in
choruses between young and mature forest.
For every bird entering the analysis, the average absolute pitch at the end of the fee
note was calculated and compared across habitats using a repeated measures ANOVA.

3.3. Results

3.3.1. Principal components analysis
A Kaiser-Meyer-Olkin Measures of Sampling Adequacy (KMO) applied to my data
gave a value of 0.52, indicating that a Principal Component Analysis was sound. A
Sphericity test applied on my data gave a p<10"6, therefore the strength of the relationship
among variables is strong enough to perform a Principal Components Analysis.
Principal component one (PCI) accounted for more than 44% of the total variation
among individuals, and principal component two (PC2) accounted for an additional 19%.
Principal components three, four and five accounted for an additional 12%, 10% and 5%
respectively. As I included seven variables in the analysis, however, only PC1 exceeded the
value of explained variation expected by chance, using the broken-stick method of factor
significance (Jackson 1993; Legendre and Legendre 1998), so only PCI was analyzed in
subsequent tests.
The coefficient of variation of all seven variables had a strong negative contribution
to PCI (Table 3.1). As a result, an increase in PCI is interpreted as a decrease in variability
in all of the seven frequency ratios measured, so an increase in the ability to maintain an
internal song structure

51

beet

fee
02

00

—r-

,

04

00

08

beeJ
1.0

—r—
%2

nmm

ffteSwrt

B

/

/

fbeel Start

fbeelma*
- fbeel 4md

fb«e2.m**
fbe«2.sUrt

\ \—
\

00

~T—
0.2

—1
04

1—
oe

08

-r—

T
"

1.0

1.2

T«*» (•)

Figure 3.1 Extraction of eight frequency variables used to describe the song consistency in
the fee bee song of the black-capped chickadee (Poecile atricapillus). Left: spectrograph^
representation. Fee: first note of the fee bee song. Beel and bee2: the two syllables
composing the second note of the fee bee song. Right: fundamental frequency monitoring.
Ffee.start, Ffee.end: frequency variables extracted from the fee note. Fbeel.start, Fbeel.max,
Fbeelend: frequency variables extracted from beel. Fbee2.start, Fbee2.max, Fbee2.end:
frequency variables extracted from bee2.

52

Table 3.1 Contribution of the coefficient of variation of the seven frequency ratio variables to
PCI eigenvalues.

Variables

Description

PCI

Glissando ratio

Ffee.end / Ffee.start

-0.60

Internote ratio

Ffee.end / Fbeel .start

-0.73

Bee start 1 ratio

Fbeel .start / Fbeel .max

-0.60

Bee end 1 ratio

Fbeel .max / Fbeel .end

-0.61

Bee start 2 ratio

Fbee2.start / Fbee2.max

-0.61

Bee end 2 ratio

Fbee2.max / Fbee2.end

-0.79

Intersyllable ratio

Fbeel .max / Fbee2.max

-0.66

53

3.3.2. Effect of habitat on the ability to maintain internal song structure
I found no difference in the absolute pitch of songs between males in either habitat
class (repeated measures ANOVA: Fa, iS) = 0.04, N= 16,/? = 0.84). I did find, however,
that the ability of males to maintain internal song structure was higher in mature than in
young forest (repeated measures ANOVA: Fa, isj - 5.42, N= 16,/> = 0.03, Fig 3.2). The
effect size of my data (d=0.72) results in a power of 60% with an alpha level of 0.05.

54

1.5

10

&
§•=•

05

3S
M O

c « 00
o «•
°o
cno.
G * "l"

o

w

-05

-10

-1.5

Young forest

Mature forest

Habitat

Figure 3.2 Comparison of the PCI value (song consistency) from birds recorded in young
versus mature forest (General Linear Model: F(l, 15) = 5.42, N = 16, p = 0.03). Increasing
values of PCI indicate higher ability to maintain consistent frequency ratios within songs
across the chorus. Plots show mean value (square), and standard error (lines) of the data.

55

3.4. Discussion
I found that males in young forests produce dawn songs that have more variable
internal frequency structure than do males occupying neighbouring mature forests. Because
there is no difference in pitch across habitat, the differences in song structure cannot be
explained by the fact that higher pitch songs may be more challenging to produce than lower
pitch songs. Also, no differences in quality of recording can be detected across habitat. This
suggests that the distance between the observer and the birds being recorded was not affected
by the structure of the habitat (small vs. big trees) and therefore did not affect the results. As
these two habitat types have been shown in past studies to differ in quality to chickadees, this
suggests that the ability to maintain controlled frequency intervals in the song may be
influenced by ecological parameters. The fact that the ability to maintain an internal song
structure does not seem to be affected by short-term food supplementation suggests that the
difference observed across habitat is entrenched into the individual and cannot be alleviated
by an increase in energy available at the time of singing.
Previous work has shown that the ability to maintain frequency ratios across songs in
chickadees is correlated with the singer's quality; dominant males having a higher ability to
maintain an internal song structure than subordinate males (Christie et al. 2004). Further,
birds in young forests matching those used in my study are associated with lower
reproductive success (Fort and Otter 2004b), reduced territorial behaviour (Fort and Otter
2004a), lower song output (van Oort et al. 2006) and lower body condition of males (van
Oort et al. 2007). I now add that males in these habitats have song structure that is associated
with subordinate males in other studies, despite my design of matching males in similar rank
categories across habitats.

56

Interestingly, difference in the ability to maintain an internal frequency ratios did not
appear to be influenced by short-term food supplementation, which suggests that the effect of
habitat may reflect longer-term differences in singer condition across habitats. One possible
mechanism that could explain the differences in the ability of males to maintain internal song
ratios is differences in food availability during the song-learning periods (Developmental
Stress Hypothesis - Nowicki et al. 2002). Such an effect was demonstrated in swamp
sparrows (Melospiza georgiana) that were nutritionally stressed during their development;
such males produce less accurate copies of the model songs with which they were tutored
(Nowicki et al. 2002).
In chickadees, early development of the song center nuclei in the brain likely occurs
between the post-fledging period and their first breeding season. The fall and winter
following dispersal of fledglings are correlated with settlement into their first winter flocks
(Smith 1991), and subsongs are commonly heard during the period between early fall and
mid winter (personal observation). Even though the production of subsongs starts before
fledglings are fully independent (day 20; Baker et al. 2003), the crystallization does not seem
to occur before the end of winter. Therefore, the first fall and winter may be a critical period
in chickadee neural development, as the juvenile birds have to forage by themselves for the
first time. The developmental stress hypothesis suggests that brain development required for
song learning is linked with the amount of food birds can find during this period (Nowicki et
al. 2002). The quantity or quality of food available may differ substantially depending on the
habitat the birds settle in. Wintering chickadees in the young forest sites carry higher levels
of fiircular fat than do birds in mature forest (van Oort and Otter 2005); such high fat levels
in other members of the Paridae family occur among birds that experience decreased or

57

inconsistent access to food (Gosler 1996; Gosler and Carruthers 1999). If these differences in
fat loading reflect differences in food availability, this may coincide with the period of song
learning for the juvenile birds. Birds in young forest may be less able to invest resources in
overall brain development, and this may compromise development of song center nuclei,
which might explain the differences in song production I found in the current study.
My results suggest habitat may indirectly affect male reproductive success through
altering their song structure. Maintenance of internal structure of notes within and between
songs is known to accurately reflect male social rank in chickadees, and is also known to be
attended to by female within this species (Hoeschele et al. 2010). If the ability to maintain
consistency in frequency ratios represents a vocal skill used by the female to assess male
quality, males with high consistency should be preferred over males with low consistency.
Because males occupying low-quality habitats produce songs with lower consistency in song
structure, they may not be able to adequately compete in territorial song contests against
males able to occupy high-quality habitats; as a result, they may have a more difficult time
attracting mates or extra-pair partners or have to expend extra effort defending their
territories from intruding males.
An alternate possibility to explain witnessed differences in vocal skill across habitats
is that individual chickadees that settle in either habitat differed before they settled in both
habitats. For example, dispersing juveniles in better condition may be more likely, or better
able, to integrate themselves into flocks in mature forest than in young forests, creating a
settlement bias that is later revealed by differences in ability of males to maintain song
structure, van Oort and Otter (2005), however, detected no difference in the condition of
feather growth rates (ptilochronology) of males settling in either habitat types used in the

58

current study. Studies are currently underway to look at other metrics of condition among
juvenile birds settling in either habitat class, but even if differences among settling birds
exist, differences in habitat quality between the two forest classes (Otter et al. 2007b) are
likely to amplify this disparity.
My results imply a strong effect of habitat quality on individual behaviour and
reproduction. Habitat quality does not only appear to affect a bird's prospects of successfully
nesting during the breeding season, but may also play a role in how they signal their quality
throughout their lifetime. A similar long-term effect of habitat on song learning has been
described by Gorissen et al. (2005), who showed that male great tits learning their song in
areas with elevated heavy-metal pollution display a less stereotyped song than birds in less
polluted habitat. In this case, the stress comes from a high exposure to contaminants known
to interfere with brain development. Human disturbances often alter habitats - such as the
mosaics of abutting young and mature forest patches within northern Canada that result from
commercial logging. My results suggest that these may not only impose short-term effects
on immediate condition, but also impose potentially permanent effects on communication
and development of sexually-selected signals. Conservation measures designed to
compensate for decreased habitat quality, such as supplemental feeding during the breeding
season, may not alleviate long-term effects of habitat quality on avian communication.

59

4 - HABITAT QUALITY AFFECTS THE PERCEPTION OF DOMINANT MALES
BASED ON THEIR RELATIVE SONG STRUCTURE

Abstract- The ability to maintain internal song structure in black-capped chickadees is
known to be affected by both rank and habitat quality. Further, lab studies show that females
discriminate between songs of dominant and subordinate males that vary in acoustic
structure. This study was set up to investigate whether males also rely on acoustic structure
to assess males quality during diurnal interactions and whether habitat quality affects the
perception of male social rank, based on their relative song structure. I conducted a playback
experiment to simulate an intrusion by dominant males recorded either in low-quality (young
forest) and high-quality (mature forest) environment into the territory of dominant males
breeding in both habitats. The playbacks reflected the difference in song structure already
known to exist across habitat. My results show that stimuli from young forest elicited less
territorial response from dominant males in either habitat. I also found that males in mature
forest responded less to young-forest stimuli compared to mature-forest stimuli, despite the
stimuli in both cases being recorded from dominant males within their own winter flocks. I
demonstrate in this study that the ability to maintain internal song structure in the blackcapped chickadee constitutes a signal that appears to be used by males to assess the level of
threat of perceived intruders, and that this perception is affected by habitat quality in which
the stimulus was recorded.

60

4.1. Introduction
Among species with hierarchical social organization, dominance rank can have a
major impact on resource access and fitness. This is evident in the higher reproductive
success experienced by dominant males among many non-human animals (see Ellis 1995 for
review). In birds, most studies on dominance effects on reproductive success and condition
focus on the Paridae family (titmice and chickadees) in which many of the species have a
stable and hierarchical social organization within flocks. Dominant males have a preferential
access to resources (Ekman 1987 (willow tits, Poecile montanus); Desrochers 1989 (blackcapped chickadee, Poecile atricapillus)), enhanced survival (Ekman and Askenmo 1984
(willow tits); Koivula 1994 (willow tits)) and enhanced reproductive success (Ellis 1995;
Otter et al. 1998 (black-capped chickadee)). However, ranks relations are a relative measure;
hierarchies will be established within flocks regardless of the absolute condition of
individuals in the population.
The condition of individuals within a population, however, can be altered by habitat
quality. For example, several studies have shown that birds in poor-quality habitat may have
lower overall condition than their counterparts in good-quality habitat. Strong and Sherry
(2000) used feather regrowth (ptilochronology) and biomass to show that ovenbirds (Seiurus
aurocapillus) in habitat with low access to resources were in poorer conditions than those
where resource access was higher. Grubb and Yosef (1994) also used ptilochronology to
assess nutritional condition, and found that loggerhead shrikes (Lanius ludovicianus) were
affected by habitat quality. Similarly, Carlson (1998) found that the condition of Whitebacked woodpeckers' (Dendrocopos leucotos) was a reflection of their territory quality.
Wintering chickadees in poor-quality habitat carry higher levels of furcular fat than do birds

61

in high-quality habitat (van Oort and Otter 2005); such high fat levels in other members of
the Paridae family occur among birds that experience decreased or inconsistent access to
food (Gosler 1996; Gosler and Carruthers 1999).
These differences in condition across habitats can alter different condition-dependent
behaviours. Within European parids, habitat quality affects the breeding performances of
Great tits (Parus major) (Sanz et al. 2010) and blue tits (Cyanistes caeruleus) (Tremblay et
al. 2003; Lambrecht et al. 2004; Tremblay et al. 2005). Among the new-world parids, blackcapped chickadees in poor-quality habitat have a decreased territorial behaviour (Fort and
Otter 2004a) and a lower reproductive success (Fort and Otter 2004b) than birds in goodquality habitat. Song output in the black-capped chickadee - which reflects resource access of
the signaller (chapter 2) - is also lower in poor-quality habitat compared with high-quality
habitat (van Oort et al. 2006).
Males of the same relative stature within their population may still differ in overall
condition when comparing between populations. Population health cannot necessarily be
measured simply by the presence/absence of dominant/subordinate relationships and stable
hierarchies within a population. Should emigration of individuals from one population to
another occur, the ability of residents to determine the status of intruders may rely on
assessment of traits that vary on a continuous scale, as they may not have information on the
relative rank status of the intruder. In most songbirds, evidence suggests that song features
can be used by both males and females to assess a signaller's condition. For example, among
black-capped chickadees (a common forest songbird in northern North America with a linear
dominance hierarchy within flocks), dominant birds are known to have a higher song output
than subordinate males (Otter et al. 1997). The study of the fine acoustic structure of the

62

songs in the black-capped chickadee showed that dominant birds have a higher ability to
maintain consistent, within-songs internal frequency ratios (aka song consistency) than
subordinate males (Christie et al. 2004). Both of these signals are therefore conditiondependent and may be used by males and females to assess a signaller's condition. However,
habitat is also known to affect both of these features. Males in poor-quality habitat (young
forest) have both lower song output and lower song consistency than males of the same rank
in good quality habitat (mature forest). When controlling for both rank and habitat quality,
only song output is affected by immediate access to food resources (chapter 2). Song
consistency may reflect a longer-term conditional signal (chapter 3) and therefore might be a
useful metric for males to assess the relative condition of unknown rivals. It has been shown
that features inherent in the song of male black-capped chickadees are indicative of their
condition (Hoeschele et al. 2010), and that those signals are attended by females during
playbacks. However, whether inherent features in the song of the black-capped chickadee are
used by males to assess a signaller's condition, and whether differences in such traits across
habitat affect the relative perception of males within rank classes, are not known.
In this study, I tested whether song consistency can be used by dominant males to assess an
intruder's condition and whether the differences in song consistency within rank class across
habitat affect the perceived status of the simulated intruder.

4.2. Methods
4.2.1. Study sites
The study took place in the John Prince Research Forest, Fort St James, British
Columbia, Canada (54°40' N, 124°24' W). Four plots of mature forest and four plots of

63

young forest were chosen as study sites. Each plot consisted of approximately 1600m2 (16ha)
of forest of homogenous age separated by at least 500m and held between three and seven
pairs of chickadees.
Details of the sites are provided in chapters 1 and 2. The four mature sites were
characteristic of the mixed woodlands of the sub-boreal spruce sub-zone in Northern British
Columbia. Further, all the sites were similar in age structure, with no commercial logging
for more than 80 years. Dominant deciduous species in these sites are trembling aspen
(Populus tremuloides), paper birch (Betula papyrifera), and black cottonwood (Populus
balsamifera ssp trichocarpa). Subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus
contorta) and hybrid spruce (Picea glauca x P. engelmannii) form the predominant conifer
species in these mixed forests, but tend to be in lower relative abundance than deciduous
species. All these sites were characterized by an average trunk diameter of 25cm and a
canopy at approximately 25m.
The four young sites were forests that have undergone near-complete clearing in the
past thirty years. All those sites had characteristic flora of a young, regenerating sub-boreal
forest. Dominant deciduous tree species and conifer species were similar to those described
above for the mature sites, but early successional species, such as lodgepole pine, dominated
the coniferous component. These sites were characterized by an average trunk diameter of
approximately 10cm and a canopy at approximately 8m.
These forest classes match those previously reported to affect reproductive success
and song output levels in this species (see Otter et al. 2007b), but are located approximately
200km from those previous sites.

64

4.2.2. Winter banding and dominance assessment
Birds were attracted to temporary feeders filled with sunflower seeds using playback
of chickadee mobbing calls during February 2010.1 captured male and female chickadees on
my study sites using mist nets and marked birds with numbered aluminum bands (Canadian
Wildlife Service) and three colored leg bands. Each bird was given a unique color
combination, allowing me to recognize individuals in the field. Sex was determined by body
measurements (Desrochers 1990) and behaviour during the breeding season. Ages were
determined by rectrix shape (Meigs et al. 1983). Birds were classified as either second-year
(SY; i.e. entering their second calendar year and therefore approaching their first breeding
season) or after-second-year (ASY; i.e. entering their third or later calendar year and second
or later breeding season).
The composition of flocks and relative dominance ranks of birds were determined as
outlined in chapter 2. Briefly, three behaviours were used to identify the flock hierarchy the focal bird was considered dominant to its opponent if: (1) the focal bird supplanted or
chased its opponent, (2) the focal bird gave a display that elicited a submissive posture by the
opponent, or (3) the opponent waited for the bird to leave before approaching a feeder (Otter
et al. 1997). A linear dominance matrix was determined for each flock; birds were classified
as low-, mid-, or high-ranking, depending on their position within the flock. In flocks
consisting of three pairs, the male submissive to the alpha male but dominant over the lowranking male was considered mid-ranking.

65

4.2.3. Playback design
I compared the reaction of focal dominant birds to both a young-forest stimulus
(songs recorded from a dominant male from a territory in a young forest) and to a matureforest stimulus (songs recorded from a dominant male from a territory in a mature forest). To
produce my stimuli sets, I used songs recorded from dominant males recorded from 2000 to
2003 in young and mature forest around UNBC in Prince George, BC (54° 53' N, 122°
48'W), and dominant birds recorded in 2006, 2008 and 2009 in the young and mature forest
in the JPRF (see study site). I did an initial screening of stimuli for recordings that contained
both good recording-quality of songs within individual males as well as of comparable
recording-quality between males. For each male, I then analysed song consistency as
described in chapter 3. This entailed measuring seven frequency ratios from nine fee bee
songs extracted from the dawn chorus of the stimulus males and calculating a coefficient of
variation of those ratios for each male. I then conducted a Principal Components Analysis
(PCA) on those coefficients of variation to obtain a single value (PC score) describing song
consistency. I ranked stimuli in each habitat based on their relative song consistency (highest
to lowest within an individual male). I then paired stimuli between habitats for these
rankings; e.g. males in either habitat with most consistent songs against each other, males
with 2nd most consistent songs against each other etc. Finally, I standardized playbacks for
amplitude so that these matched both between dyadic pairs and across all playbacks. Using
these criteria, ten dyadic stimuli sets (young-forest vs mature-forest stimuli) were constructed
from recordings involving 20 stimuli males.
Playbacks were standardized for length, average song rate and rate of frequency
shifting to reflect average singing patterns witnessed in my study site. Each playback was

66

composed of 12 different songs extracted from a male's chorus- six songs were extracted
from a frequency in the lower range of the bird's chorus, and six were extracted from the
higher frequency range of the bird's chorus. I then randomized whether the playback would
start with the low or high frequency. Six songs at the first frequency were broadcast every
five seconds, then the playback switched to the other frequency for the next six songs. This
pattern was then repeated to create a 2 min playback with 12 songs per minute and three
frequency shifts. The volume of all playbacks was normalized at 80db 1.5m from the
speaker.

4.2.4. Playback experiment
For each subject male, I randomly chose one of the dyadic stimuli sets for the
playback trials, and then randomized whether I started with the young-forest or mature-forest
stimuli. The focal birds were tested in the middle of their territories. I set up the playback
speaker (Honeytone, Danelectric) on a tripod at about 1.2m from the ground. Before I started
the playback I made sure the bird was within hearing distance from the speaker. When the
birds could not be spotted, I used a mobbing call to attract the bird toward the playback
speaker. When the birds either approached or vocally responded to the mobbing call, I ceased
broadcasting, waited 30 seconds, and then began the playbacks. I used a 40m rope marked
off at five meter intervals centred on the playback speaker to help estimate the distances of
the bird relative to the speaker during the playback. Using a directional microphone
connected to a digital recorder, I recorded all of the bird's vocalization during the trial.
Following presentation of the first stimulus, I waited at least one hour and then tested the
same bird with the other playback of the dyad. All birds were tested with both playbacks on

67

the same day. I extracted ten variables potentially describing the strength of the response of
the tested males to the playbacks - seven variables on the spatial position and movement of
the birds during the trial and three variables of the vocal behaviour of the tested birds (See
table 4.1)
I tested a total of 18 subject males - all 18 were dominant males within their own
flocks, but ten subject males were occupying territory in young forest and eight in mature
forest. To select the dyadic stimuli to be used on a focal male, I tried to pick a dyad that had
not previously been used, while also making sure that none of the birds used to create
stimulus are known by the subject male. This resulted in two different dyadic stimuli that had
to be used twice in both habitats, but otherwise the stimulus sets were used only once per
habitat.

4.2.5. Statistical analyses
The ten variables used to quantify the territorial response were entered into a
Principal Component Analysis (statistica version 6.0, StatSoft, Inc.), to create a multivariate
measure of response to playback. I then used a mixed repeated-measures ANOVA model to
compare the principal components for both within-subject effects (response to young-forest
vs mature-forest stimuli) and between-subject effects (whether response to stimuli differed
depending on whether the subject males themselves were occupying either mature or young
forest). I did mean comparison, using a post hoc Fisher's LSD test, where significant
differences exist.

68

Table 4.1 Contribution of the seven variables of song consistency to PC eigenvalues and the
total percent of variation explained by each of the two first factors. Variables having
significant contribution to the PC (>0.33, Ho et al 2006) are bolded.

PCI

PC2

Latency to first response

-0.60

-0.13

Closest approach

-0.77

0.54

Time spent between 0 and 5m

0.60

-0.63

Time spent between 5 and 10m

0.43

0.12

Time spent between 10 and 15m

0.05

-0.65

Time spent between 15 and 20m

0.62

0.26

Time spent more 20m

-0.84

0.48

Number offee bee songs

0.60

0.62

Number of frequency matching

0.75

0.43

Number of overlapping

0.54

0.46

% Total Variation explained by
each Principal Component

38.10

22.34

Variables

69

4.3. Results
4.3.1. Principal Component Analysis
Both a Kaiser-Meyer-Olkin Measures of Sampling Adequacy (KMO >0.05) and
Bartlett's test of sphericity (P<0.05) applied to my factors indicated that the Principal
Component Analysis was sound.
Principal component one (PCI) accounted for more than 38% of the total variation
among individuals (Table 4.1), and principal component two (PC2) accounted for an
additional 22%. As I included ten variables in the analysis, both PCI and PC2 exceeded the
value of explained variation expected by chance using the broken-stick method of factor
significance (Jackson 1993; Legendre and Legendre 1998), so both PCI and PC2 were
interpreted and analyzed in subsequent tests.
As a rule of thumb, Ho et al. (2006) suggest that variables with individual
contributions of 0.33 or higher should be considered as contributing significantly to the
individual principal component measure. Nine of the ten variables exceeded this contribution
to PCI, and seven to PC2 (Table 4.1). Out of the nine variables, three variables had a strong
negative contribution to PC1 (Latency to first response, closest approach and time spent
further than 20m from the speaker), and six had a strong positive contribution to PC1 (time
spent less than 5m, between 5 and 10m, and between 15 and 20m from the playback, the total
number of songs, the number of songs matching the playback in frequency and the number of
songs overlapping the playback in time). Thus, higher values of PCI indicate rapid
responses, closer approaches and higher aggressive vocal responses to the playback, and thus
would be interpreted as strong aggressive response.

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Out of the seven variables contributing to PC2, two had a strong negative contribution (time
spent less than 5m and between 10 and 15m from the speaker) and five had a strong positive
contribution (closest approach, time spent further than 20m from the speaker, total number of
songs given during the playback and total number of songs overlapping the playback in time
as well as in frequency). As a result, an increase in PC2 indicates birds maintaining a larger
distance from the playback speaker and increasing their songs rates. This would likely be
interpreted as a more tentative or weaker response to playback than PC 1, but still indicative
of a territorial response to the stimuli.

4.3.2. Effect of habitat on playback response
Overall, mature-forest stimuli elicited a more aggressive response than young-forest
stimuli among subject males (repeated measure ANOVA, F(i>i6)=7.01, N=18, p=0.02), but
there was also a between-subject effect of habitat on how subject males responded to stimuli
(repeated measure ANOVA, F(iii6)=4.55, N=18, p=0.049). Subject males occupying mature
forest had a significantly greater response to mature-forest stimuli than young-forest stimuli
(Post hoc Fisher's LSD test, df=26.2, N=8, p=0.005; Fig 4.1). Conversely, there was no
significant difference in response to either stimuli class among subject males that occupied
young forests (Post hoc Fisher's LSD test, df=26.2, N=10, p=0.70; Fig 4.1). No differences
were detected to the response of the young-forest stimuli across habitat (Post hoc Fisher's
LSD test, df=26.2, N=18, p=0.25; Fig 4.1) nor in the response to the mature-forest stimuli
across habitat (Post hoc Fisher's LSD test, df-26.2, N=18, p=0.32; Fig 4.1). The response of
subject males in young forest to both stimuli seems to be intermediate compared to the
response of subject males in mature forest to both stimuli (see Fig 4.1).

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I found no significant difference in PC2 scores for the within-subject (comparison of
response to two stimuli types-General linear model on PC2, F(i,i6)=0.7, N=18, p=0.42), or
between-subject (habitat) effects (General linear model on PC2, F(i,i6)=0.41, N=18, p=0.53).

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-2.5

Young forest

Mature forest

Habitat

Figure 4.1 Dominant birds in mature forest respond more aggressively toward a dominant
intruder from mature forest than from an intruder from a young forest (Post hoc Fisher's
LSD test, df=26.2, N=8, p=0.005). There was no significant difference in response to either
stimuli class among focal males in young forests (Post hoc Fisher's LSD test, df=26.2, N=10,
p=0.70). Circles and squares stands for the response to young forest and mature forest
stimuli, respectively. Plots show mean and bars show standard errors.

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4.4. DISCUSSION
I observed differences in the strength of response among subject males depending on
song consistency of playback stimuli and whether the stimuli originated from either young or
mature forest. Subject males respond more strongly towards stimuli with a high song
consistency (young-forest stimuli) than toward stimuli with a low song consistency (matureforest stimuli). This differential response occurred despite controlling for relative rank of the
subjects and stimuli - all tested subject males were dominant males within flocks in their
populations, and the stimuli were taken only from dominant males as well. Since song
consistency is known to vary with male rank (Christie et al. 2004), it is very likely that song
consistency is correlated with a signaller's underlying condition. Although song consistency
has been shown to differ across rank classes (Christie et al. 2004) and females show
differential response to songs of dominant and subordinate males that differ only in internal
structure (Hoechele et al. 2010), my study provides the first evidence that this conditiondependent signal appears to be used by males to assess the quality of other males.
Song consistency in the black-capped chickadee is suggested to reflect the bird
condition at the time of song learning (Christie et al. 2004; chapter 3). Birds who face higher
developmental stress, because they are not able to forage effectively by themselves after
fledging for example, may experience lower development of the brain nuclei involved in
song learning, and therefore would not be able to accurately learn songs from tutors
(Nowicki et al. 2002). This in turn may lead to either inability to accurately produce speciesspecific songs or high variability in renditions of songs. This has already been demonstrated
in other species in the Paridae such as in the great tits (Doutrelant et al. 2000; Gorissen et al.
2005), and also in other species outside of the Paridae (swamp sparrow Melospiza georgiana

74

fNowicki et al. 2002), zebra finch (Zann and Cash 2008) and European starling Sturnus
vulgaris (Buchanan et al. 2003; Spencer et al. 2004)). Thus, in the black-capped chickadee,
song consistency could provide reliable information about the ability of the signaller to
effectively find and secure resources during the post-fledging period. However, this signal is
entrenched into the signaller and cannot be altered subsequently, even if its body condition
improves.
I further found that the response of subject males was context-specific: subject males
in mature forest respond more strongly toward the mature-forest stimuli than toward the
young-forest stimuli, despite the fact that these stimuli both arose from dominant males
within their individual populations. By comparison, subject males that were tested in young
forests showed no differential response to either stimulus class, and the magnitude of their
overall response was intermediate to those of subject males tested in mature forests. The
intensity of response of territorial males to intruders is expected to be correlated with the
threat represented by the intruders. If the intruder signalling is in poorer condition, subject
males may perceive the threat to its territory as minimal, and the predicted response would be
lower. Subject males in mature forest may not have responded as strongly to young-forest
stimuli because they perceived the intruder as being a lesser threat than the mature-forest
stimuli. Subject males tested in young forest, however, responded with similar levels to both
stimuli, suggesting that they consider both "intruders" as of being of a similar condition. The
overall response level of these males fell between that given to mature-forest stimuli versus
young-forest stimuli by subject males tested in mature forest. This does not mean that
subject birds tested in young forest do not use song consistency to assess an intruder's
quality, but it means that the two stimuli presented to these subject males may be perceived

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as of equivalent threat. This may stem from subject males, themselves, being in poorer
condition due to occupying poorer habitat quality. Such males may be limited (either by
energy or motivation) to mount as strong a response on a perceived threat as did subject
males in mature forest to mature-forest stimuli, but simultaneously perceived young-forest
stimuli to be more equivalent to themselves in condition, and thus requiring a higher level of
response.
This interpretation of the results is predicated on the metrics used as constituting a
high-aggression response. To describe the response of subject males to the playback, I
recorded the movement as well as the vocal behaviour of the birds. It is well accepted that a
closer approach toward an intruder is considered as an aggressive response. However,
whether some vocal signals (such as frequency matching and song overlapping) can be
considered as aggressive signalling is the subject of current debate (Searcy and Beecher
2009; Naguib and Mennill 2010; Searcy and Beecher 2011). Among the variables used in
my study, closer approaches toward and sustained activity close to the playback speaker was
correlated with both high frequency matching and song overlapping (all contributing strongly
to PCI). These latter two signals (frequency matching and signal overlapping) were
performed by the subject male towards a static playback, indicating that the male may be
altering his own signalling behaviour to intentionally match and overlap that of the perceived
intruder. Past criticism of these metrics has pointed to a lack of such matching and
overlapping in natural encounters as evidence of these not constituting escalating signals in
birds (Searcy and Beecher 2009). However, natural encounters allow signallers to
dynamically adjust their signals if rivals attempt to match or overlap them, and thus these
patterns in natural encounters may not be clearly evident. My study suggests that males do

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attempt to match and overlap rivals, and that such behaviours are correlated with other
traditional metrics of aggressive response, and may be used to accentuate aggressive
behaviour toward an intruder.
The ability to produce songs with consistent internal structure may be linked to early
neural and motor development. This in turn may be dependent upon ability to secure
resources early in life, and thus song consistency in chickadees may constitute a conditiondependent trait that reflects information on early development. Even though males may
achieve the status of high-ranking males in young forest, thanks to their relatively higher
condition, their song consistency may continue to reflect limited resource availability.
Chickadees who settle in mature forest might face less developmental stress, and achieve
higher levels of condition, and song consistency. As my results show that males appear to
differentiate between rivals on variation in song consistency, and this is context-dependent, it
suggests that males are able to use song consistency as a measure of absolute condition of
rivals regardless of the rival's social rank. While social rank remains a strong metric for
comparing males within populations (Ratcliffe et al. 2007), the ability to use traits such as
song consistency that vary on a more continuous scale may allow comparisons of the relative
condition of birds across habitats that differ in resource quality. This would allow males to
assess relative condition of unknown rivals where habitat patches intersect - such as the
mosaics of abutting young and mature forest patches within northern Canada that result from
commercial logging. As suggested by Godfrey (2003), condition-dependent signals such as
song consistency may also provide researchers with metrics that vary on a continuous scale
to compare the relative condition of birds breeding in different habitats, and thus be used as a
mean of assessing habitat health.

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5 - EFFECT OF HABITAT QUALITY ON EARLY CONDITION AND NEURAL
DEVELOPMENT OF SONG CENTER IN JUVENILE BLACK-CAPPED
CHICKADEES

Abstract- In songbirds, the ability to learn and render the species-specific song is influenced
by the development of both the song nuclei in the brain and of the syrinx (bird's vocal
apparatus) early in the bird's life. In black-capped chickadees, habitat quality is known to
affect song structure, with birds in high-quality habitat (mature forest) having a greater
ability to render songs with consistent internal frequency ratios than birds in low-quality
habitat (young forest). Although this difference is suspected to stem from differences in
development, the developmental status of juvenile birds in either habitat remains unexplored.
In this study, I used ptilochronology and feather corticosterone (CORT) to compare the
condition of juvenile chickadees in young and mature forest during two distinct periods of
song learning - the memorization period, which occurs prior to settlement, and the
sensorimotor learning period, which occurs post-settlement. A sample of juvenile males was
«

captured and euthanized several weeks prior to their first breeding season to compare the
development of song center nuclei and syrinx in both habitats. The natal - feather CORT
levels were greater among birds that settled in mature than young forests - as these feathers
were grown pre-settlement, they might reflect differences in condition during the
memorization period. This difference in condition is reflected by differences in syrinx and
song center nuclei development later during the sensorimotor learning period - birds in
young forest have smaller syrinx and HVC, and bigger RA, than birds in mature forest.
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Those differences could possibly be responsible for the difference in consistency in song
structure observed across habitats. Birds settling in young forest seem to differ from birds in
mature forest prior to settlement - as this difference is equated with lower condition in other
species, and young forests are known to be lower in quality to chickadees, this finding
suggests a despotic distribution of black-capped chickadees across habitats. This difference
in condition across habitats, combined with potential compounding effect of differences in
winter resources between habitats, could influence the difference in syrinx and neural
development seen in juveniles males during the early spring, and influence the male's ability
to learn and render their species-specific song.

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5.1. Introduction
In songbirds, the learning, perception and production of song are controlled by a set
of interconnected brain nuclei known as the 'song system' (Doupe and Kuhl 1999; Brainard
and Doupe 2002; Bolhuis and Gahr 2006; Bolhuis et al. 2010). The song system is present in
species that learn their song(s) by the imitation of species-specific communication sounds
(Doupe and Kuhl 1999; Brainard and Doupe 2002). The imitative learning of song occurs
during a sensitive period early in development (Catchpole and Slater 2008; Bolhuis et al.
2010) that is divided into two sub-periods - initially, there is a memorization learning period,
which is then followed by the sensorimotor learning period (Hultsch and Todt 2004; Bolhuis
and Gahr 2006). In this second sensorimotor phase, the bird repeats the song initially learned
during the memorization phase, and tries to match this produced song to its internalized
template (Bolhuis and Gahr 2006). In this phase, the brain nuclei of the song system develop
intensively until the first breeding season, where the whole brain development is set and the
song crystallized (Hultsch and Todt 2004). This phase is critical for normal song
development; if there are perturbations to their development, birds do not properly produce
their songs (Nowicki et al. 2002).
The developmental stress hypothesis proposes that the condition of the birds during
the period of song learning may affect the birds' neural development and consequently the
song production quality (Nowicki et al. 2002). Nowicki et al. (2002) found brain nuclei of
the song system, the robust nucleus of the arcopallium (RA) and HVC (Higher Vocal Center;
Reiner et al. 2004; Jarvis et al. 2005), were both significantly smaller in nutritionally-stressed
birds. Such condition-dependence may explain the acquisition of song types in birds with a
vocal repertoire. For example, Doutrelant et al. (2000) showed that tarsus length correlated

80

positively with song repertoire in blue tits (Cyanistes caeruleus) and Nowicki et al. (2000)
demonstrated that a significant relationship existed between feather growth and song
repertoire in the great reed warbler (Acrocephalus arundinaceus). Food availability is an
important factor dictating a bird's condition and birds suffering from low food availability at
the time of song learning have lower song learning abilities, whether this happens when the
birds are nestlings (swamp sparrow Melospiza georgiana, Nowicki et al. 2002), during the
early post natal period (zebra finch, Zann and Cash 2008) or when the birds are juveniles
(european starlings Sturnus vulgaris, Buchanan et al. 2003; Spencer et al. 2004).
One environmental factor that may limit food availability, and thus impact the
condition of the birds, is habitat quality. Strong and Sherry (2000) used rates of feather
regrowth (ptilochronology) and biomass to show that ovenbirds (Seiurus aurocapillus) in
habitat with low access to resources were in poorer condition than those where resource
access was higher. Grubb and Yosef (1994) also used ptilochronology to assess nutritional
condition, and found that loggerhead shrikes (Lanius ludovicianus) were affected by habitat
quality. Similarly, Carlson (1998) found that the condition of white-backed woodpeckers'
(Dendrocopos leucotos) was a reflection of their territory quality. In the Paridae family
(chickadees and titmice), wintering black-capped chickadees (Poecile atricapillus) in poorquality habitat carry higher levels of fiircular fat than do birds in high-quality habitat (van
Oort and Otter 2005); such high fat levels in other members of the Paridae occur among birds
that experience decreased or inconsistent access to food (Gosler 1996; Gosler and Carruthers
1999).
Habitat quality is also known to impact condition-dependent behaviours in
chickadees. Birds in young forest have a lower reproductive success (Fort and Otter 2004a),

81

lower territorial behaviour (Fort and Otter 2004b) and lower song output (van Oort et al.
2006). These condition-dependent behaviours appear to reflect low food access in poorquality habitat, but have been measured primarily during the breeding season. In support of
this hypothesis, Otter et al. (2007a) found subordinate female chickadees in young forest had
a higher food-solicitation calling rates than subordinate females in mature forests. Because
these calling rates were correlated with immediate hunger levels (Otter et al. 2007a), the
results suggest that birds in young forest tend to have reduced access to food relative to birds
in mature forest. In a comparison of somatic condition in chickadees during the breeding
season, van Oort et al. (2007) found that there was greater disparity in provisioning rate
among dominant and subordinate males among birds occupying poor-quality vs. goodquality habitat, suggesting that resources may be limited and males in low condition may
reduce parental behaviour to compensate. Further, reduced song output at dawn that is seen
across habitat (van Oort et al. 2006) is alleviated by supplemental feeding in the early spring
(Grava et al. 2009; Chapter 2), suggesting a link between song output and the condition of
the birds during the breeding season.
Although song output is increased with supplementation of resources, other
condition-dependent signals in the chorus are not affected by supplemental feeding in the
early spring, even though this likely increases the immediate condition of the birds. This is
the case with the fidelity with which birds render their songs: subordinate birds are less able
than dominant birds to maintain internal frequency parameters between successive renditions
of their songs (Christie et al. 2004), and birds in high-quality habitat also show a higher
ability to maintain consistent note structure between songs than do birds in poor-quality
habitat (Grava et al. 2012; Chapter 2). I have also shown that males assess rivals on this

82

ability to accurately maintain internal note structure; males show lower response to perceived
intruders with low consistency in song structure than they do to playbacks that maintain
higher consistency in song structure (Grava et al. in prep; Chapter 3). These combined results
suggest that the ability to maintain consistent song structure throughout the chorus is a
condition-dependent signal in the black-capped chickadee. However, food supplementation
at the time of singing has no effect on this ability, suggesting that it does not reflect the shortterm condition of the birds at the time of singing (Grava et al. 2012; chapter 2). Rather,
internal song structure consistency may reflect the condition of the birds at the time of song
learning, but no study has tried to estimate the early condition of the birds and its implication
on song structure in the black-capped chickadee. Further, it is unknown whether habitat
affects the birds during the sensorimotor learning period (post dispersal period) or whether
the birds that settle in young and mature forest already differ prior to settlement.
To address these questions, I followed a cohort of chickadees settling in either young
forests (poor-quality habitat) or mature forests (high-quality habitat) during their first winter.
First in the fall and then subsequently during mid-winter and again in early spring, birds were
captured and feathers removed for analysis of growth rates and levels of corticosterone
(CORT) because both can reflect nutritional status and environmental variation, including
habitat (Suorsa et al. 2003; Grubb 2006; Fairhurst et al. 2012; for reviews see Breuner et al.
2008, Bonier et al. 2009a). A subset of males caught in early spring was then euthanized to
assess neural and muscular development of the song nuclei and syrinx. I used feather growth
rates (ptilochronology) and feather CORT levels as proxies of conditional state during two
potentially important periods of song acquisition and development within the first year of

83

male chickadees: the natal period, when it is thought that the birds might be memorizing
song, and the post-dispersal/first winter period, when the neural song system is developing.
This study investigated whether a difference in condition between juvenile blackcapped chickadees exist across habitats during these two song learning periods and whether
this difference affects the development of the organs involved in song learning and song
production. This work contributes importantly to my understanding of the interplay of
environment, physiology and signalling behaviour in passerine species.

5.2. Methods
5.2.1. Study species
Black-capped chickadees are resident songbirds of northern North America. They
over-winter in stable flocks composed of two to five mated pairs during the non-breeding
season, typically from August to early May in my study site. The breeding season typically
starts in May when the flocks break up and the mated pairs defend their own territory. The
pairs raise on average five to seven chicks, which fledge at ca. 18 days posthatch (late June
to early July). After fledging, the chicks are fed by their parents in the territory surrounding
the nest for an additional two weeks. By mid July to late August, the juveniles disperse
randomly and can travel as far as 10km before settling into a flock (Smith 1991; Foote et al.
2010). Once settled, these birds will typically remain and attempt to breed in this settled
habitat for the remainder of their lives. The song of the black-capped chickadee is a simple
two-note fee bee song, which is commonly heard during the breeding season. As with other
songbirds, black-capped chickadees undergo a complex learning process in order to produce

84

their species-specific song (Baker et al. 2003; Hultsch and Todt 2004; Bolhuis and Gahr
2006).

5.2.2. Song learning in black-capped chickadees
In wild populations of black-capped chickadees, juvenile birds starts subsong at
approximately day 20 postfledging (Baker et al. 2003; personal observation), which
corresponds to the date they become independent from their parents. This likely represents
the starting point of the sensorimotor learning in this species, and that memorization
therefore occurs prior to this date when the birds are still fed by their parents (i.e. from
hatching to independence). During this first phase, the chickadees memorize the template of
their species-specific songs that they will try to reproduce in the next phase. Once the
chickadees disperse, the males enter the sensorimotor learning period. During this phase, the
young males practice singing songs they have previously memorized, attempting to match
their subsong attempts with their learned template songs (Shackleton and Ratcliffe 1993).
This sensorimotor phase lasts until the birds enter their first breeding season (Shackleton and
Ratcliffe 1993; Baker et al. 2003; Hultsch and Todt 2004).
During the entire sensitive period, the song system develops in parallel to the
acquisition of adult song (for review see Bolhuis et al. 2010). As a songbird, the template for
the tutor's song in the black-capped chickadee is thought to be encoded in the neural
substrate of NCM (caudomedial nidopallium) and CMM (caudomedial mesopallium; Bolhuis
et al. 2006). During development, the anterior forebrain pathway (AFP) is involved in the
acquisition of song as well as auditory-vocal feedback processing (for review see Mooney
2009). The AFP connects the brain nuclei for learning of song including Area X (Area X of

85

the striatum), HVC, LMAN (lateral magnocellular nucleus of the anterior nidopallium), and
RA (Bolhuis et al. 2010). The posterior pathway or song motor pathway (SMP) controls the
production of song and includes HVC and RA, which innervate nXIIts (tracheosyringeal
portion of the hypoglossal nucleus) that control the syrinx (Mooney, 2009). Normal
development of the song system is crucial during the sensitive period for normal
development of song (Nowicki et al. 2002; Christie et al. 2004; Grava et al. 2012; Chapter 3).

5.2.3. Study site
The study took place in Prince George, British Columbia, Canada (53°55"01'N
122°44"58'W) on three plots of mature forest, at least 850m apart, and four plots of young
forest at least 3.7km apart. Plots of both types were intermixed throughout the study region
(which covered 16.35km from N to S andl3.9km from E to W, centered on the city), and
each study plot consisted of forest of homogenous age.
The three mature sites were characteristic of the mixed woodlands of the sub-boreal
spruce sub-zone in Northern British Columbia. Further, all sites were similar in age
structure, with no commercial logging for more than 80 years. Dominant deciduous species
in these sites are trembling aspen (Populus tremuloides), paper birch {Betula papyrifera), and
black cottonwood (Populus balsamifera ssp. trichocarpa). Subalpine fir (Abies lasiocarpa,
lodgepole pine (Pinus contorta) and hybrid spruce (Picea glauca x P. engelmannii) form the
predominant conifer species in these mixed forests, but tend to be in lower relative
abundance than deciduous species. All these sites were characterized by an average trunk
diameter of 25cm and a canopy at approximately 25m.

86

The four young sites were forests that have undergone near-complete clearing in the
past thirty years. All these sites had characteristic flora of a young, regenerating sub-boreal
forest. Dominant deciduous tree species and conifer species were similar to those described
above for the mature sites, but early successional species, such as lodgepole pine, dominated
the coniferous component. These sites were characterized by an average trunks diameter of
approximately 10cm and a canopy at approximately 8m.

5.2.4. Feather collection
I caught birds during three distinct catching sessions during the period of Aug 2009 to
April 2010. The first catching session lasted from August 28th to September 6th 2009 (Fall),
the second catching session was conducted between November 11th 2009 and January 21st
2010 (Winter), the third between March 23rd and April 12th 2010 (Spring). I used a variety of
techniques to attract the birds into mistnets, including: playback of chick-a-dee calls
recorded in the context of either food-finding or mobbing; presentation of feeders filled with
sunflower seeds; presentation of stuffed chickadee (intruder) or saw-whet owl (predator). In
most circumstances, combinations of these stimuli were used to catch birds.
Only birds that hatched during the preceding spring (identified by shape and wear of
outer rectrices (Meigs et al. 1983)) were sampled for this study. I plucked the two outer tail
feathers at the beginning of the first fall (August) of 120 birds settling in both young and
mature forests; these "natal" feathers reflect those grown in the nest and during early postfledging in juvenile chickadees. This plucking induced feather regrowth during the fall song
learning period. In the winter period, I re-caught 30 of the birds from the fall sample, as well
as an additional 30 new birds. For recaptured birds, I plucked the induced feathers to

87

measure growth rates during the first fall period. For new birds, I plucked feathers so as to
induce regrowth in the winter across habitats; plucked feathers from these birds also added to
my sample of natal-grown feathers. Finally, in spring sampling, I returned to the same catch
locations and recaught as many of the previously banded birds as possible. This constituted
six birds captured in all three periods, 14 birds captured only in fall and winter periods, six
birds caught in winter and spring and six caught in fall and spring (but not in winter). In
addition, a sample of 16 birds was sampled for the first time in the spring collection period,
but I did not attempt to recapture these birds a second time. Out of the 32 birds for which I
managed to induce feather re-growth, 26 re-grew their feathers in fall and six re-grew their
feathers in winter. For each bird, I measured growth bands on one tail feather, using
ptilochronology, and the other feather was sent to collaborators at the University of
Saskatchewan for CORT analysis. All feathers were mature (i.e., fully-grown) when
collected.
Finally, out of the 48 birds for which I collected feathers grown in the natal territory,
26 had settled in their post-dispersal habitat as early as September, and 22 were definitely
settled by the onset of winter. Because CORT is known to affect the juvenile birds dispersal
and settlement behaviour in at least one other species of old-world Parids (willow tits Poecile
montanus, Silverin 1997), I addressed time of settlement in the subsequent analysis.

5.2.5. Ptilochronology
Ptilochronology is the study of the bird daily growth through the study of the tail
feathers (Grubb 1989). The tail feathers record evidence of their daily growth in visible
"bands" that can be measured, similar to growth rings on tree. The size of these daily growth

88

bands correlates with food availability, and thus ptilochronology can be used as an index of
relative condition of individuals under different ecological conditions (Grubb 2006).
The methods for measuring daily growth bars (DGB) on the feathers were taken from
Grubb (1989). Briefly, the feathers were placed on a card and pinned at their extremities. I
measured the total length of the feathers and calculated and marked the location of the 2/3rds
position along the feather's length from the proximal end. I then pushed a pin perpendicularly
through the feather at the distal edge of each growth bars. I then determined the average
growth bar width by calculating the mean length of ten growth bars centered on the 2/3rd
point with four additional growth bars proximal to and five distal to the central growth bar.
This measure gave me the average daily growth bar of the natal feathers that grew during the
first few weeks after the birds fledge, and of the induced feathers of the birds I re-caught at
least once.

5.2.6. CORT analysis
Feather CORT assays followed Bortolotti et al. (2008). Briefly, I extracted CORT
from feathers using a methanol-based technique. The length of the feather was measured, the
calamus was removed and discarded, and then the sample was cut into pieces <5 mm2 with
scissors. I then added 10 mL of methanol (HPLC grade; Fisher Scientific, Fairlawn, New
Jersey, USA) and placed the samples in a sonicating water bath at room temperature for 30
min, followed by incubation at 50° C overnight in a shaking water bath. To separate the
methanol from the feather material, I used a vacuum filtration system that included a plug of
synthetic polyester fibre in a filtration funnel. The methanol extract was placed in a 50° C
water bath and subsequently evaporated in a fume hood. Extract residues were reconstituted

89

in a small volume of phosphate buffer system (PBS; 0.05M, pH 7.6) and frozen at -20° C
until analyzed by radioimmunoassay (RIA). I assessed the recovery of the methanol
extraction by including feather samples spiked with a small amount (approximately 5000
CPM) of 3H-corticosterone in the extraction. Samples were extracted in two batches, and
92% and 96% of the radioactivity was recoverable in the reconstituted samples, respectively.
Final values were adjusted by recoveries. For more information about validation, see
Supplementary Appendix S1 available in online version of Bortolotti et al. (2008).
Feather CORT levels were determined by RIA as in previous studies (Wayland et al.
2002; Bortolotti et al. 2008; Fairhurst et al. 2011; Fairhurst et al. 2012). Measurements were
performed on reconstituted methanol extracts and were duplicated. Samples were measured
in two assays with an intra-assay coefficient of variation of 8.2%, an inter-assay coefficient
of variation of 9.6%, and mean (± SD) limit of detection (ED80) of 10.63 ± 0.26 pg
CORT/assay tube. Data values are expressed as pg CORT per mm of feather, which gives a
valid estimate of CORT per unit time of feather growth (for validation see Bortolotti et al.
2008;Bortolotti et al. 2009a; Bortolotti 2010). CORT assays were performed at the
University of Saskatchewan, Canada.

5.2.7. Brain and syrinx analysis
Histology
During the spring sampling, and after the flocks broke up (after song crystallized), 20
second-year males hatched the previous summer (the 15 additional birds from the previous
analysis, plus five birds which were part of the re-grown feather analysis) were euthanized
(Animal Care approval - A2009 0119 013) for brain and syrinx histology. Ten males from

90

either of the two habitat classes were sampled. Sex was confirmed via inspection of gonads
following euthanization. Birds were euthanized with 0.03 ml of 100 mg/ml ketamine and 20
mg/ml xylazine given intramuscularly (1:1) and then transcardially perfused with heparinized
0.1M phosphate buffered saline (PBS) and then 4% paraformaldehyde. Following perfusion,
the brain, gonads and syrinx were removed and placed in 4% paraformaldehyde and
transported to the University of Alberta for further analysis. After one week, the brain,
gonads and syrinx were weighed three times each and the average value was recorded. The
average syrinx weight was then corrected for body size for statistical analysis. The length
and width of the gonads was also measured and all measurements were carried out blind to
the habitat condition of the specimens. The brain was then placed in 30% sucrose in PBS for
approximately 24 h until saturated. The brains were then snap-frozen in isopentane at -80°C
and stored at -80°C. Brains were then warmed to -20°C and the entire brain was sectioned
coronally at 40|am using a cryostat and mounted on gelatin-embedded slides in four series.
The first series of mounted sections, every fourth section, were then stained with Thionin
(0.25%) and coverslipped.

Brain morphometry
To calculate total brain volume, I used methods similar to Phillmore et al. (2006) and
Smulders et al. (2006). First, each slide was scanned into ImageJ, using an Hp 4470c
ScanJet. The total area of the brain as well as the total areas of the brain nuclei for HVC and
RA in every section in which they appeared were traced using a WACOM tablet. The total
volume of the brain, HVC and RA were calculated by multiplying the given surface area by

91

the distance between measurements (Smulders et al. 2006). All imaging and tracing was
done by a research assistant unaware of the habitat condition of the specimens.

5.2.8. Statistical analysis
Condition during memorization phase
To study the condition of the birds during the memorization period, I analyzed the
width of daily growth bars (DGB) and CORT levels in the natal feathers collected.
I used general linear models (STATISTICA version 6.0, StatSoft, Inc) to compare the
levels of CORT in the natal feathers (dependent variables) between habitats in which the
birds were sampled (categorical variables), controlling for period of known settlement (fall
or winter - categorical variables) as well as for the DGB in the same natal feathers
(continuous variables).
To detect an effect of habitat on DGB in the natal feathers, I used general linear
models to compare the natal DGB (dependent variables) between habitats in which the birds
were sampled (categorical variables), controlling for period of settlement (categorical
variables) as well as for the CORT in the same natal feathers (continuous variables).

Condition during sensorimotor learning phase
To study the condition of the birds during the post-settlement period, I performed
analysis on DGB and CORT in the induced feathers.
I used general linear models to compare the levels of CORT in the induced feathers
(dependent variable) between habitats in which the birds were sampled (categorical variable),
controlling for the period in which the feather grew (winter or fall) (categorical variables) as
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well as for the DGB in the same feathers and for the DGB and CORT in the natal feathers
previously sampled from the same birds entering this analysis (continuous variables).
I used general linear models to compare the levels of DGB in the induced feathers
(dependent variable) between habitats in which the birds were sampled (categorical variable),
controlling for the period in which the feather grew (winter or fall) (categorical variable) as
well as for the CORT in the same feathers and for the DGB and CORT in the natal feathers
previously sampled from the same birds entering this analysis.

Brain and syrinx measurement after song crystallization
Because my sample size of ten birds euthanized per habitat was limited by ethical
considerations, my analyses lacked sufficient power to detect statistically meaningful
patterns. Hence, I used Cohen's d to compare the effect size of differences in brain and
syrinx measurements between birds that had settled in either young or mature forests. I used
the guidelines of Cohen (1988) who suggests that a d=0.1 represents a small effect, d=0.3 an
intermediate effect and d=0.5 a large effect. When the power of the analysis was above 60%
I performed an ANOVA to compare the measurement across habitats.

5.3. Results
5.3.1. Condition during memorization phase
When controlling for time of establishment as well as for the DGB in the same natal
feathers, I found that the CORT level in natal feathers were higher in birds that settled in
mature forests compared to birds settling in young forests (ANOVA, F(i;43)=4.48, N=48,
p=0.03, d=0.35; Fig 5.1).
93

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MATURE

YOUNG

habitat

Figure 5.1 CORT level in natal feathers are higher in birds that settled in mature forests
compared to birds settling in young forests (ANOVA, F(i;43)=4.48, N=48, p=0.03, d=0.35).
Dots represent mean and whiskers error bars.

94

However, when controlling for CORT in natal feathers, there was no significant
difference in average growth bar widths (DGB) in the original feathers of birds that settled in
either mature or young forests (ANOVA, F(i,45)=0.35, N=48, p=0.55, d=0.004; Fig 5.2).

5.3.2. Condition during sensorimotor learning period
I found no differences in CORT in induced feathers among birds settling in young
versus mature forest (ANOVA, F(ij25)=0.34, N=32, p=0.57, d=0.05) and no difference in
DGB across habitats (ANOVA, F(I,25)=0.43, N=32, p=0.51, d=0.05).

5.3.3. Brain and syrinx measurement after song crystallization
Syrinx weight in the first spring correlated significantly with CORT levels of the
natal feathers (regression analysis, R2=0.2, F(i>ig)=4.9, N=20, p=0.03; Fig 5.3) but not with
DGB in these feathers (regression analysis, R2=0.003, F(i,i8)=0.7, N=20, p=0.79). Neither
HVC nor RA volumes were correlated with CORT levels (regression analysis, R2=0.08,
F(i jg)=1.5, N=20, p=0.23; regression analysis, R2=0.04, F(i)18)=0.75, p=0.38; respectively) or
DGB in natal feathers (regression analysis, R2=0.003, F(i>i8)=0.7, N=20, p=0.79; regression
analysis, R2<0.0001, F(i,ig)<0.001, p=0.98; respectively). As several of these males were
only caught in the spring sampling period, there was insufficient data to compare syrinx or
brain measures to CORT or DGB of re-grown feathers.

95

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p=0.55, d=0.004). Dots represent mean and whiskers error bars.

96

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Figure 5.3 Syrinx weight of black-capped chickadee in their first spring is correlated with
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97

The power of the analysis comparing syrinx weight across habitats gave a value of
63% allowing me to perform statistical comparisons. However, all comparisons between
song nuclei across habitat have a power below 14%. I found that birds from young forests
have a smaller syrinx mass than birds in mature forest (ANOVA, F(i i7)=5.86, N=20, p=0.02,
d=0.60; Fig 5.4a). Using a Cohen's d index of effect size, an intermediate effect was found
between RA volume in young vs. mature forests; birds in young forests tend to have a bigger
RA volume than birds in mature forest (d=0.38; Fig 5.4c). HVC volume differs very slightly
across habitats (d=0.13; Fig 5.4b) with birds in young forests tending to have a smaller HVC
than birds in mature forests.

98

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Figure 5.4 (a) Birds from young forests have a lighter syrinx than birds in mature forest
(ANOVA, F(i i7)=5.86, N=20, p=0.02, d=0.60). (b) HVC volume differ slightly across habitat
(d=0.13) with birds in young forests tending to have a smaller HVC than birds in mature
forests, (c) An intermediate effect was found between RA volume in young vs. mature
forests; birds in young forests tend to have a bigger RA volume than birds in mature forest
(d=0.38). Dots represent mean and whiskers error bars.

99

5.4. Discussion
I found that the birds that ultimately settled in young forests had lower CORT in their
natal feathers than did birds that established in mature forests. This result seems at odds with
the idea that birds in poorer condition are predicted to be under higher levels of stress,
reflected by higher CORT level, than birds in better condition (Bonier et al. 2009). All
previous studies of birds settling in similar habitat types in the region would indicate that
birds in young forest would be likely to encounter higher levels of resource limitation (van
Oort et al. 2006; Fort and Otter 2004a, 2004b; Otter et al. 2007; Grava et al. 2009). This
discrepancy, however, might be associated with my metric for measuring CORT levels. I
assessed CORT using feathers, which provides a longer-term perspective compared to
instantaneous blood sampling typical of other studies. CORT from feather samples provides
an integrated measure (Bortolotti et al. 2008), and current studies have found a positive
relationship between feather CORT and nutritional status (Fairhurst et al. 2012), rather than
the inverse relationship proposed by previous work (Bonier et al. 2009). Moreover, the lack
of consistency in the relationship between CORT and condition across taxa questions the
generality of assuming high CORT is associated with poor condition. In particular, in the
Paridae family, the relationship between CORT and condition is not consistent across two
closely related species - the great tit (non-significant relationship - Eeva et al. 2003) and the
blue tit (positive relationship - Muller et al. 2007). My results differ from Lucas et al. (2006)
who documented an elevated CORT levels in disturbed habitat in the Carolina chickadees
(Poecile carolinensis) based on fecal CORT on both adult and juvenile chickadees. The fact
that my results differ from theirs is likely based on the difference in the experimental

100

designs, and is representative of the problem of generalization when studying condition and
CORT levels.
My results, however, show similar patterns to other studies using feather CORT to
assess relative nutritional condition among individuals. Fairhurst et al. (2012) for example,
found that Cory's shearwater (Calonectris diomedea) nestlings, facing conditions of
diminished provisioning rates, had lower feather CORT, which shows a link between lower
energetic condition and lower CORT in nestling birds, a result also found using blood CORT
levels (Kitaysky et al. 2005). My findings that birds settling in young forest have lower
CORT in their natally-grown feathers may indicate that these birds were in poorer energetic
state than those that ultimately settled in mature forests, but the factors responsible for CORT
differences need to be addressed with additional research. As previous research has
demonstrated a strong positive influence of mature forests on reproductive success in these
birds (Otter et al. 2007b), any settlement bias based on competitive advantage of juvenile
birds would tend to favour settlement in the mature forests. Once the birds establish in their
habitat, however, my results suggest they do not differ in CORT deposited into induced
feathers.
My results also suggest that the condition of the black-capped chickadee nestlings
may predict whether the birds will establish in good or poor-quality habitat, and suggest that
a despotic distribution occurs when black-capped chickadee juveniles disperse ~ the birds in
better condition tend to establish in higher-quality habitat, while birds in lower condition are
pushed towards sub-optimal habitat (Godfrey 2003). A similar study in the black-capped
chickadee showed that birds across habitat did not differ in DGB before they settled in their
habitat (van Oort and Otter 2005). The authors concluded that there was no evidence for

101

differential settlement (at least among males) based on individual condition. By adding the
CORT level as another measure of condition in this study, I was able to detect a difference in
condition between juvenile birds settling in young versus mature forest through a study of the
feather CORT, even though I confirmed the previous results of van Oort and Otter (2005),
that there were no differences across habitats in DGB of settling birds.
During sensorimotor learning period, I found no effect of habitat on either CORT or
DGB in induced feathers. However, my study of the condition of the birds during the
sensorimotor learning period suffers from a smaller sample size compared to the study of the
song memorization period, which could explain the lack of consistency between those
results.
The song memorization period occurs when the birds are still nestlings. This period is
extremely important for song development. It has been shown that birds in lower condition
during the song memorization have a lower ability to learn their species-specific songs
because of a lower development of the organs involved in song learning (Nowicki et al.
2002). Because I found a physiological difference between birds that eventually settle in
young forest compared to those in mature forest, these differences in feather CORT may
reflect general differences among the birds during the song memorization period. If these
differences in CORT are correlated with other physiological measures associated with later
song production, it may indicate a connection between natal condition and song learning.
This connection appears in the correlation between CORT levels in natal feathers and
the syringeal mass in juvenile males entering their first breeding season. I also found
differences in song center nuclei across habitats. Birds in young forest tend to have a smaller
HVC volume and a bigger RA volume. Other studies suggest that HVC is affected more by

102

early condition than RA. In zebra finch, birds who have undergone developmental stress
during song learning (through nutritional and hormonal stress, or parasitic infections) have a
lower development of the HVC region while RA volume or brain size was not affected
(Buchanan et al. 2004, Spencer et al. 2003, Spencer et al. 2005). My results show that even
though birds in young forest tend to have a bigger RA volume, they still have a lower ability
to maintain internal song structure (Grava et al. 2012; chapter 3).
My results also suggest that birds occupying habitats associated with lower
consistency in song rendition also tend to have a lower HVC volume. The relationship
between HVC volume and song learning ability has been well demonstrated (DeVoogd et al.
1993; Szekely et al. 1996; Airey and DeVoogd 2000; Airey et al. 2000; Garamszegi and
Eens 2004, although see Gahr et al. 2008 for a recent study that found no difference in
learning ability among males that differ in HVC volume).
Syrinx mass was smaller among my young-forest birds than in the mature-forest
birds. Because a smaller syrinx is associated with less accurate imitation of a tutor song
(Solis and Doupe 2000), my results may explain why motor production of songs among
young-forest birds tends to be less consistent than those of mature-forest birds (Grava et al.
2012; chapter 3). The fact that syrinx weight is correlated with natal CORT level may
suggest a link between the condition of the birds during the song memorization period and
later development of the neuro-motor pathways responsible for song learning in the blackcapped chickadee. However, it is possible that there is no causality between development and
condition and that birds in both habitats differ genetically - the birds having a better genotype
(higher CORT, higher DGB, faster neural and syringeal development) preferentially settle
into optimal habitat, while birds with a lower-quality phenotypes are pushed towards sub-

103

optimal habitat. In any case, it seems that the establishment of the birds into good and poorquality habitat may be linked to the natal condition of the birds, and that settlement within
these habitats leads to reduced development of neural centers and motor pathways associated
with song development. This current study may provide a mechanistic connection as to why
adults birds in these same habitats differ in the ability to produce songs with consistent
frequency ratio (Grava et al. 2012; chapter 3), and the ability of maintaining these
components of song structure appear to positively influence male perception of intruder
condition (Grava et al. in prep.; chapter 4). Song consistency may therefore be a conditiondependent signal. That I did not find differences in condition in birds in both habitats during
the sensorimotor period might reflect a trade-off where birds with limited energy reserves
invest first in somatic maintenance (body condition) and less in the development of sexually
selected traits (Godfrey 2003). This may explain why I see little difference in condition of
birds between good and poor habitats, but major differences in the expression of sexuallyselected signals.

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6- GENERAL DISCUSSION

6.1. Synopsis
My thesis explored how habitat quality affects individual birds at different stages in
their life-history and consequently their vocal performance. In northern BC, intensive
forestry activity in the last half century has created a mosaic of habitat types across a
landscape that was traditionally continuous sub-boreal forest. Although forest practices
advocate mimicking natural disturbance events (forest fires and insect epidemics), the extent
of disturbance over much of northern BC is larger and more regular than natural events, and
creates a diversity of small, adjacent patches of forest differing in ages. Patches of oldgrowth forest now abut newly clear-cut patches of forest, and/or patches of forest
regenerating from a ten, 20 or 30 year-old clear-cut. Black-capped chickadees are a
generalist forest species that can occupy forests in a range of ages (Smith 1991; Otter et al.
2007). The juvenile birds dispersing in this habitat mosaic can settle in territory that can
range in age from 20-year old regeneration to old-growth forest. However, previous
assertions that such generalist species were not sensitive to habitat disturbance are too
simplistic. Chickadees occupying low-quality habitat have lower breeding success than
those in high-quality habitat (Fort and Otter 2004a; Otter et al. 2007), and their behaviour is
significantly altered by habitat quality (van Oort et al. 2007; Fort and Otter 2004a; Fort and
Otter 2004b; Otter et al. 2007). However, previous studies had found no difference in
condition between juvenile males that settled in either habitat type (van Oort et al. 2007).
Thus, the influence of habitat on condition was considered to begin post-settlement (Otter et
al. 2007). By adding data about the energetic condition using feather CORT, I showed that

105

juvenile black-capped chickadee settling in young forest had lower CORT levels prior to
settlement compare to juvenile birds settling in mature forest (Chapter 5). As low CORT
levels are associated with lower energetic condition in some other species (Fairhurst et al.
2012), my results may suggest that young forests are occupied by birds in greater energetic
stress during a period that also corresponds to the song memorization period. At the onset of
the breeding season, I also found that these birds had lesser development of the brain and the
syrinx (Chapter 5). This may account for my finding that birds in young forest are less able
to maintain consistent frequency ratios within their songs during the chorus (Chapter 3). This
difference in song consistency may be compounded by differences in habitat quality once the
birds have settled (Chapter 5). At the time of chorusing, all breeding males start singing at
dawn and this "dawn chorus" represents a vocal performance in term of vocal vigour and
vocal skills. In fact, in terms of performance, the ability to maintain internal frequency ratios
represents vocal skill (as defined by Byers et al. 2010), but it does not appear to reflect the
immediate condition of the birds at the time of signal production (Chapter 3), but rather
reflects their condition during the development during the song learning period (Chapter 5).
On the other hand, once the birds enter their first breeding season, their song crystallizes, and
their vocal skill is set for each individual. Another form of vocal performance reflects direct
condition at the time of singing and is known as vocal vigour (song output), van Oort et al.
(2006) showed that birds in young forest had lower song output than birds in mature forest. I
demonstrated that song output at dawn is positively affected by short-term feeding (Chapter
2), but this increase in resources does not restore the song consistency differences between
habitats (Chapter 3). The difference observed previously in song output across habitat can
be attributed to lower food availability at the time of singing in young forest. In this case,

106

habitat quality has a short-term effect on the bird's condition and consequently on their vocal
vigour. To conclude, while vocal skills may reflect the condition of the birds at the time of
song learning (chapter 5), vocal vigour is known to reflect the bird's immediate condition at
the time of vocal production (Chapter 2), and both of these measures of performance differ
across habitat (van Oort et al. 2006, Grava et al. 2012, Chapter 3).
This habitat effect on song consistency is likely to alter the perception of the birds'
signals relating to condition with other individuals across habitat. Females are known to
attend to elements of the song structure related to male quality (Hoeschele et al. 2010) and
dominant males tend to sing with higher consistency than subordinate males (Christie et al.
2004). My results show that the ability to sing with consistent internal song structure is used
by males to assess other males' condition during diurnal interactions (Chapter 4). Dominant
males in young forest are not considered as an equivalent threat compared to dominant males
from mature forest, based only on the consistency of the broadcast songs. This reflects the
long-term effect of habitat on the bird's ability to signal their status. No matter whether
males in young forests are competitively similar to their rank counterparts in mature forest,
young-forest males may be unable to signal this vocally, and thus may face greater numbers
of challenges from intruders. From these results, I can anticipate that birds in young forest
may be less able to defend their territories. In fact, Fort and Otter (2004) already showed that
birds in young forest have a lower territorial behaviour than birds in mature forest. By
showing that the relative perception of the males rank is condition- and habitat-dependent,
my results give a good understanding of the territorial dynamics in the black-capped
chickadee.

107

6.2. The effect of human activities on the production of sexual signal
In this thesis, I explored the short-term and long-term effect of habitat quality, due to
intensive forestry practices on a generalist songbird, the black-capped chickadee. Human
activities, today more than ever, are responsible of a high degradation of the natural habitats
of many bird species through a variety of activities. I focused my work on the effect of forest
practices on vocal performance in the black-capped chickadee but there are a variety of
examples that demonstrate how human activities affect individual condition and
consequently the production of sexual signals that convey information about their quality.
Vocalizations are not the only sexual signals used by birds to convey information
about their condition. In numerous bird species, visual signals are condition-dependent and
are used by females to assess males' condition. For example, the expression of carotenoidbased plumage coloration reflects the food provisioning rates of the male blue tit and great tit
(Hill 1991; Linville et al. 1998; Senar and Escobar 2002; Senar et al. 2002; Isakson et al.
2006). Many studies have also shown that females prefer males with brighter carotenoiddependent colours (e.g. zebra finch, Burley and Coppersmith 1987; house finch, Hill 1991).
At least in blue tits, the colour is assumed to be subject to sexual selection because it is
correlated with male parental quality (Senar et al. 2002; Johnsen et al. 2005). One study in
great tits has shown that such colour differences appear once the nestling hatch (phenotypic
traits), and therefore reflect habitat quality rather them genetic quality (Horak et al. 2000),
while others have demonstrated both environmental and genetic effects in both tit species
(Fitze et al. 2003a; Johnsen et al. 2003). As a result, habitat quality could have a profound
impact on the expression of visual signals in those species. Yellow feathers of the underparts
of blue tits and great tits are paler in a range of habitats assumed to be of poorer quality than

108

deciduous woodland (Ferns and Hinsley 2008). For example, Slagsvold and Lifjeld (1985)
found that great tits are more yellow in deciduous than in coniferous forest, and it is known
that birds in deciduous forest are bigger than birds in coniferous forest (Ulfstrand et al.
1981).
For acoustic signals, the relationship between forest structure and the production of
acoustic signals is largely reviewed in the present work. Both song output and song
consistency are higher in dominant than in subordinate males, suggesting the conditiondependency of those signals (Otter et al 1997; Christie et al. 2004). In this species, the study
of the effect of forest structure on singing was based on a comparison of young and mature
forest, van Oort et al. (2006) showed that birds in young forest had lower song output than
birds in mature forest. My work showed that this difference likely results from a difference in
condition across habitat at the time of singing (Grava et al. 2009, chapter 2), and may also
reflect lower amounts of caterpillar prey in these habitats during the early part of the
breeding season (as quantified by relative frass abundance - chapter 1). I also demonstrated
that song consistency was affected by forest structure with birds in young forest having lower
song consistency than birds in mature forest (Grava et al 2012, chapter 3). This difference is
not alleviated by short-term food supplementation (Grava et al. 2012, chapter 3) and
therefore more likely reflects the condition of the birds at an earlier period in their
development prior to the breeding season, during the period of song learning (chapter 5).
In other species, habitat fragmentation alters repertoire size (e.g. blue tits, Doutrelant
et al. 2000; house sparrow, Reid et al. 2005), a characteristic of signalling that is known to be
associated with a better fitness in numerous birds' species. By increasing the distance
between neighboring males, habitat fragmentation might affect song variation in some bird

109

species (white crowned sparrow Zonotrichia leucophrys nuttalli, Trainer 1983; corn bunting
Miliaria calandra, Holland et al., 1996). Studies on Dupont's lark (Chersophilius duponti;
Laiolo and Telia 2005; 2007) found that isolation caused song repertoires to decrease
drastically in fragmented habitats. The authors suggest that cultural diversity should be taken
into consideration the same way as biodiversity when looking at the effect of landscape
alteration on bird populations (Laiolo and Telia 2007).
By affecting the environment (food quality and/or availability) and the physiology of
the organisms, heavy metal pollutant released in the environment by industry can have a
profound effect on a bird's condition and production of both visual and acoustic signal. Eeva
et al. (1998) compared plumage of birds living close to an air pollution source to those of
birds living further from the source. They found that the plumage brightness was negatively
affected by air pollution, and that this reflects lower food abundance in the polluted area.
Also, aberrant tail morphology in barn swallow is another visual signal associated with a
decrease in condition. For example, males with increased ectoparasitism show higher
asymmetry in the length of outer tail feathers (Moller 1992b) than other birds. Moller (1993)
studied the effect of the highly contaminated site of Chernobyl on fluctuating asymmetry
(perturbations in symmetry of bilaterally-symmetrical traits) in the barn swallow. He
compared birds sampled from the contaminated site to birds sampled both from the site
before the catastrophe and museum specimens from the area. After the catastrophe, the level
of asymmetry in male tail length was higher than before the catastrophe. Similarly, Gorissen
et al. (2005) found that great tits from sites polluted with heavy metal sang less at dawn and
had a smaller repertoire of song types than birds from the non-polluted area. The authors

110

suggest that long-term effect of heavy metal exposure during song learning could alter the
development of the birds and consequently their song complexity.
The effect of human activities on natural landscape can also be considered in terms of
urban and agricultural activities. For the past 10,000 years, humanity has been transforming
the landscape through development of both agriculture and urban areas. The relative quality
of those environments from a bird's perspective is still under investigation, as generalization
about the relative quality of urban vs. rural environment is extremely difficult and therefore
studies hard to compare. For example, great tits in mixed forest have higher carotenoid level,
which are typically associated with higher condition, than birds in sub-urban area (Figuerola
and Senar 2005), demonstrating that the latter areas are perceived as lower quality than
natural environment. However, the study of plumage colour in house finch show that birds
from sub-urban areas are more colourful than birds nesting further into rural areas with
intensive agriculture, suggesting that sub-urban areas might hold more food resources than
polluted rural areas (Hill 1995). The study of the effect of urban area on birdsong to date has
focused primarily on the effect of urban noise on birdsong frequency. An increased in
minimum frequency of the songs of birds has been detected in response to urban noise
(Luther and Baptista 2009; Hu and Cardoso 2010). Slabbekoom and Peet (2003) showed that
birds in urban areas sang at higher frequency to ensure their song would transmit through the
urban noise. In a 30-years study, Luther and Baptista (2009) showed that the songs of the
white-crowned sparrow increase in frequency along with the increase of urban noise.

Ill

6.3. Implications of my results on bird's life history and conservation
My results show the implications that habitat quality has on the bird's life history.
Previous research showed that habitat had a direct effect on the bird's territoriality (Fort and
Otter 2004a), reproduction (Fort and Otter 2004b, Otter et al. 2007) and signalling (van Oort
2006). My results demonstrate the developmental and ecological mechanisms linking those
differences. First, I demonstrated that habitat has a direct effect on chickadee behaviour and
reproduction via resource limitation during the breeding season. I showed in this thesis that
the difference in song output across habitat observed previously (van Oort et al. 2006) is
directly related to a difference in food availability at the time of chorusing (Grava et al. 2009,
Chapter 2). My results also demonstrate that the difference in food availability during the
breeding season does differ across habitat (Chapter 1). This difference in food availability
can explain other differences in life history across habitat. All behaviour that represents nonmaintenance activities requires extra energetic investment. As a result, it will be undertaken
only if the bird has enough energy to sustain its other primary life-sustaining activities
(Godfrey 2003). If birds in young forest have access to scarcer resources during the breeding
season than birds in mature forest, it could explain why they do not invest as much energy in
non-maintenance activities. Those activities range from singing, but also breeding and
defending territory. It has been demonstrated that birds in mature forests have higher
reproductive success, at the UNBC study sites (Fort and Otter 2004b) but also at the JPRF
study sites (unpublished data). In my sites, the low reproductive success was particularly due
to high nest abandon during the early breeding season. I showed in chapter 1 that the
caterpillar population is significantly lower in young forest during the incubation period,
which correspond to first stage of breeding. The scarcer food resources during the early

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breeding season could therefore help understanding the low reproductive success observed
across habitat. Territoriality is another non-maintenance activities that birds will invest into if
they have extra energy. It has been shown that birds in young forest are less territorial than
birds in mature forest at the UNBC study sites (Fort and Otter 2004b), and this result has
been confirmed by personal observations for four consecutive field seasons at the JPRF study
sites. Again, the low resource access in young forest that I demonstrated interfere with
chorusing (Chapter 2), could similarly explain the low territorial behaviour across habitat.
On the other hand, my results also showed that habitat quality can indirectly impact
the bird's behaviour. I showed that through habitat selection during flock establishment and
effect of habitat on brain and syrinx development, males black-capped chickadee in young
forest have a lower ability to maintain internal song structure than do birds in mature forest
(Chapter 3 and 5). Through altering their song structure, habitat quality can consequently
impact male's reproductive success. If males occupying poor-quality habitats produce songs
with lower consistency, poorer renditions of song-types, or decreased repertoires (globally
considered as decreased "vocal skill"), they may have more difficult time attracting mates or
extra-pair partners or have to expend extra effort defending their territories from intruding
males. Maintenance of internal structure of notes within and between songs (song
consistency) is known to accurately reflect male social rank in chickadees, and is known to
be attended to by female within this species (Hoeschele, 2010). I also demonstrated that
song consistency is a condition-dependent signal used by males to assess the condition of
male intruders and that, based on this signal, dominant birds in mature forest do not consider
dominant birds from young forest as equally ranked than dominant birds from mature forest
(Chapter 4). This relative perception of the bird's rank across habitat can potentially interfere

113

with the bird's territorial behaviour, with birds from young forest being less able to defend
their territory against intruder than birds in mature forest. Also, if song consistency is a vocal
skill used by the female to assess male quality, males with high song consistency should be
preferred over males with low consistency. Byers (2006) demonstrated such an association in
chestnut-sided warblers (Dendroica pensylvanica), where males exhibiting a high song
consistency have higher access to extra-pair copulations. Similarly, Botero et al. (2009)
demonstrated that highly-consistent male mocking birds have a better reproductive success
than less-accurate singers. If the singer's vocal skill reflects the inherent quality of the singer,
it could act as a reliable signal with which females may be selecting among males. Thus,
reduction in the ability of males to maintain consistent song structure when occupying young
forests may compromise their perceived attractiveness, and thus their reproductive potential.
The results of my thesis can help investigating the use of the fee-bee song as a
measure of habitat quality. If song output and structure is affected by habitat quality, it seems
possible to use those signals to assess the relative quality of habitat, as measured through its
short-term and long-term effects on resident species. Godfrey (2003) suggested that
condition-dependent signals can be used as a measure of habitat quality. In the black-capped
chickadee, song output gives a measure of the birds' condition (Otter et al. 1997) and this
signal is affected by the quality of the habitat (van Oort et al. 2006). My work demonstrates
that song output is directly related to food limitation of the signaller at the time of singing
(Grava et al. 2009; Chapter 2). This signal gives a proxy of the perceived environment by
males. By comparing the song output at dawn of male black-capped chickadees of the same
rank in different habitat, I should be able to detect whether a difference in quality exists
between the two habitats. The ability to sing with consistency, on the other hand, gives me

114

long-term information on the conditions available to birds during their early developmental
period. My results suggest that song consistency reflect the conditions faced by birds during
the learning period, which occurs from when the birds are still nestlings to the time of the
first settlement (Chapter 5). The birds with lower feather CORT, during the learning period,
indicative of lower condition in other species (Fairhurst et al. 2012), are more likely to settle
in low-quality habitat. This habitat may then compound effects that resulted in these birds
having lower neural development of song centers and syringeal mass when it was time for
song production (Chapter 5), and consequently lower ability to sing with high consistency
(Chapter 3). As a result, the ability to sing with consistent internal frequency ratios could
give me additional information about the quality of the habitat in which a bird is recorded.
By combining the study of the song output with the study of song consistency across
habitats that may differ in quality, one may be able to compare the quality of the habitat as
perceived by the birds at multiple times in its life history and therefore in the year. For
example, I could consider two adjacent habitats that differ in structure, such as two forest
types differing in tree species, in which black-capped chickadees are present and breed (a
scenario close to the study done on blue tit in France by Blondel et al. (1993) who showed
that the behaviour of birds in evergreen oak (Quercus ilex) forest differ from birds in
deciduous forest (Quercus pubescens) due to difference in food availability). Those habitats
might be of very different quality for the birds if the amount of food available for breeding is
lower in one habitat compare to the other (e.g. lower amount of caterpillar). At the time of
singing, birds recorded in low-food habitat would have a lower song output than birds
recorded in the rich-environment. However, those habitats might not differ in quality to birds
during the song learning period. The amount of food available when the birds are dispersing

115

and learning their song may be similar (e.g. same amount of seed/insects during the fall and
winter), which could result in birds recorded in both habitats showing no difference in the
ability to maintain internal frequency ratios of their songs. This example demonstrates that
the signals entrenched into the fee-bee song of the black-capped chickadee gives two
measures of habitat quality that can help biologists to assess the relative quality of habitat
over two independent time periods in the bird's annual cycle.
Finally, the study of the consequences of habitat quality on condition and behaviour
in generalist species is part of a new concept in conservation biology. Scientists are now not
only taking into consideration the diversity of species in an ecosystem, but also the
abundance and health of individuals within a population. While diversity is important, as it
provides information about the importance to protect specific environments, the presence and
condition of generalist species populations give reliable information about the quality of
those environments. I now realize that the ecological services provided by species to
humanity are mostly provided by the mass of individuals in generalist species rather than by
the few individuals in rare species populations.

6. 4. Future research
My thesis demonstrates the effect of habitat quality on vocal performance and
explores the mechanisms through which habitat affects the information entrenched within the
fee-bee song of the black-capped chickadee. I suggest that the fee-bee song can be used as a
measure of signaller-condition by the receiver through two means: the song output at dawn
gives information about the condition of the signaller at the time of singing, whereas the
ability to maintain internal frequency ratios provides information on the condition of the

116

birds during the song learning period. This thesis brings a holistic approach to understand the
link between the production of signals, the condition of the birds and the quality of the
habitat in which the birds is settling and/or breeding.
However, we are still investigating whether food availability differs naturally across
habitat during the breeding season. Analysis of frass data, presented in Chapter 1, suggests
that differences in caterpillar abundance differs between years, but more importantly to my
thesis, also differs between habitat classes during the incubation period following the dawn
chorus. I suggest in Chapter 2 that birds in young forest sing less than in mature forest
because they have access to less food resources, but I could not directly quantify the amount
of food available at this time of the year. Effectively, the chorus period typically occurs prior
to the leaf burst period, so it is difficult to monitor caterpillar frass between habitats at this
time of the year. Birds during this period tend to forage on trunk invertebrates, which I was
unable to quantify. However, looking at patterns of prey availability in general during the
spring may provide insight into habitat differences. During the breeding season, the blackcapped chickadee relies mostly on caterpillar resources they find under the tree leaves, as
they represent a high-protein resource the birds need to sustain their energetic cost while
providing sufficient food for the nestlings. It has been shown in a wild population of blue tits
living in two habitats where evergreen (Quereus ilex) or deciduous oak (Q. Pubescens)
predominate, that the main difference in resources during the breeding season occurs because
the caterpillar peaks in evergreen oak forest was delayed of few weeks (Blondel et al. 1993).
We are currently monitoring the relative abundance of caterpillars in both habitats to see
whether a difference exists in the absolute amount of caterpillar during the peak or whether
the hatching peak is delayed between habitats. Preliminary results tend to show that the

117

caterpillar peak seems to happen at the same time in both habitats, but that the amount of
caterpillar available in young forest is lower than in mature forest. More data are needed to
detail the exact caterpillar population trend in both habitats.
Also, although I demonstrated that birds differ in measures associated with condition
prior to settlement, and my results suggest that this can explain the lower ability to maintain
consistent frequency ratio in the song of birds in young forest, I need more information about
the exact links between the condition of the juvenile birds at the time of song learning and
their singing ability. More data are needed to monitor the birds' condition in the first fall and
winter and consequently their song structure, and such studies may need to focus on a single
long-term population to track cohorts over a number of years.
Finally, it seems that habitat quality differs on a temporal scale. Birds tend to
perceive the quality of their environment differently during years with particularly harsh or
mild winters. It seems plausible that food availability (i.e. caterpillar abundance) depends
largely on winter temperature or snow condition. Models predicting future climate change
anticipate higher winter a decrease in temperature in North-Central BC, which would tend to
increase insect winter survival. However, an increase in snow fall is also predicted, which
would potentially increase the length of time the ground is snow-covered and make seeds and
insects less available during late winter and early spring. As forest structure is different
between habitats, it is very likely that climate change will differentially affect my forest
classes, and create complex interactions on bird behaviour and condition. Using
meteorological stations at multiple spots across habitats may help clarify the relationship
between the effect of climate change and future bird populations.

118

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