Plumage Characteristics In Tree Swallows Tachycineta bicolor: Honest Signals Of
Condition, Reproductive Investment And Mating Strategies

Pierre-Paul Bitton
B.Sc, University of Northern British Columbia, 2004

Thesis Submitted In Partial Fulfillment Of
The Requirements For The Degree Of
Master Of Science
In
Natural Resources And Environmental Studies
(Biology)

The University Of Northern British Columbia
September 2007

© Pierre-Paul Bitton, 2007

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Plumage characteristics are known to honestly reflect individual quality in many species
of birds, and influence inter- and intra-sexual interactions between conspecifics. In this
study, I investigated the ecological significance of plumage characteristics of male and
female tree swallows (Tachycineta bicolor). Plumage characteristics were related to age
in both males and females. Female plumage attributes were positively associated with
mean mass of the eggs laid, and their ability to fledge young. Also, I found that breeding
pairs mated assortatively by plumage brightness. Furthermore, males that produced extrapair young were brighter and older than males that did not produce extra-pair young.
Collectively, my results demonstrate that plumage characteristics in tree swallows convey
information about individual quality and influence the behaviour of conspecifics. These
findings are important, because this is the first study on the signalling value of plumage
characteristics in this species.

Acknowledgements
This research project would not have been possible without the constant mentoring and
support provided by my supervisor, Dr. Russell D. Dawson. For over six years, Russ has
been patient, understanding, and has allowed me to progress at my own pace. My time spent
in his research lab has taught me much, and I have gained personally and professionally from
our relationship. I am also grateful to my committee members Drs. Katherine Parker and
William Owen for valuable input on this project.
I am also highly indebted to my co-worker Erin L. O'Brien, not only for sharing
paternity data without which producing Chapter 4 would have been impossible, but also for
putting up with me during the course of rather demanding field seasons. Our difficult
existence in the "cave" was made easier by her presence, and I hope that we have the
opportunity to collaborate in the future.
I thank all individuals who have helped with field work. These include: Erin O'Brien,
Dan Baxter, Ben Schonewille, Mark Bidwell, Mecah Klem, Cheyenne Lawrie, and Kristy
Hillen. Alicia Goddard was particularly helpful with statistical analyses. I also thank A.
Castle, G. Sanders, and the late Dr. R. Dykes for giving me access to their properties to
conduct these studies.
Conducting research was made easier by those surrounding me in the lab. I consider
myself lucky to have shared space with Mark Bidwell, Erin O'Brien, Jennifer Greenwood,
Alicia Goddard, Cory Ochs, and Mecah Klem.
I am also grateful to my parents who have constantly supported me not only
financially but also by believing in me. It is because of them that I have a passion for the
outdoors and the nature that surrounds us.
Funding support for this project was provided by an Undergraduate Student Research
Award and a Canada Graduate Scholarship to myself, and a Discovery Grant to R. D.
Dawson from the Natural Sciences and Engineering Research Council of Canada. Additional
funding was provided by the Canada Foundation for Innovation, British Columbia
Knowledge Development Fund, and the University of Northern British Columbia. All
research protocols were approved by the Animal Care and Use Committee of UNBC.

iv

Table of Contents
Abstract

ii

Acknowledgements

iv

Table of Contents

v

List of Tables

viii

Lists of Figures

ix

1. General Introduction

1

1.1 Study species and study area

3

1.2 General objectives

4

1.2.1 Plumage characteristics as honest indicators of quality

4

1.2.2 Plumage characteristics, reproductive investments, and social pairing

6

1.2.3 Plumage characteristics and male extra-pair fertilization success

7

2. Age-related differences in plumage characteristics of male tree swallows: colour and
brightness signal different aspects of individual quality

9

2.1 Abstract

9

2.2 Introduction

10

2.3 Methods

12

2.3.1 Study area and field procedures

12

2.3.2 Plumage characteristics

13

2.3.3 Statistical analysis

14

2.4 Results

16

2.5 Discussion

22

2.5.1 Within-individual changes in plumage attributes

v

25

2.5.2 Plumage colour and return rate

27

3. Plumage characteristics, reproductive investment, and assortative mating in tree
swallows

30

3.1 Abstract

30

3.2 Introduction

31

3.3 Methods

33

3.3.1 Study area and field procedures

33

3.3.2 Plumage characteristics

34

3.3.3 Statistical analysis

35

3.4 Results

36

3.5 Discussion

40

4. Plumage brightness and age predict extra-pair mating fertilization success of male
tree swallow

48

4.1 Abstract

48

4.2 Introduction

49

4.3 Methods

51

4.3.1 Study area and field procedures

51

4.3.2 Plumage characteristics

52

4.3.3 Parentage analysis

52

4.3.4 Statistical analysis

53

4.4 Results

56

4.4.1 Patterns of paternity and male reproductive success

56

4.4.2 Plumage characteristics

58

vi

4.4.3 Extra-pair paternity and male characteristics

58

4.5 Discussion

62

5. General discussion

68

5.1 Plumage attributes and individual quality

68

5.2 Plumage attributes in relation to past and current reproductive investments

72

5.3 Plumage attributes, social pairing, and extra-pair mating strategies

75

5.4 Conclusions

77

References

78

Appendix I: Haematological characteristics

91

vn

List of Tables
Table 2.1 Plumage characteristics of male tree swallows captured near Prince George
(British Columbia, Canada) in three different years. Ris reflectance

21

Table 4.1 Analysis of covariance model investigating the influence of characteristics of
male tree swallows on the proportion of extra-pair offspring found within their nests
(n = 31 broods)

59

Table Al. Summary of leukocyte statistics for adult tree swallows captured in 2003 and
2004

91

viii

List of Figures

Figure 2.1 Plumage colour in relation to capture status in male tree swallows
(mean ± SE). Plumage colour is a unitless score (PCI) extracted from a principal
components analysis of 4 spectral curve descriptors (see Methods) with higher scores
representing shorter wavelength hues and greater UV chroma. Capture status is a
surrogate measure of age; newly captured birds are assumed to be relatively young
individuals, recaptured birds are relatively older individuals. Sample sizes are shown
above error bars

17

Figure 2.2 Relationship between feather investment index and plumage brightness scores
in newly captured (open circles and dashed line, n = 54) and recaptured (open triangles
and solid line, n = 34) male tree swallows. Plumage brightness is a unitless score (PC2)
extracted from a principal components analysis of 4 spectral curve descriptors (see
Methods) with brighter birds receiving larger scores. The feather investment index is
calculated as the contribution of the ninth primary feather to the wing chord. Lines of
best fit are included to illustrate trends
19
Figure 2.3 Relationship between a) plumage colour (PCI) and b) plumage brightness
(PC2) of individual male tree swallows between the first year of capture (year x) and
the following year (year x+1). Solid line represents 1:1 reference line; n = 18

20

Fig. 2.4 Probability of return to the study area by male tree swallows in relation to
plumage colour (PCI) for dull individual (low PC2 score; open circles and solid line),
and bright individuals (high PC2 score; open triangles and dashed line). Removal of the
2 extreme data points did not qualitatively change the results and therefore were kept in
the analyses
23
Figure 3.1 Plumage colour in relation to age in female tree swallows (TY = Third year,
ATY = After-third year). Plumage colour is a unitless score (PCI) extracted from a
principal components analysis with higher scores representing shorter wavelength hues
(i.e., bluer). Boxes show median (50th percentile), interquartile range (25th to 75th
percentile); whiskers indicate the 95% confidence intervals. Open circles are data
points outside the 95% CI range, and sample sizes are shown above each box
38
Figure 3.2 Mean egg mass in relation to plumage brightness of female tree swallows
(n = 20). Plumage brightness is a unit-less score (PC2) from a principal components
analysis, with brighter birds receiving larger scores. Line of best fit is included to
illustrate trend

39

Figure 3.3 Number of young fledged in relation to plumage colour (PCI) of female tree
swallows (2003: open circles and dashed line, n = 20; 2004: open triangles and full
line, n = 32). Lines of best fit are included to illustrate trends

ix

41

Figure 3.4 Relationship between plumage brightness (PC2) of male and females tree
swallows in social pairs (n = 49). Line of best fit is included to illustrate trend

42

Figure 4.1 Mass of male tree swallows in relation to the Julian date of capture (n = 41).
Line of best fit is included to illustrate trend

54

Figure 4.2 Number of young fledged by males that obtained extra-pair paternity and
males that did not obtain extra-pair paternity on the study site in the same breeding
population of tree swallows (mean ± SE). Sample sizes above error bars indicate
number of males

57

Figure 4.3 Spectral curves, averaging rump and mantle feathers, of male tree swallows
that did not sire extra-pair young on the study site (WP; n = 24) and males that did sire
extra-pair young (EP; n = 17). Error bars, incorporated periodically throughout the
curve to show the degree of variance over the whole spectrum, represent one standard
error
60
Figure 4.4 Male extra-pair fertilization success (WP = did not sire extra-pair young,
EP = sired extra-pair young on the study site) in relation to plumage brightness (PC2)
and capture status. Boxes show median (50th percentile), and interquartile range (25th
to 75th percentile); whiskers indicate 95% confidence intervals. Open circles are data
points outside the 95% CI range and sample sizes are shown above each box

x

61

1. General Introduction

Many hypotheses have been proposed to explore the mechanisms responsible for the
evolution of secondary sexual traits (Andersson 1994). Among these are models suggesting
that the expression of secondary sexual characteristics depends on the condition of the bearer
and, therefore, honestly signals individual quality (Kodric-Brown and Brown 1984). These
models assume that there is a trade-off between allocating limited energy and other resources
(e.g., colourful pigments) to the production of these traits and to other costly functions such
as metabolic regulation. In addition, the expression of ornamental traits can be costly for
survival since an increase in conspicuousness may attract predators (Zahavi 1975).
Individuals that can bear relatively more exuberant traits and survive would therefore be of
higher quality. As a consequence, a particular trait such as the colourful plumage of birds,
would reflect the overall genetic superiority of an individual and be considered an honest
signal of quality.
Feather colours are the result of pigments or organized structural features of the
feather's surface. Pigments include carotenoids, melanins, and porphyrins which produce
blacks, grays, browns, reds, oranges and yellows (Brush 1990; Lucas and Stettenheim 1972;
McGraw et al. 2004). In contrast to pigment-based colours, structural colours like most blues
and greens are the result of constructive interference, with short wavelengths of light
reflecting off the keratin-based barbs and barbules of feathers, and longer wavelengths being
absorbed (Prum et al. 1998). The nanostructure of feathers of most species also reflects
ultraviolet (UV) wavelengths. Unlike humans, which have three types of colour
photoreceptors (red, green, and blue), birds possess four types of cones (red, green, blue, and
UV) that allow them to perceive UV wavelengths (Chen et al. 1984). In addition, each cone

1

cell has an oil droplet that filters light and has the potential to increase spectral sensitivity
(Hart 2001). Accordingly, birds are capable of discerning small differences in colours,
especially in the UV-blue range (Burkhardt and Maier 1989).
Although the costs of producing pigment-based plumage colouration has been fairly
well documented (Hill 2006a), the cost of producing quality structural plumage is still under
debate (Doucet et al. 2006). Nonetheless, it has been suggested that the accurate production
of fine keratin structures is energetically costly and, since finer nanoscale structures reflect
shorter wavelengths (Prum et al. 1998), better quality individuals are expected to exhibit
plumage that reflects shorter wavelengths when compared to conspecifics (Shawkey et al.
2003). Experimental evidence also suggests that individuals in better condition at the time of
moult produce brighter plumage (McGraw et al. 2002). Therefore, structural plumage
colouration, like pigment-based colours, has the potential to be an honest signal of individual
quality (Hill et al. 2005; Sheldon et al. 1999). To date, research on honest signals in birds has
mostly focused on bright, pigment-based plumage colouration (e.g., Hill and Farmer 2005;
Massaro et al. 2003), but the signalling potential of structurally-based colours has recently
attracted much interest (e.g., Doucet 2002; Hill et al. 2005; Keyser and Hill 1999a).
Structural plumage characteristics have been shown to influence male and female intrasexual interactions (Alonso-Alvarez et al. 2004; Amundsen and Parn 2006), as well as intersexual interactions (Andersson and Amundsen 1997).
The objective of this research was to investigate the ecological significance of
structural plumage colouration in tree swallows (Tachycineta bicolof). More specifically, I
examined whether plumage characteristics of this species could 1) be an honest indicator of

2

individual quality, 2) reflect the potential for reproductive investment, and 3) influence
mating strategies.

1.1 Study species and study area
Tree swallows are small aerial insectivorous passerines, which breed throughout much of
North America. As secondary cavity nesters, their ecology is thought to be largely influenced
by competition for nest sites (Robertson et al. 1992). As they readily use nest-boxes, and are
resilient to human presence and disturbances, these birds have been the subject of intensive
ecological and biological research.
Tree swallows moult shortly after fledging nestlings, and all breeding males display
iridescent green to ultraviolet plumage on their dorsal surface. On the other hand, females
exhibit delayed plumage maturation (Robertson et al. 1992); sub-adult females (i.e., in their
second year) bear dull brown plumage whereas adult females have iridescent plumage, which
usually reflects more green than males (Hussell 1983). Although this difference in plumage
characteristics between female age groups has been hypothesized to influence intra-sexual
competition for nest sites, variation in the iridescent plumage of mature females may also
serve as inter- or intra-sexual signals. Therefore, both male and female tree swallows are
appropriate for the study of plumage signalling in iridescent species.
Male tree swallows normally arrive on the breeding grounds a few days before
females and start defending a suitable cavity (Robertson et al. 1992). Females are known to
prospect many sites before pairing, potentially selecting quality males, quality nest sites, or
both. Tree swallows are a socially monogamous species (with a pair bond lasting usually a
single season) in which females readily engage in extra-pair copulations. In fact, this species

3

shows one of the highest levels of extra-pair paternity known in birds (38% to 69% of all
young being extra-pair, review in Barber et al. 1996; O'Brien and Dawson 2007). Females
appear to deter unwanted copulations and seem to have great control over extra-pair mating,
yet the factors influencing female mate choice are poorly understood (Kempanaers et al.
2001; Robertson et al. 1992). The influence of male plumage attributes on social and extrapair mating has been investigated in other species (Hill 2006b), but this type of research has
never been published for tree swallows.
The study site used for this research was located south of Prince George where an
established colony has been studied since 2001. Every year, about 60 pairs of tree swallows
breed in nest boxes distributed over an area composed mostly of open agricultural fields with
patches of mixed deciduous and coniferous trees.

1.2 General objectives
1.2.1 Plumage characteristics as honest indicators of quality
In many bird species, the brightness of a male's plumage has been demonstrated to reflect
good health and potential fitness (Saks et al. 2003; Wiehn 1997). This relationship may be
partially attributed to the fact that brighter individuals have access to more resources because
they maintain larger territories (Keyser and Hill 2000), or possess greater foraging ability
(Senar et al. 2005). These skills are usually more developed in older individuals and, in
accordance, plumage attributes have been shown to be age-related in a number of species
(e.g., Siefferman et al. 2005). If females prefer to mate with older individuals because they
are better at providing resources (Marchetti and Price 1989), or because they can provide

4

good genes for their offspring (Hansen and Price 1995), then females could use the
expression of these secondary sexual traits to help them find older mates.
Second year sub-adult female tree swallows display dull brown plumage, in contrast
to mature females that possess male-like iridescent plumage on their dorsal surface (Hussell
1983). Although males of this species do not show delayed plumage maturation, it is
unknown whether younger males display a subdued plumage version of older males. In
Chapter 2,1 investigate the relationship between male plumage attributes and age. When
studying the effects of age on sexually selected traits, it is useful to combine both crosssectional and longitudinal analyses (Siefferman et al. 2005). Whereas cross-sectional
analyses compare the characteristics of different age classes at the population level,
longitudinal analyses follow the same individuals for more than one year. Since individual
survival may be positively (Jennions et al. 2001) or negatively (Hunt et al. 2004) correlated
with the expression of secondary sexual characteristics, differential mortality may confound
the results of a cross-sectional study. Therefore, I also investigate the relationship between
male plumage attributes and return rates to the breeding ground. In addition, I test the idea
that plumage attributes may signal individual quality, independent of age, by examining the
influence of investment in feathers and chronic stress on plumage quality. Resistance to
parasites and immunocompetence in general have been associated with sexual selection
(Doucet and Montgomerie 2003b) and may play a key role in the evolution of plumage
colouration (Hamilton and Zuk 1982), life-history strategies (Gustafsson et al. 1994), and
reproductive decisions (McNamara and Houston 1996). Studies of ecological immunity,
therefore, have developed various immunoassay and haematological health indices to
investigate proximate causes of animal behaviour and ecology. Of these, the blood

5

heterophile/lymphocyte (H/L) ratio, which is known to increase in response to various
stresses, has been deemed one of the most promising health indices for ecological research
(Ots et al. 1998). This measure of stress has been shown to be influenced in the short term by
reproductive effort (Shutler et al. 2004), but also, may represent long-term chronic stress
(Horak et al. 2002). Together, these analyses allowed me to determine whether plumage
characteristics could be honest indicators of individual quality in male tree swallows.

1.2.2 Plumage characteristics, reproductive investment, and social pairing
Historically, the signalling value of female ornaments has been mostly ignored by
evolutionary ecologists. The colourful, often conspicuous, plumage of females was mainly
considered to be caused by male genetic effects, without the influence of individual quality
(Amundsen 2000). However, recent studies have shown that female plumage attributes can
be honest indicators of quality and influence inter- and intra-sexual interactions (Amundsen
and Pain 2006). For example, female plumage attributes have been shown to reflect the
potential for reproductive investment, and influence male mate choice (Komdeur et al. 2005).
Female birds also have been shown to vary their investment in reproduction based on
the "attractiveness" and quality of their mates. If attractiveness is heritable, then females
would gain indirect fitness through their offspring (Trivers 1972). In some cases, females
will lay more eggs when mated to an attractive mate (Parker 2003; Petrie and Williams
1993), while in other cases, females have been shown to lay heavier eggs (Rutstein et al.
2004). By increasing resources allocated to eggs fertilized by quality males, females can
enhance the performance and survivorship of their offspring (Styrsky et al. 1999).

6

In Chapter 3,1 present results pertaining to the relationship between plumage
characteristics of both males and females, and past reproductive investment and success, as
well as present reproductive investment and success. Investigation of these questions allowed
me to evaluate if there is a cost of past reproductive effort to plumage, and in turn, if plumage
characteristics could indicate the potential for current effort. Furthermore, I determined
whether social pairs mated assortatively based on plumage attributes.

1.2.3 Plumage characteristics and male extra-pair fertilization success
When female tree swallows arrive on the breeding grounds, they compete for nest sites
defended by males (Robertson et al. 1992). In many animal species including birds, male
characteristics advertising quality have been shown to influence female mate choice (Nfeller
1988; Johnsen et al. 1998), yet in tree swallows, no such morphological characteristics of
males have been found (Dunn et al. 1994). Territory owners, which successfully attract
females, are usually smaller but in better condition than non-territorial males (Barber et al.
1998). However, even when paired with these males in good condition, a high percentage of
females readily engage in extra-pair copulations (Lifjeld et al. 1993), usually with other local
territory owners (Kempanaers et al. 2001; O'Brien and Dawson 2007). Extra-pair copulations
are often initiated by females and, in tree swallows, do not seem to be random as females will
resist mating attempts by certain males (Lifjeld and Robertson 1992). This behaviour
suggests that females choose specific mates that may offer certain benefits. The good genes
hypothesis argues that females choose males based on their genetic quality (not necessarily
compatibility) so that the fitness of offspring will be increased. As supporting evidence,
extra-pair young in some avian species have been shown to have increased survival rates

7

(Hasselquist et al. 1996), growth (Petri 1994) and immunocompetence (Johnsen et al. 2000).
Females should be able to select males that possess good genes by comparing secondary
sexual characteristics that are honest indicators of quality (Johnsen et al. 1998). In some
species, females will choose extra-pair mates of higher quality than their own mates (e.g.,
Otter et al. 1998) but again, no male characteristic has been identified as a criterion for
female mate choice in tree swallows. It is possible that female tree swallows, like North
American barn swallows (Hirundo rustled) (Safran and McGraw 2004), choose males based
on plumage colouration. However, this idea has never been tested.
In Chapter 4,1 present the results of a paternity study conducted in 2004 in which I
determined which characteristics, if any, could predict male extra-pair fertilization success.
Since a social mate's phenotypic attributes may influence female extra-pair mating strategy, I
also investigated the relationship between male characteristics and the proportion of extrapair young in their own nest. Furthermore, to determine if females mate outside the pair bond
with individuals of greater quality than their social mate, I performed pairwise comparisons
between the characteristics of the resident male and the cuckolding male.

8

2. Age-related differences in plumage characteristics of male tree swallows: colour and
brightness signal different aspects of individual quality

2.1 Abstract
Age-related differences in plumage characteristics of birds can be the result of: 1) differential
survival of more ornamented individuals, 2) within-individual changes in plumage attributes
with age, or 3) a combination of both. In this study, I investigated age-class related
differences in plumage colour and plumage brightness of male tree swallows (Tachycineta
bicolor) by performing both cross-sectional and longitudinal analyses. Male tree swallows in
their first breeding season do not display delayed plumage maturation, and possess the same
metallic green to metallic blue iridescent plumage as experienced breeders. My results
showed that, at the population level, recaptured (older) males were brighter and reflected
light maximally at shorter wavelengths (i.e., were bluer) than newly captured adults. These
differences in plumage brightness were most likely caused by changes within individuals, as
males increased in brightness between the first time they were captured and the subsequent
year. However, differences in colour were not due to within-individual changes, but rather
appear to be the result of greener individuals having lower survival and/or philopatry. Indeed,
relatively dull, greener birds had a lower probability of being recaptured in subsequent years.
In contrast, if birds captured in their first year as breeding adults were relatively bright,
colour did not seem to influence subsequent recapture probability. These results suggest that
plumage attributes in tree swallows have the potential of being an honest signal of quality.
Furthermore, plumage brightness and plumage colour might signal different aspects of male
quality in this species.

9

2.2 Introduction
Theory predicts that the expression of secondary sexual characters is dependent on condition,
and that such characters can honestly signal individual quality (Kodric-Brown and Brown
1984). In birds, as well as other taxa, these traits are usually more developed in older
individuals (Andersson 1994) because they may be more efficient at foraging and in better
condition at moult (Keyser and Hill 1999a; Doucet and Montgomerie 2003a). If females
prefer mating with older individuals because they hold better territories (Keyser and Hill
1999b), provide better parental care (Siefferman and Hill 2005b), or because they can
provide good genes for their offspring (Brooks and Kemp 2001), then females could use
plumage attributes of males to help them choose older mates.
In several species of birds, breeding adult males progress through distinct age-specific
plumages; males breeding for the first time have delayed plumage maturation and display
dull, often female-like, plumage. These individuals can be sexually mature but are usually
less successful at securing a mate (Thompson 1991). However, in most species, plumage
characteristics vary on a continuous spectrum as birds age, with younger birds often
displaying drabber or less elaborate plumage in comparison to older birds. In general, the
greatest change in ornamentation usually takes place between the first year and second year
(e.g., Delhey and Kempanaers 2006). Whether the plumage changes in colour, brightness, or
both seems to be species-specific (e.g., Komdeur et al 2005; Siefferman et al. 2005). At the
population level, age-related differences in plumage characteristics can also be caused by
differential survival of more ornamented individuals. Males that display more elaborate traits
have higher survival rates (Jennions et al. 2001), and are over-represented in the older portion
of the population (Forslund and Part 1995). However, if the production of secondary sexual

10

characteristics is too costly, the expression of these traits could be negatively correlated with
survival (Hunt et al. 2004).
In this study, I investigated age-related differences in plumage attributes of male tree
swallows (Tachycineta bicolor). Males and mature (two years of age or older) females of this
species possess bright iridescent contour feathers on their dorsal surface that range in colour
from metallic green to metallic blue. Although females in their second year display brown
dorsal plumage (Hussell 1983), males do not exhibit this delayed plumage maturation.
However, it is unknown whether the plumage characteristics of younger males differ
significantly from those of older males. To determine if there are age-class related
differences in plumage attributes at the population level in male tree swallows, I performed
cross-sectional analyses on data collected during 2 field seasons. In addition, I tested the
relationship between investment in feather production and stress levels (using the heterophil
to lymphocyte ratio), and plumage characteristics. If producing elaborate plumage
colouration is costly, then individuals in poor physiological condition at the time of feather
production should not be able to invest in quality plumage to the same extent as individuals
in good physiological condition. Doucet (2002) showed that birds that invested less in feather
growth produced duller, less saturated plumage.
Various immunoassay and haematological health indices have been developed to
investigate questions related to life-history evolution. Ots et al. (1998) suggested that blood
heterophil/lymphocyte (H/L) ratio, which is known to increase in response to various
stresses, is an appropriate index of "health" for ecological research. The H/L ratio has been
found to covary with reproductive stress (Shutler et al. 2004), blood parasite load (Figuerola
et al. 1999), and the production of elaborate ornaments (Bortolotti et al. 2006). Therefore, it

11

would be expected that individuals with higher levels of stress (i.e., high H/L ratios) would
produce less extravagant plumage.
In addition to the cross-sectional analyses, I investigated whether plumage attributes
changed with age within individuals by conducting longitudinal analyses. Furthermore, I
tested the idea that plumage characteristics can be used as predictors of survival or dispersal
probability by comparing the plumage attributes of males that returned to the study area in
years following capture to those that did not.

2.3 Methods
2.3.1 Study area and field procedures
I conducted this research on a population of tree swallows that has been breeding in nest
boxes near Prince George B.C., Canada (53°N, 123°W) since 2001. The study area consisted
of open agricultural fields mixed with patches of coniferous and deciduous forest, and many
small wetlands. The site had 125 nest boxes mounted on fence posts, approximately 20-30 m
apart. During the breeding seasons of 2002 to 2004, when this study was conducted, adult
males were captured in the nest using a swing-door trap. I measured length of the right wing
and ninth primary flight feather (nearest 0.5 mm) with a ruler. Birds were banded with a
standard U.S. Fish and Wildlife Service aluminum leg band. A blood smear was made from a
drop of blood extracted by puncturing the brachial vein (Bennett 1970). Smears were air
dried, fixed in 100% ethanol, and stained with Wright-Giemsa (Cameo Quik Stain II, Fort
Lauderdale FL, USA). Counts were made of heterophils, eosinophils, basophils, monocytes,
and lymphocytes by scanning for a minimum of 100 white blood cells with a compound
microscope using 10X ocular and 100X oil-immersion objective lenses.

12

Male tree swallows cannot be aged using morphological measurements. Although
nestlings do not show natal philopatry, adult male tree swallows have relatively high rates of
fidelity to nest sites (Robertson et al. 1992), with fewer than 5% of all breeding males
changing breeding sites during their lifetime (Winkler et al. 2004), males breeding on the
study area for the first time are probably relatively young individuals in comparison to males
banded as adults in previous years. Therefore, I used capture status (relatively young or
"newly captured" versus relatively old or "recaptured") as a surrogate for age in my analyses.

2.3.2 Plumage characteristics
At the time of capture, I collected feather samples from the rump and mantle of all males.
Feathers were kept in envelopes at room temperature until analysis. Four feathers (from the
same area and individual) were taped to a black piece of cardboard in an overlapping pattern
that simulated a bird's natural plumage, creating a solid surface from which measurements
could be obtained. Spectral data were collected using an Ocean Optics USB2000 (Dunedin,
Florida, USA) with a deuterium tungsten halogen light source (Avantes, Broomfield,
Colorado, USA). I used a bifurcated probe held in a cylindrical sheath that excluded ambient
light and kept the probe tip at a 90° angle to, and 6 mm from, the feather surface. I took three
readings at random locations on the feathers; spectral data were recorded between
wavelengths of 300-700 nm as the proportion of light reflected relative to the reflectance by a
pure white standard (Ocean Optics). To reduce electrical noise, each of the measures was an
average of 20 spectral readings taken at 100-msec intervals and smoothed with a boxcar
function using a 5-data-point average (Montgomerie 2006).

13

I summarized the spectral curves of each body region by quantifying measures of
brightness, hue, and chroma. Average brightness (Rav), which represents the amount of light
reflected by a surface, was calculated as the average percent reflectance between 300 nm and
700 nm (Doucet et al. 2004). As an index of hue (the main colour perceived), I used the
wavelength of maximum reflectance (XR.Max)- For chroma, since the iridescent plumage of
tree swallows peaks in the blue range of the spectra, I calculated blue chroma as the relative
contribution of the blue range as a percentage of the overall brightness (R4oo-5i2nm / R300700nm)-1 also calculated ultraviolet (UV) chroma (R3oo-4oonm / R3oo-700nm)- Although tree
swallow plumage does not peak in the ultraviolet range of the spectrum, studies in other
species have shown that UV chroma of male plumage influences female mate choice
(Bennett et al. 1997; Hunt et al. 1999) and that ultraviolet reflectance may be a biologically
and ecologically relevant signal in birds (Hausmann et al. 2003).
Since the rump and mantle feathers did not differ within individuals for any of the
four plumage descriptors that were measured (all P < 0.1), I used the average values of both
feather types for all analyses (Doucet 2002). The four plumage characteristics were entered
in a principal components analysis to eliminate the multiple correlations among the
measures. The first principal component, PCI, was interpreted as a measure of colour
whereas the second component, PC2, was interpreted as a measure of brightness (see
Results). I used the interpretation of these components in all subsequent analyses for this
study.
2.3.3 Statistical analysis
The length of a bird's wing is determined in part by the length of the boney structures as well
as the length of feathers. I derived an index of relative feather investment by calculating the

14

contribution of the ninth primary feather to the total length of the wing chord (length of ninth
primary / length of wing chord). Length of ninth primary feathers are known to increase with
body size in a number of species (e.g., Arevalo and Heeb 2005) and, in tree swallows, both
the length of the ninth primary and length of wing chord are positively correlated with
various measures of body size (unpublished data). Therefore, this index can be considered a
measure of feather length relative to body size, with higher scores associated with birds that
produced comparatively long feathers in relation to their body size.
To investigate the influence of age on plumage characteristics, I performed both
cross-sectional and longitudinal analyses. For the cross-sectional analyses I used data from
birds captured in 2003 and 2004. For birds captured in both years, I randomly selected one of
the observations to include in the analysis. I used general linear models with plumage
characteristics (either PCI or PC2) as the dependent variables and I included capture status
and year as fixed factors, index of feather investment and H/L ratio as covariates, plus all
first-order interactions in both models. For these analyses, I used a backward stepwise
procedure to eliminate non-significant interactions and variables, keeping only significant
terms in the final models.
For longitudinal analyses, I used data from 2002 to 2003 for newly captured birds that
were subsequently captured in the next breeding season (2003 and 2004). I analysed changes
in plumage characteristics (PCI and PC2) between the two years using repeated-measures
ANOVA, with plumage characteristic in year x and x + 1 as the within-subject factor, and
year as a between-subjects factor. To assess whether the pattern of change within individuals
was consistent, or whether it was influenced by the years the data were collected (either 2002

15

to 2003, or 2003 to 2004), I included the interaction between the within-subject factor and
year in the models.
To test whether plumage characteristics at the population level could be influenced by
differential survival and/or dispersal based on plumage attributes, I used a logistic regression
model with a binary dependent variable (returned / did not return), and plumage
characteristics (PCI and PC2), and the interaction as explanatory variables. For this analysis,
I included data from all newly captured birds that were recaptured the subsequent year, and
an identical number of randomly selected newly captured birds that never returned to breed
on the study site. All statistical analyses were performed using SPSS (Norusis 2000). Results
were considered significant at the 0.05 level, and I present means ± SE unless stated
otherwise.

2.4 Results
Principal component analysis of plumage characteristics showed that the first component,
PCI, explained 52.0% of the total variance and was heavily weighted by the hue, blue
chroma, and UV chroma, but very little by the brightness (rotated matrix values: -0.98, 0.56,
0.90, and -0.08 respectively); PCI was therefore interpreted as a measure of colour. The
second component, PC2, explained 25.5% of the total variance and strongly represented
differences in brightness among individuals (rotated matrix values: 0.09, 0.33, -0.18, and
0.93 for hue, blue chroma, UV chroma, and brightness, respectively) and was interpreted as a
measure of brightness. Older males were bluer and brighter than newly captured males,
which were generally greener (PCI; Figs = 5.94, P = 0.02; Fig. 2.1) and duller (PC2; Fi85 =

16

0.8 !

34

0.6

o

0.4

3

o
o
o

0.2

<D

ro
Q.

0.0

54

-0.2

ro
-0.4
-0.6

Newly
captured

Recaptured

Fig. 2.1 Plumage colour in relation to capture status in male tree swallows (mean ± SE).
Plumage colour is a unitless score (PCI) extracted from a principal components analysis of 4
spectral curve descriptors (see Methods) with higher scores representing shorter wavelength
hues and greater UV chroma. Capture status is a surrogate measure of age; newly captured
birds are assumed to be relatively young individuals, recaptured birds are relatively older
individuals. Sample sizes are shown above error bars.

17

4.35, P = 0.04). Plumage brightness was also influenced by the feather investment index
(Fi,85 = 7.26, P < 0.01), with birds that invested more in feathers displaying brighter plumage
(Fig. 2.2). In contrast, H/L ratio (range: 1.2 - 95.0; mean: 20.4 ± 1.9 SE; Table Al) was not a
significant variable in the model.
Longitudinal analyses of male plumage attributes revealed that plumage colour (PCI)
was highly correlated between the first capture and the subsequent year capture (r = 0.74, P <
0.001, n = 19), and that this plumage characteristic did not change within individuals from
one year to the next (Fi;n = 0.22, P = 0.65; Fig. 2.3a). These results suggest that plumage
colour (PCI) is a trait partially inherent to individuals. In contrast, although plumage
brightness (PC2) between the first capture and the subsequent year capture was correlated (r
= 0.48, P = 0.038, n = 19), males became brighter with age (repeated measure ANOVA, F i n
= 16.76, P = 0.001; Fig. 2.3b). Together, these results indicate that while individual males
become brighter with age, those that were relatively bright the first time they were captured
remained relatively bright the subsequent year.
Plumage characteristics were slightly different from year to year (Table 2.1). To
control for these differences, I standardized the four measures extracted from the original
spectra for each year using z-scores (mean = 0, standard deviation =1). The new scores were
reduced to measures of colour (PCI) and brightness (PC2), which were used for the final
analyses, using principal components analysis as described above. The analysis of returns to
the study area suggested that neither plumage colour (PCI; Wald = 0.01, df = 1, P = 0.98, n =
44) nor plumage brightness (PC2; Wald = 0.43, df = 1, P = 0.51, n = 44) alone were
significant predictors of probability of returning; however, the interaction between these two
terms was significant (Wald = 6.56, df = 1, P = 0.01, n = 44). To investigate the nature of this

18

3 -i
CN

o
CO
CO

0

A

0

D)

E -1
_2

-

8
.-O-

@-l" i
6

a.
q>

m -2

A

A

A

CD

(TJ

A

A

1-

c

5

o

-

A
A

8

6

o

1
-T
o 8 8
Q

A

®

o

1

1

1

1

0.74

0.76

0.78

0.80

Feather investment index
Fig. 2.2 Relationship between feather investment index and plumage brightness scores in
newly captured (open circles and dashed line, n = 54) and recaptured (open triangles and
solid line, n = 34) male tree swallows. Plumage brightness is a unitless score (PC2)
extracted from a principal components analysis of 4 spectral curve descriptors (see
Methods) with brighter birds receiving larger scores. The feather investment index is
calculated as the contribution of the ninth primary feather to the length of the wing chord.
Lines of best fit are included to illustrate trends.

19

o
i-

3

O
O
(D
O)
CO

E
Q.
®
CO

Male plumage colour (PC1yearx)

CM

o
p_,
V)
<n
CO
c

_o
d>
O)
CO

E
j3
Q.

^)
CO

-

2

-

1

1

0

2

3

Male plumage brightness (PC2yearx)

Fig. 2.3 Relationship between a) plumage colour (PCI) and b) plumage brightness (PC2) of
individual male tree swallows between the first year of capture (year x) and the following
year (year x+1). Solid line represents 1:1 reference line; n = 18.

20

Table 2.1 Plumage characteristics of male tree swallows captured near Prince George (British
Columbia, Canada) in three different years. R is reflectance.

Plumage characteristic

Year

Mean

S.D.

F

df

P

Average brightness(Rav)

2002

10.52

1.42

1.17

105

0.32

2003

10.77

1.38

2004

11.09

1.60

2002

42.31

1.99

1.28

105

0.28

2003

42.97

1.65

2004

42.41

1.81

2002

458.04

13.53

4.43

105

O.Olt

2003

468.65

12.08

2004

466.13

13.57

Ultraviolet chroma(3oo-400nm) 2002

18.84

3.28

5.15

105

O.Olf

2003

16.34

2.76

2004

17.33

2.66

Blue chroma(4oo-5i2nm)

Hue (ARmax)

f Post-hoc tests revealed that 2002 values were significantly different than 2003 values (P
0.01), but that all other pairwise comparisons were not significant (Bonferroni corrections
applied).

21

interaction, I separated the data into two groups based on whether plumage brightness was
greater than average (bright) or lower than average (dull), and analysed the effects of colour
(PCI) on probability of returning. The model for dull birds fit the data acceptably (%2i = 6.79,
P < 0.01, n = 22) with colour (PCI) as a valid predictor of survival (Wald = 3.82, df = 1, P =
0.05), suggesting that if birds were relatively dull, greener birds were less likely than bluer
birds to be recaptured in the subsequent year. In contrast, the model for bright birds did not
fit the data acceptably (x^ = 0.11, P = 0.74; n = 22), indicating that if the birds were bright,
colour did not influence their recapture probability (Fig. 2.4).

2.5 Discussion
In this study I showed that, at the population level, plumage colour and brightness of male
tree swallows were influenced by capture status, a surrogate measure of age. Relatively older
individuals were brighter (Fig 2.2) and reflected light maximally at shorter wavelengths
compared to younger males (Fig 2.1). In addition, individuals that were able to invest more in
feather growth relative to their body size possessed brighter plumage. Patterns of age-related
plumage characteristics were influenced by both year to year within-individual changes, and
colour-biased return rate. Individual birds increased in overall brightness from one year to the
next (Fig 2.3b) but did not seem to change significantly in colour (Fig 2.3a). Instead,
plumage colour was related to the probability of returning to the study area in subsequent
years; among males that were relatively dull, greener individuals were less likely to return as
breeders than bluer individuals. In contrast, if the males were relatively bright, colour did not

influence probability of returning (Fig 2.4).

22

0 ( 0 5 ) 0 CX3D
1 A

M A

A A A

A A

O)

c
'c

M—

o
_>.
!5
CO
.Q

o
OH

O O O O O CD OO
OO
A AA^ A A A A A
-

1

0

1

2

Male plumage colour (PC1)

Fig. 2.4 Probability of return to the study area by male tree swallows in relation to plumage
colour (PCI) for dull individuals (low PC2 score; open circles and solid line), and bright
individuals (high PC2 score; open triangles and dashed line). Removal of the 2 extreme
data points did not qualitatively change the results and therefore were kept in the analyses.

23

Iridescent colours of feathers are produced by the coherent scattering of light from
alternating layers of materials that have different reflective indices (Prum 2006). These
colours are known to be produced mainly at the interfaces between air, keratin and melanin.
Unlike non-iridescent structural colours that are produced by the feather barbs, iridescent
colours are produced by the barbules (Doucet et al. 2006; Prum 2006). Although the cost of
producing quality iridescent plumage is poorly understood, general trends have been
emerging from recent research on structural colours. Differences in hue within species have
been attributed to variation in feather cortex thickness, whereas differences in UV chroma
have been attributed to the percentage of melanin found below the keratin (Doucet et al.
2006). In contrast, variation in brightness is thought to be related to the number or condition
of barbules (Shawkey et al. 2003; Doucet et al. 2006). Hence, susceptibility to feather
damage, abrasion, and even keratinolytic bacteria could influence variability in brightness
(Osorio and Ham 2002; Shawkey et al. 2007). Together, these lines of evidence imply that, if
the production of quality barbules is influenced by genetic quality or physiological stress
during feather development, iridescent colours may in fact honestly signal individual quality.
My results show that birds that invested more in feather production displayed brighter
plumage (Fig. 2.2). The feather investment index I used takes into account the size of the
individual and therefore represented the relative investment in relation to structural size. This
relationship between feather investment and brightness was independent of age, as the two
age classes did not differ in relative investment (P.-P. Bitton, unpublished data). If plumage
brightness in tree swallows is a function of feather barbule density, as research on other
structurally coloured species suggests (Shawkey et al. 2003; Doucet et al. 2006), then birds
that produced relatively long feathers may also have produced feathers with greater barbule

24

density. This would not necessarily indicate that there is no trade-off between allocating
energy to feather length or to feather quality, but rather that the investment in barbules is
proportional to investment in feather length. Individuals in poor condition would produce
short, dull feathers, whereas individuals in good condition would produce long feathers with
a high density of barbules. Similar findings have been reported in other species using feather
growth bars as an index of investment in feathers during moult. In both blue-black grassquits
(Volatinia jacarina) and satin bowerbirds (Ptilonorhynchus violaceus), two structurally
coloured species, feather growth rates were positively correlated with plumage brightness
(Doucet 2002; Doucet and Montgomerie 2003a). Furthermore, this relationship has also been
observed in species that possess pigment-based plumage coloration (e.g., Senar et al. 2003).
Therefore, my results support the idea that individuals in good body condition at the time of
moult are able to produce relatively long, brightly coloured feathers, and that plumage
brightness in tree swallows can potentially be an honest indicator of quality.
2.5.1 Within-individual changes in plumage attributes
I found evidence to suggest that plumage characteristics in tree swallows are partly inherent
to the individual. Plumage colour was highly correlated between the first time a bird was
captured and its recapture the subsequent year, and did not significantly change from one
year to the next (Fig. 2.3a). Plumage brightness, on the other hand, did increase from one
year to the next but, as for colour, was also correlated between the first and second year of
capture (Fig 2.3b). These results show that birds that were relatively dull when they were

first captured as adults were still relatively dull as returning breeders, albeit brighter than the
first time they were captured.

25

Age-related differences in structural plumage attributes are not consistent among
species. In both European starlings (Sturnus vulgaris) and blue tits (Cyanistes caeruleus), hue
and brightness have been shown to change with age (Komdeur et al. 2005; Delhey and
Kempenaers 2006). However, the structurally coloured rump of eastern bluebirds (Sialia
sialis) increases in brightness over the lifetime of an individual but does not change in colour
(Siefferman et al. 2005). Although all structural colours are produced by coherent scattering,
the physical composition and structural arrangements of feather barbules are very different
among species (Prum 2006). There is a large variety of developmental and physiological
mechanisms that produce various feather structures, and therefore, colour production may be
more flexible in some species than in others. As such, it is not surprising that plumage hue
has been found to change with age in certain species, but not others. On the other hand, if
plumage brightness is mostly a function of density and quality of barbules, this structural
plumage attribute may be more variable among individuals of the same species.
Developmental mechanisms may constrain the ability of individuals to vary plumage colour
from year to year, but investment in feather barbules may be more flexible. If this is the case
in tree swallows, an increase in plumage brightness with age might simply indicate, as in
other species, that older birds are in better condition during moult (e.g., Limmer and Becker
2006; Heise and Moore 2003). However, the fact that plumage brightness from one year to
the next was correlated among birds may indicate that plumage brightness could also signal
inherent qualities of the individual. Indeed, relatively dull newly captured birds were also
relatively dull the subsequent year. This implies that by comparing plumage brightness
among individuals, it would be difficult to differentiate dull, experienced breeders from

26

bright inexperienced ones. If brightness is partially indicative of inherent quality, young birds
would be able to convey that information.

2.5.2 Plumage colour and return rate
My results also showed that the age-related structure in plumage colour was partially driven
by biased return rates of bluer males. However, not all relatively green individuals had lower
probability of returning. When including only relatively dull birds in my analyses, greener
birds were less likely to return. In contrast, when considering only relatively bright birds, I
did not find any evidence of colour-biased return rates (Fig 2.4). Whether or not a bird was
recaptured could have been influenced by a variety of factors including the ability to capture
them, breeding site philopatry, and overwinter survival. Since birds were captured in the nest
after the eggs had hatched, a few of the returning birds may not have been captured if their
clutch did not successfully hatch. If this is the case, my results would reflect the fact that
greener males may have been mated to females of lower quality, since the ability to incubate
eggs is related to female age and condition in tree swallows (Ardia and Clotfelter 2007). This
further implies that greener individuals may themselves be of lower quality, as there is
evidence for assortative mating in this population (Chapter 3) and in other populations of this
species (Robertson and Rendell 2001). It is also possible that greener individuals may
disperse to different breeding sites more readily than bluer individuals. Such behaviour is
rather unlikely; male tree swallows have very high nest-site fidelity and rarely disperse after
having bred at one location (Winkler et al. 2004). Furthermore, there is little evidence to
support the hypothesis of colour-biased dispersal in other species (Delhey and Kempenaers
2006). More probable is the possibility that plumage colour reflects individual quality and the

27

likelihood of surviving from one year to the next. Overwinter survival has been associated
with quality structural plumage (Sheldon et al. 1999; Griffiths et al. 2003), melanin-based
plumage (Bize et al. 2006), and carotenoid-based plumage (Horak et al. 2001). Even in
species that contain specific morphs, plumage attributes have been shown to influence adult
survival (Brommer et al. 2005). If elaborate sexual ornaments are extremely costly to
produce, current sexual selection theory predicts that the expression of these ornaments can
be negatively correlated with survival (Kokko et al. 2002). However, in many cases, plumage
attributes have been shown to correlate positively with some aspect of individual quality, and
thus covary with apparent survival. In tree swallows, overwinter survival has been positively
associated with individual immunocompetence (Ardia et al. 2003), and longer telomeres
(Haussmann et al. 2005). In both cases, the ability to survive from one year to the next was
argued to reflect the capacity of individuals to fair well in high-stress situations.
The fact that greener individuals had lower return rates than relatively bluer
individuals suggests that plumage colour, independent of plumage brightness, is an indicator
of individual quality. Yet, my results from the longitudinal analyses strongly imply that
plumage colouration in this population is inherent to the individual, making it difficult for
this trait to honestly reflect condition of the individual. Although seemingly contradictory,
the lack of environmental influence on a plumage attribute does not preclude its possible
signalling function. The study of the evolution and function of colour traits has mostly
focused on traits that are condition-dependent and that can be influenced by environmental
conditions, arguing that these traits are more likely to honestly signal quality. However, it is
also possible for genetically inherited traits to reveal information about individual quality
(reviewed in Roulin 2004). Different colour morphs, for example, are associated with

28

different life-history strategies, which may, to a certain extent, influence survival. Although
some species display non-sexual plumage dimorphism, plumage characteristics that vary on a
continuous spectrum can also be under genetic control (Bize et al. 2006; Py et al. 2006). If
plumage attributes are co-inherited with either behavioural or physiological traits that
increase the likelihood of an individual surviving overwinter, plumage attributes may convey
information about individual quality. Unfortunately, there is no information currently
available on colour-specific behaviour or the heritability of plumage characteristics in tree
swallows, making it difficult to strongly infer the signalling value of plumage colour in this
species.
Overall, my results lend support to the idea that both plumage colour and plumage
brightness, independently of one another, have the potential to signal quality in male tree
swallows. Plumage brightness seems to reflect an individual's age and the ability to invest in
feather growth, suggesting that bright birds are in better condition at time of moult. On the
other hand, plumage colour may be indicative of an individual's inherent quality, although
the exact mechanisms that would make this possible are uncertain. Future studies on plumage
coloration of tree swallows should investigate the proximate mechanisms that influence
plumage colour, so that a better link may be established between an individual's colour and
its ability to survive. Furthermore, there is a need for a better understanding of the costs
associated with developing the structures that produce colour. An extensive survey of the
light-reflecting structures found among birds may be required, since the diversity of
nano structures makes it difficult to generalise the cost of producing them.

29

3. Plumage characteristics, reproductive investment, and assortative mating in tree
swallows

3.1 Abstract
Elaborate ornamental plumage has been associated with various measures of individual
quality in many species of birds. Male plumage characteristics, which have been relatively
well studied, can reflect past reproductive investment as well as the potential for reproductive
investment in the current breeding attempt. In contrast, the signalling functions of female
traits remain largely unexplored. In this study I investigated the relationship between
plumage attributes of breeding adult tree swallows and past reproductive investment, current
reproductive investment, and social mate pairing strategy. Both male and mature females
possess metallic green to metallic blue iridescent plumage on their dorsal surface, making
tree swallows a suitable species for this type of investigation. I did not find any effects of
past reproductive investment and success on the plumage attributes of returning breeders. In
contrast, female plumage colour covaried with fledging success in the two years of my study,
and female plumage brightness was positively associated with mean clutch egg mass. In
addition, I found that social pairs mated assortatively with respect to plumage brightness. I
argue that, since plumage colouration also varies with age in both male and female tree
swallows, plumage attributes in this species are honest signals of quality and indicative of
breeding experience. Positive assortative mating could be the result of mutual mate choice or
intra-sexual nest site competition by both males and females.

30

3.2 Introduction
The feathers of birds can be coloured by deposition of pigments, such as carotenoids and
melanin, or through the physical interaction between light and the nanostructures of barbs
and barbules (Prum 2006). Although pigments are responsible for most black, brown, red,
orange, and yellow feathers, colours associated with the feather nanostructure, known as
structural colours, include blue, violet, ultraviolet, and iridescent plumage. These colours are
produced when light strikes the quasi-ordered matrix of keratin and air within feather barbs
or alternating layers of material with different refractive indices such as keratin, melanin, and
air in feather barbules (Doucet et al. 2006).
The cost of incorporating pigments in feathers to produce saturated and bright
plumage has been relatively well studied both in the wild and in aviaries (Hill 2006a), but the
cost of producing brilliant structural plumage is poorly understood. Indeed, there is
considerable variation among species in the nanostructure arrangement in feathers such that it
is difficult to generalize the costs associated with producing quality plumage (Prum 2006).
Nonetheless, experimental evidence suggests that producing fine nanostructures in feathers is
costly (Hill 2006b), and therefore, these traits have the potential to be honest signals of
individual quality (Kodric-Brown and Brown 1984). For example, structural plumage
characteristics have been shown to be affected by endoparasites (Hill et al. 2005) and
nutritional stress (McGraw et al. 2002). In males, plumage attributes also have been
associated with differences in age (Dehley and Kempenaers 2005) and body condition at the
time of moult (Keyser and Hill 1999b). There even seems to be a cost of past reproductive
effort on structural plumage characteristics, as an experimental increase of parental effort

31

during brood rearing has been shown to influence plumage attributes, following moult, in
eastern bluebirds {Sialia sialis; Siefferman and Hill 2005c).
Although much work has focused on the ecological and behavioural importance of
elaborate ornamentation in males, the signalling potential of plumage in females has, until
recently, largely been ignored (Amudsen and Parn 2006). Plumage characteristics of females
were often considered a by-product of selection on males, uninfluenced by the pressures of
sexual selection (Amundsen 2000). However, recent studies have presented strong evidence
suggesting plumage attributes of females may also honestly signal quality. Variation in
structural plumage characteristics in females has been associated with age (Komdeur et al.
2005), various indices of individual quality such as body condition (Siefferman and Hill
2005a), and immunological status (Hanssen et al. 2006). In addition, there is evidence for
mate choice of elaborately ornamented females by males (Griggio et al. 2005). Clearly, our
understanding of the functions of female plumage warrants more attention.
Tree swallows {Tachycineta bicolor) are rather unique among North American
passerines in that it is the female, not the male, that displays delayed plumage maturation.
Subadult females in their second year (SY) of life possess dull brown plumage on their dorsal
surface that usually contains less than 50% bright iridescent feathers (Hussell 1983). In
contrast, mature after-second year (ASY) females display male-like bright metallic green to
metallic blue iridescent feathers. The ecological reasons for this delay in plumage maturation
are unclear, but it has been hypothesized that in the context of competition for nest sites, dull
brown plumage may allow younger females to suffer less aggression from older females
(Robertson et al. 1992). Regardless of its merit, this explanation does not preclude the
possibility that female plumage characteristics also serve as inter-sexual signals.

32

In this study I investigated the relationship between plumage attributes of breeding
adult tree swallows and past reproductive investment, current reproductive investment, and
social mate pairing strategy. More specifically, I investigated 1) the influence of past
reproductive investment and success on the plumage characteristics of returning breeders, 2)
the relationship between breeding plumage characteristics and reproductive investment and
success within the same breeding season, and 3) whether individuals paired with mates
displaying similar plumage (assortative mating).

3.3 Methods
3.3.1 Study area and field procedures
I conducted this study on a population of tree swallows breeding in nest boxes near Prince
George BC, Canada (53°N, 123°W) since April 2001. The study area consisted of open
agricultural fields mixed with patches of coniferous and deciduous forest, and many small
wetlands. The site he Ids 125 nest boxes mounted on fence posts, and placed approximately
20-30 m apart. During the breeding seasons of 2002 to 2004 when this study was conducted,
I started visiting nest boxes every other day in early May, keeping track of the nest building
progress until the first egg was laid. From then on all nests were visited daily. I recorded the
clutch initiation date and, once egg laying was completed, clutch size. In 2002 and 2003 (but
not 2004), each egg was weighed on the day it was laid using an electronic balance (nearest
0.0lg), and numbered with a non-toxic permanent marker according to laying sequence.
Soon after the eggs had hatched, I captured adults in the nest using a swing-door trap.
Females were aged as either SY subadult or ASY adult, based on plumage characteristics
(Hussell 1983). All adults were banded and a blood smear was made from a drop of blood

33

extracted by puncturing the brachial vein (Bennett 1970). Blood smears were subsequently
fixed in 100% ethanol and stained with Cameo Quik Stain II (Fort Lauderdale, Florida,
USA), a Wright-Giemsa stain. Counts were made of heterophils (H), eosinophils, basophils,
monocytes, and lymphocytes (L) by scanning for a minimum of 100 white blood cells with a
compound microscope using a 10X ocular and an oil-immersion 100X objective lens. Ratios
of H/L were calculated as a measure of stress (Ots et al. 1998). Nests were monitored
throughout the brood-rearing period to determine how many nestlings fledged.
3.3.2 Plumage characteristics
For birds captured in 2003 and 2004,1 collected feather samples from the rump and mantle
of all males and from the rump of mature (ASY) females. The feathers were kept in
envelopes at room temperature until analysis (described in Chapter 2). Briefly, four feathers
(from the same area and individual) were taped to a black piece of cardboard in an
overlapping pattern that reproduced a bird's natural plumage. Spectral data were collected
using an Ocean Optics USB2000 spectrometer (Dunedin, Florida, USA) with a deuterium
tungsten halogen light source (Avantes, Broomfield, Colorado, USA). I summarized the
spectral curves of each body region by quantifying measures of brightness, hue, blue chroma
and UV chroma.
For males, since the rump and mantle feathers did not differ within individuals for
any of the four plumage descriptors that were measured, I used the average values of both
feather types for all analyses (Doucet 2002). The four plumage characteristics were entered

in a principal components analysis to eliminate the multiple correlations among the
measures, and I used the interpretation of these components in all subsequent analyses for
this study. Analyses were conducted separately for males and females because the

34

relationships between the four descriptors were different for the sexes. The first principal
component, PCI, was interpreted as a measure of colour; PC2 was interpreted as a measure
of brightness (see Results).

3.3.3 Statistical analysis
I investigated the influence of past reproductive investment and success on the plumage
characteristics of returning breeders separately for males and females, with analysis of
covariance. Using only ASY1 females, I conducted two tests, one with plumage colour (PCI)
as the dependent variable and a second with plumage brightness (PC2) as the dependent
variable. In both cases, I included reproductive variables collected the previous year as
independent predictors. These included clutch initiation date (corrected for annual
differences by defining date of first egg in each year as day "0"), mean egg mass, clutch size,
and number of young fledged. In these models, I also included the age of each female (SY or
ASY) in the year the reproductive variables were recorded, the year the feathers were
collected (2003 or 2004), and the interactions between age and all other factors as
independent variables. For males, I conducted two tests with plumage colour (PCI) and
plumage brightness (PC2) as dependent variables. Because males should not be affected by
female-related investments such as clutch size and egg mass, I did not include these variables
in the models. Instead, these models included (from the previous year) clutch initiation date,
number of young fledged, the year the feathers were collected (2003 or 2004), and all first
order interactions as independent variables. I used a backward stepwise procedure to
eliminate all non-significant terms, keeping only significant variables in the final model.
1

This group includes birds that were caught as SY the previous year and are considered third year (TY) in the
analysis, and birds that were caught as ASY the previous year and are considered after-third year (ATY) in the
analysis.

35

I also used analysis of covariance to investigate the relationship between plumage
characteristics of breeding adults and reproductive investment and success (clutch size, mean
egg mass, and fledging success) in the same year. Since male characteristics have been
shown to influence female investment in some species (e.g., Uller et al. 2005), I included
both male and female attributes in these analyses. The models for investment in clutch size
and mean egg mass (dependent variables) included year the feathers were collected (2003 or
2004), clutch initiation date, male and female plumage colour (PCI), male and female
plumage brightness (PC2), and all interactions between year and the other factors as
independent variables. When testing for the influence of parental attributes on fledging
success, I also included male and female H/L ratio (log-transformed to correct for nonnormal distribution) in the analysis. The H/L ratio is a measure of chronic stress in birds and
has been shown to correlate with reproductive success in some species (Friedl and Edler
2005).
Finally, I used correlations to compare plumage colour (PCI) and brightness (PC2)
between social mates. Tree swallows will sometimes remain paired over more than one
breeding season. In such cases, I used only the initial pairing when testing for assortative
mating. For the other tests, I included birds that bred in both years, considering the data
points to be independent from one another. All statistical analyses were performed using
SPSS (Norusis 2000). Results were considered significant at the 0.05 level, and I present
means ± 1 SE unless stated otherwise.
3.4 Results
Principal component analysis of male plumage characteristics showed that the first principal
component, PCI, explained 52.0% of the total variance and was heavily weighted by the hue,

36

blue chroma, and UV chroma, but very little by brightness (rotated matrix values: -0.98, 0.56,
0.90, and -0.08, respectively); PCI was therefore interpreted as a measure of colour. The
second component, PC2, explained 25.5% of the total variance and was heavily loaded by
brightness (rotated matrix values: 0.09, 0.33, -0.19, and 0.93 for hue, blue chroma, UV
chroma and brightness, respectively); PC2 was therefore interpreted as a measure of
brightness. For females, the first principal component explained 61.1% of the total variance
and was also interpreted as a measure of colour (rotated matrix values: -0.97, 0.89, 0.83, and
0.15 for hue, blue chroma, UV chroma, and brightness, respectively). The second
component, which accounted for 26.5% of the variance, strongly represented differences in
brightness among individuals (-0.16, 0.04, 0.25, 0.98 for hue, blue chroma, UV chroma, and
brightness, respectively), and as for males was interpreted as a measure of brightness.
I did not find any effects of past reproductive investment or past reproductive success
on female plumage colour (PCI; all Ps > 0.10) and plumage brightness (PC2; all Ps > 0.10),
or on male plumage colour (PCI; all Ps > 0.10) and plumage brightness (PC2; all Ps > 0.10).
Female plumage colour was influenced by female age (F120 = 5.61, P = 0.03, R2 = 0.22), with
after-third year individuals displaying bluer plumage than third-year conspecifics (Fig. 3.1).
Clutch initiation date for nests used in the analyses ranged from 23 May to 19 June
(mean: 2 June ± 0.8 days) in 2003, and 19 May to 8 June (mean: 28 May ± 0.7 days) in 2004.
Although birds that laid earlier in the season had larger clutches (F194 = 6.89, P = 0.01, R2 =
0.07), female plumage characteristics were unrelated to clutch size (all Ps > 0.10). However,
I did find a relationship between female plumage characteristics and investment in egg mass,
and reproductive success. There was significant positive association between female plumage
brightness (PC2) and mean clutch egg mass (F U 8 = 5.27, P = 0.03, R2 = 0.26; Fig 3.2).

37

2 -,

14
O

1

_o
o
o

0

_=Q=_

0

D)

as

O

E -1 H
a.
05

^

-2

TY

ATY

Fig 3.1 Plumage colour in relation to age in female tree swallows (TY = Third year, ATY =
After-third year). Plumage colour is a unitless score (PCI) extracted from a principal
components analysis with higher scores representing shorter wavelength hues (i.e., bluer).
Boxes show median (50th percentile) and interquartile range (25th to 75th percentile);
whiskers indicate the 95% confidence intervals. Open circles are data points outside the
95% CI range, and sample sizes are shown above each box.

38

1.95 -i

1.90 -

oo

V 185

°

w
re

°^

GD

E

o o

en 1.80
O)
0

I

o
1.75
1.70

°8

o

O)

o

0

- o°

1.65

I

-

2

-

1

0

i

1

2

i

3

Female plumage brightness (PC2)

Fig 3.2 Mean egg mass in relation to plumage brightness of female tree swallows (n = 20).
Plumage brightness is a unit-less score (PC2) from a principal components analysis, with
brighter birds receiving larger scores. Line of best fit is included to illustrate trend.

39

Significantly more young were fledged in 2004 (3.68 ± 0.32, n = 57) than in 2003 (2.09 ±
0.31, n = 45; Fi,49 = 12.74, P < 0.001), females reflecting light maximally at shorter
wavelengths (i.e., bluer) were more successful at fledging young in both years of my study
(PCI: Fi,49 = 6.61, P = 0.01; R2 = 0.24; Fig 3.3). In contrast, neither of the two male plumage
attributes included in these models were significant predictors of female reproductive
investment or the pair's reproductive success (all Ps > 0.10). Similarly, neither male H/L
ratios (range: 1.2 - 95.0; mean: 20.4 ± 1.9; Table Al) nor female H/L ratios (range: 1.1 104.5; mean: 25.3 ± 2.9) were significant variables in the models. When testing for
assortative mating, I found that tree swallows did not pair in relation to plumage colour (PCI;
r = -0.13, P = 0.38, n = 49), but did mate assortatively by plumage brightness (PC2; r = 0.39,
P < 0.01, n = 49; Fig 3.4).
3.5 Discussion
My results show that plumage colour of ASY females reflects age in tree swallows (at least
between TY and ATY) and that this plumage characteristic is associated with the ability to
successfully fledge young (Fig. 3.3). Females displaying bluer feathers were generally older
and, in both years of this study, fledged more young than relatively green females.
Plumage attributes are reliable signals of age in many species. Older female bluethroats
(Luscinia svecica), for example, are brighter than juveniles (Rohde et al. 1999; but see
Amundsen et al. 1997), and the same pattern has been observed in European starlings
(Sturnus vulgaris; Komdeur et al. 2005). In tree swallows, at the population level, older

males are also bluer than younger conspecifics (Chapter 2). Although, in males, the agerelated differences in plumage colour seem to be caused by lower return rates to the breeding

40

8 i

7 A
•o
„
0
6
TO
T3
q= 5
O)
C
4

§

o 3 H

1 2

AD

.<<§ AD

n
Z

1

/

A A ^ A AO ADO

OH
-

3

-

2

-

1

GD O
0

1

2

Female plumage colour (PC1)

Fig 3.3 Number of young fledged in relation to plumage colour of female tree swallows
(2003: open circles and dashed line, n = 20; 2004: open triangles and solid line, n = 32).
Lines of best fit are included to illustrate trends.

41

3 -i

CM

O

o

2 -

o ° °
o ° ° 8o

a^
(A
CO

,

0

o oo
a ) O J *o o o

C

x:
JQ

°
°cP0o ° o

0)
D)
CD

E
_3
Q.

-1 --

CO

o

o

J)

o

-2 -

3

§>

i

1

-

o

-

2

n

-

1

1—

0

1

1

2

1

3

Female plumage brightness (PC2)

Fig 3.4 Relationship between plumage brightness of male and female tree swallows in social
pairs (n = 49). Line of best fit is included to illustrate trend.

42

grounds, and therefore perhaps overwinter survival, of greener individuals, I have no data for
females that would allow me to determine if the observed differences in colour are caused by
within-individual changes, or by biases in rates of philopatry or survival. Regardless,
plumage colour of females of this species is related to age and seems indicative of their
ability to raise young. It is possible that older, more experienced birds are more efficient at
feeding and caring for their nestlings. Although there is little evidence to suggest that
nestling feeding rates are different among age groups in tree swallows (Lombardo 1991),
females for which condition was influenced by feather clipping fed their young at lower
frequencies (Winkler and Allen 1995). Furthermore, nestlings raised by clipped ASY females
fared just as well as those raised by intact ASY females. In contrast, nestlings raised by SY
females that had been clipped, did not fare as well as nestlings raised by undipped SY
females (Ardia and Clotfelter 2007). These results may indicate that older, bluer females are
better able to cope in stressful situations, or that they preferentially allocate energy to raising
their nestlings. Indeed, it is hypothesized that birds should increase their reproductive effort
as they age, since their prospects of surviving and reproducing decline (Clutton-Brock 1984;
Velando et al. 2006). Bluer females may therefore be investing more in raising their nestlings
than relatively green females.
My results also show that brighter females laid heavier eggs (Fig. 3.2). In tree
swallows, mean clutch egg mass is highly heritable (Wiggins 1990), but this measure of
reproductive investment has been found to be influenced by female age (Ardia et al. 2006;
De Steven 1978). My results may imply that older females are in better condition, or
strategically invest more in reproduction than younger females. Although I did not find agerelated differences in female plumage brightness, male tree swallows do increase in

43

brightness with age (Chapter 2). Longitudinal analyses of plumage attributes would be
required to determine the nature of age-related plumage changes in females. Nonetheless, my
results do imply that plumage brightness in females may signal individual quality.
Perhaps surprisingly, I did not detect any effects of past reproductive investment and
success on the plumage characteristics of returning breeders. Adult tree swallows begin
moulting their feathers while still on the breeding grounds (Robertson et al. 1992),
suggesting that, if plumage is reflective of condition at the time of moult, birds that raised or
fledged larger broods should bear these costs while producing feathers. As such,
experimental increase in the number of young raised by male eastern bluebirds was shown to
reduce the ability of these males to invest in plumage (Siefferman and Hill 2005c). Males
that had raised experimentally enlarged broods produced much duller plumage than males
that had raised experimentally reduced broods. However, large scale experimental studies in
tree swallows have found no evidence to suggest a cost of reproduction for adults of this
species. Survival rates of nestlings and adults in enlarged broods were no different than those
from reduced broods, and increased parental care did not influence subsequent year
investment in reproduction (Shutler et al. 2006; Murphy et al. 2000). My results therefore
concur with other studies of tree swallows concluding that there appears to be little or no cost
of reproduction for adults of this species.
In this study, I did not find any influence of male plumage attributes on female
reproductive investment, even though there are obvious advantages for females to assess
male colour displays. Quality plumage in males in some species has been found to signal the
holding potential of larger and higher quality territories (Keyser and Hill 2000), greater
nestling provisioning effort (Siefferman and Hill 2003), and good genes for offspring

44

(Sheldon et al. 1997). As such, in various species, male plumage attributes are known to
influence female investment in reproduction. For example, females mated to attractive males
lay heavier eggs (Rutstein et al. 2004; Uller et al. 2005), and may invest more antioxidants
(Williamson et al. 2006; Szigeti et al. 2007) and testosterone (Gil et al. 1999) in their yolks.
In tree swallows, older males display bluer and brighter plumage, and birds in better
condition at the time of moult also produce brighter plumage, independent of age (Chapter
2). Therefore, females could be expected to vary their investment in reproduction based on
male plumage attributes. The fact that I did not detect such trends does not imply that
females are incapable of adjusting their reproductive effort. When there is a perceived risk of
high parasitism in the nest, for example, female tree swallows have been demonstrated to
reduce their clutch size (O'Brien and Dawson 2005). In addition, females in good body
condition will skew the sex ratio of their broods towards a higher proportion of male
offspring (Whittingham and Dunn 2000). It is possible that females did not adjust their
reproductive investment in relation to male plumage attributes because of the high rate of
extra-pair paternity found in this species. Tree swallows are socially monogamous, yet
exhibit among the highest rates of extra-pair paternity identified in passerines. About 50%89% of females produce at least one extra-pair young, and at the population level, between
38%-69% of all offspring are the result of extra-pair mating (Lifjeld et al. 1993, Barber et al.
1996, O'Brien and Dawson 2007). Since more than half of the offspring contained in a brood
are often extra-pair young, investment in reproduction by females might be more dependent
on their own quality rather than that of their social mate.
In this study population, social pairs of tree swallows mated assortatively by plumage
brightness (Fig. 3.4). This type of pairing has been observed in other structurally coloured

45

species (Andersson et al. 1998; Komdeur 2005), and can theoretically occur by a variety of
behavioural mechanisms. First, social pairing can be driven by the mutual preference of
individuals with similar phenotypes along a continuum of available phenotypes (Burley
1983). This behaviour has been observed in a variety of taxa, and seems mostly attributed to
the greater compatibility of "like" individuals (e.g., morphological compatibility: Brown
1993; personality compatibility: Buston and Emlen 2003). Alternatively, assortative mating
can occur when there is mutual mate choice for similar ornaments (Johnstone et al. 1996). In
this scenario, both males and females prefer to mate with individuals that possess certain
characteristics (e.g., bright plumage). Although all individuals strive for the highest quality
mates, lower quality individuals are competitively excluded from mating with high-quality
mates. Finally, assortative mating can be driven by intra-sexual competition for nest sites,
with high-quality individuals gaining access to the best territories and pairing with higher
quality mates (Creighton 2001; Ferrer and Penteriani 2003). In tree swallows, competition for
nest sites is thought to be a major driving force behind their breeding behaviour (Robertson
et al. 1992). Males arrive on the breeding ground early in the spring and start defending
cavities several days prior to arrival of females. Even after pairing has occurred, females may
visit other males for several days and nest usurpation can occur (Leffelaar and Robertson
1985). Both sexes are known to ferociously defend their territories against conspecifics, and
this level of aggression often leaves one of the combatants dead or seriously injured
(Lombardo 1986). Indeed, it is not uncommon for dead male or female tree swallows to be
found in nest boxes (Robertson et al. 1986; pers. obs.). Therefore, it is possible that the
positive assortative mating observed in this population was driven by intra-sexual
competition for nest sites. Supporting this hypothesis, in a 25-year study of reproductive

46

performance in tree swallows, Robertson and Rendell (2001) found evidence for assortative
pairing by age. Older individuals in many species are more aggressive and win territorial
contests more often than juveniles (Landmann and Kollinsky 1995; Hyman et al. 2004). By
gaining access to good-quality territories, both males and females would find themselves
mated to individuals sharing similar phenotypic traits that honestly signal quality. It is also
possible for the pattern of assortative mating observed in this study to be driven by mutual
mate choice. Male mate choice is known to occur (Amundsen et al. 1997; Hunt et al. 1999)
and should be expected when both sexes provide parental care (Burley 1977). Studies on tree
swallows suggest that males spend much time feeding the nestlings, removing fecal sacs, and
defending the territory against intruders (Lombardo 1991; McCarty 2002). In addition, it is
not unusual to observe males of this species chasing away prospecting females in early spring
(pers. obs.). Males could be choosing to mate with females based on certain phenotypic
characteristics, such as plumage brightness. As my data do not allow me to distinguish
between the possibility that assortative mating is driven by mutual mate choice or intrasexual competition for nest sites, future studies should attempt to determine which of these
two behaviours, if not both, lead to the pattern of paring observed in this study.
This study on tree swallows is the first to show that plumage color of mature females
is related to age and has the potential of being an honest signal of individual quality. Females
reflecting light maximally at shorter wavelengths (i.e., bluer) successfully fledged more
nestlings than females with relatively green plumage, and plumage brightness was positively
associated with mean clutch egg mass. Furthermore, I presented evidence for assortative
social mate pairing according to adult plumage brightness in this species, even though a large
proportion of nestlings in a population are the result of extra-pair mating.

47

4. Plumage brightness and age predict extra-pair fertilization success of male tree
swallows
4.1 Abstract
In socially monogamous passerines, extra-pair paternity can increase the variance in male
reproductive success. If gaining extra-pair fertilizations is linked to specific secondary sexual
ornaments, the opportunity for sexual selection is enhanced. Therefore, to understand the
evolution of male phenotypic characteristics, it is important to identify traits that predict male
extra-pair mating success. Tree swallows exhibit among the highest rates of extra-pair
paternity known to occur in birds, yet it is unclear whether male extra-pair mating success is
associated with phenotypic traits that honestly advertise individual quality. I compared
morphological characteristics and plumage attributes of male tree swallows that sired extrapair offspring to those that sired only within-pair offspring in the same breeding population
to identify the characteristics that predict extra-pair fertilization success. Males that produced
extra-pair offspring had brighter plumage, and were more likely to be returning breeders,
than males that did not sire extra-pair young on the study site. However, in paired
comparisons, there was no difference between extra-pair males and the male they cuckolded.
These results suggest that female tree swallows may prefer brighter or older individuals as
extra-pair mates, but also, that older males may invest more energy in pursuing extra-pair
copulations. Furthermore, since females had extra-pair offspring in their nests regardless of
their social mate's morphometric or plumage attributes, I suggest that extra-pair mating may

be a reproductive strategy allowing females to increase the genetic diversity, while
maintaining genetic quality, of their offspring.

48

4.2 Introduction
Extra-pair paternity is common in birds, including socially monogamous species (reviewed in
Griffith et al. 2002). For females, multiple mating can enhance fertilization success or genetic
quality of offspring, circumventing the constraints associated with strict monogamy (Petrie
and Kempenaers 1998). For males, extra-pair fertilizations may increase the total number of
offspring they sire, improving their reproductive success. If all males are not equally
successful in securing extra-pair fertilizations, and phenotypic characteristics of males
predict extra-pair fertilization success, breeding outside the pair bond has the potential to
increase the variance in male reproductive success and, in turn, increase the opportunity for
sexual selection. Indeed, characteristics of males including age (Richardson and Burke 1999),
body condition (Moller et al. 2003), and plumage quality (Delhey et al. 2003; Doucet et al.
2005), have been found to influence extra-pair fertilization success in a number of species.
Moreover, males that are successful at securing extra-pair fertilizations may also lose less
paternity in their own nests (Saino et al. 1997). Consequently, variance in male reproductive
success, and thus the strength of sexual selection, may be significantly influenced by withinpair as well as extra-pair fertilizations (Webster et al. 1995). In many species, there is no
evidence for phenotypic differences between extra-pair sires and the males they cuckolded
(e.g., Charmantier et al. 2004), and in some cases, extra-pair sires are as likely to lose
paternity in their own nests as are other males in the population (Yezerinac et al. 1995).
Under these conditions, realized within-pair reproductive success of males appears to be
unrelated to their phenotypic characteristics, and differential extra-pair fertilization success
may be particularly important in determining the strength of sexual selection.

49

Tree swallows (Tachycineta bicolor) are socially monogamous passerines that exhibit
high levels of extra-pair paternity: up to 90% of broods contain extra-pair offspring, and
between 35-69% of nestlings are sired by extra-pair males (Lifjeld et al. 1993; Barber et al.
1996; Whittingham and Dunn 2001; O'Brien and Dawson 2007). The phenotypic quality of
male tree swallows does not appear to influence their within-pair fertilization success (Barber
et al. 1998), which may indicate that the benefits of extra-pair paternity for females are
independent of phenotypic characteristics of their social mate. Most previous studies of
extra-pair paternity in socially monogamous passerines have compared extra-pair males to
the resident males they cuckolded, under the assumption that females would copulate with
extra-pair males who are of higher quality relative to their social mate (but see Johannessen
et al. 2005). While there is some evidence in tree swallows that extra-pair males are in better
condition than males they cuckolded (Kempenaers et al. 2001), most studies have failed to
detect such differences (Lifjeld et al. 1993; Dunn et al. 1994; Kempenaers et al. 1999).
Nonetheless, the high variance in male reproductive success due to extra-pair paternity in this
species suggests that extra-pair fertilization success is not evenly distributed among males
(Whittingham and Dunn 2004), as would be expected if females are consistent in their
preference for extra-pair males with certain characteristics, or if different males adopt
different mating strategies. There is potential for this differential extra-pair fertilization
success to influence the strength of selection on male traits if the ability of male tree
swallows to secure extra-pair fertilizations is associated with particular phenotypic
characteristics. However, it remains unclear whether male extra-pair mating success is
associated with any morphological or behavioural characteristics in this species.

50

In this study, I examined characteristics of males that sired extra-pair offspring on the
study site in a population of tree swallows to determine which phenotypic traits of males, if
any, predict extra-pair fertilization success in this species. In addition, I performed pairwise
comparisons of extra-pair males and the males they cuckolded to determine if females chose
extra-pair mates of higher quality than their social mates. Furthermore, I investigated whether
male characteristics influenced the proportion of extra-pair young found in their nests.

4.3 Methods
4.3.1 Study area and field procedures
I studied a population of tree swallows breeding in nest boxes near Prince George BC,
Canada (53°N, 123°W). The study area consisted of open agricultural fields mixed with
patches of coniferous and deciduous forest, and many small wetlands. The site contained 125
nest boxes mounted on fence posts, and placed approximately 20-30 m apart. In 2004, the
year this study was conducted, 64 boxes were occupied by breeding pairs of tree swallows.
Beginning in May, I visited nest boxes regularly to determine clutch initiation date. Soon
after the nestlings had hatched, adults feeding their young were captured in the nest using a
swing-door trap. For each adult, I measured length of the right wing with a ruler (nearest 0.5
mm), and determined mass using a spring balance (nearest 0.25 g). I banded adults and 14day old nestlings with standard aluminium leg bands and collected blood samples, for
paternity analyses, from all individuals by puncturing the brachial vein. Blood samples were
stored in 1 ml of Queen's lysis buffer (Seutin et al. 1991) at 4°C.

51

4.3.2 Plumage characteristics
At the time of capture, I collected feather samples from the rump and mantle of all males.
After collection, feathers were stored in small opaque envelopes at room temperature, and
spectral analyses were performed as described in Chapter 2. In short, four feathers (from the
same body area and individual) were taped to a black piece of cardboard in an overlapping
pattern that simulated the way feathers are naturally arranged on birds. I assessed plumage
reflectance using an Ocean Optics USB2000 spectrometer (Dunedin, Florida, USA) with a
deuterium tungsten halogen light source (Avantes, Broomfield, Colorado, USA). I
summarized the spectral curves of each body region by quantifying measures of brightness,
hue, and chroma.
Since the rump and mantle feathers did not differ within individuals for any of the
four plumage descriptors that were measured (all P < 0.10), I used the average values of both
feather types for all analyses (Doucet 2002). Data for the four plumage characteristics were
then entered in a principal components analysis to eliminate the multiple correlations among
the measures and the interpretation of these components was used in all subsequent analyses
for this study. The first principal component, PCI, was interpreted as a measure of colour;
PC2 was interpreted as a measure of brightness (see Results).

4.3.3 Parentage Analysis
Details of the paternity analysis used in this study have been described in O'Brien and
Dawson (2007). In short, three microsatellite regions were amplified with polymerase chain
reaction (PCR) using the following primer pairs: HrU6 (Primmer et al. 1995), HrUlO
(Primmer et al. 1996) and IBI MP5-29 (Crossman 1996). PCR products were analyzed using

52

a Beckman-Coulter CEQ 8000 automated sequencer and offspring were matched with their
biological parents using the program Cervus 2.0 (Marshall et al. 1998). The exclusion
probability for all three loci (Jamieson 1994) was 0.995 with one parent known; the
probability that a randomly chosen male would share the same genotype as the extra-pair
offspring (Jeffreys et al. 1992) varied from 9.4x10"5 to 0.024 (mean ± SD: 0.0033 ± 0.0050, n
= 32).

4.3.4 Statistical analysis
Male tree swallows, unlike females, cannot be aged according to plumage characteristics.
However, since adult male tree swallows have very high nest site fidelity (Robertson et al.
1992) with fewer than 5% of all breeding males changing breeding sites throughout their
lifetime (Winkler et al. 2004), males breeding on the study area for the first time are probably
relatively young individuals in comparison to those males banded as adults in previous years.
As a surrogate for age, I included capture status (relatively young or "newly captured" versus
relatively old or "recaptured") in all analyses.
I used an analysis of covariance models to investigate whether phenotypic
characteristics of males predicted the amount of paternity lost in their own nests (dependent
variable: percentage of young in nest sired by extra-pair males). I included male plumage
colour (PCI), plumage brightness (PC2), capture status, mass, body size (estimated by wing
length), clutch initiation date, and all first-order interactions as independent variables. Since
the mass of individual males decreases over the breeding period, I calculated the residuals
from a regression of mass on Julian date of capture (Fig 4.1). This measure was used instead
of a body condition index (such as mass corrected for size) because previous

53

22 "

21

V)
(A

20

o
ooo

o
o
oooo
ooo
" - S k O O Oo o

CO

T3-vO
OO
^ - \
O O O
\ s

E
o>
15 19

o 0 0 oO

oO

o
^o

O

o
o oo

18

17

o
1

160

170

180

190

Julian date of capture

Figure 4.1 Mass of male tree swallows in relation to the Julian date of capture (n = 41). Line
of best fit is included to illustrate trend.

54

research on tree swallows suggests that body mass can be important in extra-pair mate choice
(Kempenaers et al. 2001). I used a backward stepwise procedure to eliminate non-significant
interactions and variables, keeping only significant terms in the final models.
To determine which male characteristic(s) best predicted extra-pair fertilization
success within the population (dependent variable: 0 = did not sire extra-pair young, 1 = sired
extra-pair young), I developed a logistic regression model that contained capture status,
plumage colour (PCI), plumage brightness (PC2), corrected mass, body size (wing length),
and clutch initiation date. I used a backward stepwise procedure to eliminate non-significant
variables, keeping only significant terms in the final models.
To determine whether females were fertilized by extra-pair males that were
phenotypically different than their social mates, I used paired t-tests comparing the
morphological and plumage characteristics of males that sired extra-pair young to the
characteristics of males they cuckolded. When two or more extra-pair males were found to
have sired nestlings in the same brood (n = 5 nests), I used the average value for each
characteristic compared. To compare age of extra-pair males and those they cuckolded, I
performed a McNemar change test corrected for continuity (Siegel and Castellan 1988).
Analyses were performed using SPSS (Norusis 2000), and data are presented as mean
± 1 SE unless otherwise indicated. All tests were two-tailed, and results were considered
significant at the 0.05 level.

55

4.4 Results
4.4.1 Patterns of paternity and male reproductive success
Incidence of extra-pair paternity in this study population has been reported previously in
O'Brien and Dawson (2007), and was similar to other estimates for this species. Briefly, of
the 40 nests included in parentage analysis, 85% (34) contained offspring of mixed paternity.
The average proportion of extra-pair offspring in these nests was 0.43 ± 0.04; no nests
contained exclusively extra-pair young. In total, 35% (76/216) of all nestlings genotyped
were extra-pair offspring (O'Brien and Dawson 2007). The biological fathers of 42% (32/76)
of extra-pair young were identified. Of the 34 nests containing offspring of mixed paternity,
the biological father of at least one extra-pair offspring was identified in 41% (14 nests); in
nine of these nests, all extra-pair young were sired by a single male. Two extra-pair sires
were identified in each of three broods, and three extra-pair sires were identified in two
broods. The biological father(s) of all nestlings in 32% (11) of broods with mixed paternity
were identified. Nineteen males were identified as the father of at least one extra-pair
offspring, and two of these males sired extra-pair young in two nests.
Obtaining extra-pair fertilizations did not influence the proportion of paternity that
extra-pair males (EP males) lost in their own nests (proportion of extra-pair offspring in nests
of EP males: 0.41 ± 0.08 vs. 0.32 ± 0.05 in nests of males that did not sire extra-pair
offspring on the study site (WP males); independent samples t-test, t3i = 0.97, P = 0.34).
Consequently, within-pair reproductive success of EP males (3.1 ± 0.5 young fledged) did
not differ from that of WP males (3.5 ± 0.3 young; t29 = 0.73, P = 0.47). Siring extra-pair
offspring allowed EP males to improve their total reproductive success (4.8 ± 0.5 young
fledged) relative to WP males (independent samples t-test, t29 = 2.24, P = 0.03; Fig. 4.2).

56

12
*

5

•o

0

c

14

19

.Q

E 3H
z

Did not produce
extra-pair young

Produced
extra-pair young

Figure 4.2 Number of young fledged by males that obtained extra-pair paternity and males
that did not obtain extra-pair paternity on the study site in the same breeding population of
tree swallows (mean ± SE). Sample sizes above error bars indicate number of males.

57

4.4.2 Plumage characteristics
Principal components analysis of plumage characteristics showed that the first component,
PCI, explained 53.4% of the variation and was heavily weighted by hue, blue chroma, and
UV chroma, but very little by brightness (rotated matrix values: -0.97, 0.60, 0.98, and -0.15,
respectively); PCI therefore represented plumage colour, with higher scores representing
bluer birds. The second component, PC2, explained 24.6% of the variation and generally
represented differences in brightness among individuals (rotated matrix values: -0.24, -0.30,
0.16 and -0.98 for hue, UV chroma, blue chroma and brightness, respectively); PC2 was
interpreted as a measure of brightness.

4.4.3 Extra-pair paternity and male characteristics
The proportion of extra-pair young in a nest was not influenced by clutch initiation date
(range: May 19th - June 8th; mean May 28th ± 0.7 days), age, corrected mass, body size (wing
length, range: 115 mm - 129 mm; mean 121.3 mm ± 0.4 mm), plumage colour (PCI), or
plumage brightness (PC2) of the attending male (Table 4.1).
When testing for factors that influenced male extra-pair fertilization success, I found
that both brightness (PC2; Wald = 7.67, df = 1, P < 0.01, n = 41; Fig 4.3) and capture status
were valid predictors (Wald = 5.99, df = 1, P = 0.02, n = 41; Fig 4.4). According to this
model, which correctly classified 85.5% (34/41) of males, older (recaptured) males, as well
as those with brighter plumage, were more likely to sire extra-pair offspring.
When performing pairwise comparisons, I did not find any differences between extrapair males and the males they cuckolded (mean difference plumage colour PCI: 0.15 ± 0.36,
ti3 = 0.41, P = 0.69; plumage brightness PC2: -0.53 ± 0.36, tn = -1.48, P = 0.16; wing length:

58

Table 4.1 Analysis of covariance model investigating the influence of characteristics of male
tree swallows on the proportion of extra-pair offspring found within their nests (n = 31
broods)

Independent variables

F

P

Colour (PCI)
L981
(U7
Brightness (PC2)
0.106
0.75
Clutch initiation date3 0.373
0.55
Size (wing length)
0.082
0.78
Body mass
0.002
0.97
Capture status
0.186
0.67
a
Clutch initiation date was included in the
analyses to account for the possibility that
females could engage in extra-pair copulations
at different rates throughout the breeding
season.

59

300

400

500

600

700

Wavelength (nm)
Figure 4.3 Spectral curves, averaging rump and mantle feathers, of male tree swallows that
did not sire extra-pair young on the study site (WP; n = 24) and males that did sire extrapair young (EP; n = 17). Error bars, incorporated periodically throughout the curve to show
the degree of variance over the whole spectrum, represent one standard error.

60

3-i

12
CM

O

£L^

2A

JL

20
O

(0

</>
<D

WP Males
EP Males

5

c

J=

g>

.

'i_

.a
a)

-"-

4

^F

D)
(0

E -1 H
Q.

J)
as

-2

O

Newly captured

Recaptured

Figure 4.4 Male extra-pair fertilization success (WP = did not sire extra-pair young on the
study site, EP = sired extra-pair young) in relation to plumage brightness (PC2) and capture
status. Boxes show median (50th percentile), and interquartile range (25th to 75th
percentile); whiskers indicate 95% confidence intervals. Open circles are data points
outside the 95% CI range and sample sizes are shown above each box.

61

0.05 ± 0.73mm, t13 = 0.07, P = 0.95; corrected mass: 0.41 ± 0.31, ti 3 = 1.33, P = 0.21). In
addition, the capture status of extra-pair males did not differ from the males they cuckolded
(McNemar change test, °£\ = 0.90, P = 0.34), and nests in which they sired extra-pair young
were not initiated earlier than their own nests (0.56 ± 1.44 days, to = 0.39, P = 0.70).

4.5 Discussion
My results show that male tree swallows are not equally successful at breeding outside the
pair bond. When investigating which characteristic(s) best predicted extra-pair fertilization
success, my results showed that brighter males had an advantage over dull males, and that
older individuals were more likely than younger ones to secure extra-pair fertilizations (Fig.
4.4). With both characteristics included in the same model, 35 of 41 individuals were
correctly classified as WP or EP males. Although plumage brightness increases with age in
male tree swallows, there is considerable overlap in brightness between newly captured and
recaptured birds in this population (Chapter 2), and this characteristic alone cannot be used to
accurately assign individuals to one of these two groups. It is possible that some of the bright
birds that were included in the newly captured class, yet obtained extra-pair fertilizations,
were in fact older birds that bred elsewhere in previous years, thus increasing the apparent
importance of brightness in the models. This interpretation seems unlikely because it has
been estimated that fewer than 5% of all breeding males change breeding sites throughout
their lifetime (Winkler et al. 2004). Therefore, my results imply that plumage brightness and
age are independent predictors of male extra-pair fertilization success, and hint at two
possible complimentary aspects of the breeding behaviour of tree swallows. First, it is
probable that females prefer bright males, rather than dull males, as extra-pair mates.

62

Structurally based plumage ornaments have been found to be condition-dependent (e.g.,
Keyser and Hill 1999a), reflective of nutritional stress (McGraw et al. 2002; Siefferman and
Hill 2005b) and parasite load (Hill et al. 2005), and thus honestly reflect individual quality
(Kodric-Brown and Brown 1984). By choosing individuals with bright plumage, females
would be mating with high-quality individuals, potentially gaining good genes for their
offspring. Indeed, females in many species are known to assess male quality, and choose a
mate, based on plumage signals (reviewed in Hill 2006b). Second, it is possible that older
males invest more time and energy in courting females other than their social mate and/or
that they are better at competing for access to receptive females. In fact, one of the most
prominent correlative patterns in studies of extra-pair paternity in birds is the finding that
extra-pair sires are older, experienced males (Griffith et al. 2002). When compared to
yearlings, older individuals in some species often spend more time displaying to fertile
females and intruding on other males' territories (Karubian 2002; Kleven et al. 2006), thus
increasing their opportunity for extra-pair copulations. In addition, it has been argued that
even when patterns of refusal or acceptance of extra-pair copulations by females have been
linked to certain male phenotypes, the role of female choice is uncertain (Westneat 1994).
For instance, females may be receptive to persistent males because the cost of resistance is
too high. The benefits of consenting to such copulation attempts could equate with the
benefits of seeking exclusively high quality extra-pair mates, making it difficult to
differentiate between these two nonexclusive behaviours (Dickinson 2001). Unfortunately,
even in well-studied species such as tree swallows, very little is known about age-related
courtship behaviour of males, limiting the inferences that can be made from this particular
result. Future documentation of courtship displays of males of known age (i.e., banded as

63

nestlings) would contribute substantially to our understanding of extra-pair mating in this
species. Nonetheless, the finding that brighter males and older males are more successful at
obtaining extra-pair fertilizations is important, since it explains, in part, the influence of male
characteristics on the realized reproductive success of individuals.
It is important to consider that there may be error in the classification of individual
males as being either successful or unsuccessful at siring extra-pair young. While the group
composed of successful extra-pair males is well defined, the biological fathers of only 42%
of extra-pair nestlings were identified. It is possible, then, that males who were unsuccessful
at siring extra-pair offspring in this population may have in fact mated outside the pair bond
with females nesting in areas other than the study site. However, if females are consistent in
their mating preferences, and if male behaviour is a main factor in determining extra-pair
fertilization success, which seems to be the case, then this classification should be accurate.
If anything, the occurrence of female mating preferences and differences in male courtship
behaviour could increase the discrepancy between apparent and realized reproductive success
of males, since brighter and older males would most likely produce additional undetected
extra-pair young outside of the study area.
Since the actual reproductive success of a male is determined by the number of young
sired in his own nest and the number of young sired in other nests, individuals could improve
their success by preventing the loss of paternity in their own nest, in addition to maximising
their chances of obtaining extra-pair fertilizations. However, observations of male tree
swallows have led to the conclusion that males do not spend much time guarding their social
mate during the fertile period, although they might try to reduce their loss of paternity by
copulating frequently (Venier and Robertson 1991; Beasley 1996). This behaviour might not

64

be sufficient to assure paternity, as male removal experiments have failed to find differences
in the proportion of extra-pair young between experimental and control nests (Barber et al.
1998). Whether or not females have extra-pair young in their nests might therefore be an
integral component of a female's reproductive strategy, irrespective of the behaviour of their
social mate. Indeed, female tree swallows are known to actively fend off courting males and
seem to have control over whom they mate with (Lifjeld and Robertson 1992).
The factors that influence female extra-pair mating behaviour is a controversial topic
and has been the subject of many reviews (e.g., Ligon 1999; Griffith et al. 2002; Westneat
and Stewart 2003). In the population of tree swallows I studied, the proportion of extra-pair
offspring in a brood was not related to any phenotypic characteristics of the female's social
mate. This finding suggests that the benefits of extra-pair paternity for female tree swallows
are not necessarily dependent on the quality of their social mate. It is unlikely that females in
this population mated outside the pair bond to insure against male infertility, as none of the
broods in the study contained exclusively extra-pair young (but see Kempenaers et al. 2001;
Whittingham and Dunn 2001). Rather, this behaviour is consistent with the idea that females
may seek extra-pair mates to increase the genetic diversity of their offspring (Williams
1975). Such a strategy would be especially beneficial in unpredictable environments, or when
the quality of potential mates is difficult to assess, and would increase the probability that at
least some offspring would be successful (Westneat et al. 1990). Supporting this hypothesis,
the benefits of extra-pair mating in this population have been shown to manifest themselves
only under certain conditions (O'Brien and Dawson 2007).
In studies of extra-pair mating strategies, female choice is usually considered to have
occurred when extra-pair sires are shown to be of higher quality than the social mate, based

65

on the expression of traits that honestly reflect individual quality (Andersson 1994). The
assumption is that females compare their social mates to potential partners and choose higher
quality individuals to gain genetic benefits for their offspring (Jennions and Petrie 2000). In
studies of tree swallows, pairwise comparisons between extra-pair sires and the males they
cuckold have produced mixed results. While Kempenaers et al. (1999; 2001) found that
extra-pair males are in better condition or produce more sperm than the males they
cuckolded, other studies have found little similar evidence (e.g., Dunn et al. 1994).
Furthermore, it is not certain that these two characteristics are easily gauged by females and,
therefore, represent reliable indicators of genetic quality. In this study, I did not detect any
differences between extra-pair males and the social mate they cuckolded, further supporting
the idea that the benefits of extra-pair paternity for female tree swallows are not necessarily
dependent on the quality of their social mate. Indeed, some of the females in this population
had extra-pair offspring in their nests even though their social mates were very bright, and
their extra-pair mate(s) were therefore duller by comparison (albeit brighter than average).
Instead of "trading up", females could simply be actively pursuing extra-pair mating
opportunities with males of high absolute genetic quality.
While foraging, female tree swallows can travel up to 10 km from the nest site during
their fertile period, and may find extra-pair mates at these distant locations (Dunn and
Whittingham 2005). Floater males that are difficult to capture are also known to sire extrapair offspring in this species (Barber and Robertson 1999). For these reasons, it is difficult to
accurately assess the fertilization success for each male in a breeding population, and this
limits our ability to estimate variance in male realized reproductive success. The best
estimates produced for tree swallows so far were obtained by Kempenaers et al. (2001), who

66

showed that 38% of the variance in reproductive success for their study population was due
to the production of extra-pair young. This estimate was slightly greater than the variance
due to the production of within-pair young. Although not all sires of extra-pair offspring
were identified, my results still suggest that males that sire extra-pair offspring fledge more
young than males siring only within-pair offspring (Fig. 4.2), supporting the idea that extrapair paternity has the potential to increase the opportunity for sexual selection in this species
(Webster et al. 1995; Whittingham and Dunn 2005; Kleven et al. 2006). Moreover, my
finding that plumage brightness predicts male extra-pair fertilization success suggests that
this may be a sexually selected trait in tree swallows.
Overall, my results suggest that female tree swallows do not evaluate their social
mate when engaging in extra-pair copulations. This conclusion is supported by the findings
that 1) the proportion of extra-pair paternity in females' nests was not influenced by any
measured characteristics of their social mates and 2) I did not detect any differences between
EP males and the males they cuckolded. Instead, my results suggest that extra-pair mating
behaviour in tree swallows may be a combination of age-dependent mating investment by
males and female mating preference based on plumage quality. Future experimental work
should be directed toward determining the relative importance of female and male behaviour
in extra-pair mating. Such experiments would improve our understanding of extra-pair
mating behaviour of tree swallows and inter-sexual conflicts in socially monogamous mating
systems.

67

5. General discussion
Birds arguably possess some of the most extravagant and conspicuous secondary sexual
ornaments in the animal kingdom. In males, the evolution of elaborate traits is mainly
thought to be driven by female choice for characteristics that would honestly reflect
individual quality (Andersson 1994). By choosing mates of high quality, females would be
gaining either direct benefits, such as a territory with abundant food resources, or indirect
benefits, such as good genes for their offspring. In females, the display of brightly coloured
plumage was, for a long time, mainly considered to be genetically linked to male
characteristics (Amundsen 2000). However, in the last decade, the signalling value of female
plumage attributes has been explored and found to indicate individual quality, and to
influence male mating strategies (Amundsen et al. 1997). The objective of this research was
to examine the signalling potential of plumage colouration in tree swallows (Tachycineta
bicolor), a structurally coloured, iridescent species. Furthermore, I investigated the
relationship between plumage attributes of males and females, and reproductive investment,
reproductive success, within-pair mating strategy, and extra-pair mating behaviour.

5.1 Plumage attributes and individual quality
In Chapters 2 and 3,1 showed that plumage attributes of male and female tree swallows are
related to age, and reflect individual quality. In males, both plumage colour and plumage
brightness were associated with age and, in addition, individuals that invested more in feather
growth, independently of age, produced brighter plumage. In females, plumage colour was

68

associated with age and the ability to fledge young, while plumage brightness was correlated
with mean clutch egg mass.
It is difficult to determine why female plumage brightness was not related to age
since this plumage attribute was correlated with mean clutch egg mass, a measure of
reproductive investment normally associated with age in tree swallows (Ardia et al. 2006; De
Steven 1978). It is possible that plumage brightness in females also correlates with an index
of health or condition at the time of moult that I did not measure. If this is the case, highquality females, even relatively young ones, may produce bright, quality feathers, and lay
relatively heavy eggs. Both egg mass and egg composition have been associated with insect
abundance just prior to laying (Ardia et al. 2006), suggesting that foraging skills may be of
great importance to the production of large eggs. Furthermore, individual quality in tree
swallows has been found to correlate with flight speed (Bowler and Winkler 2004), an ability
linked to wing loading (Norberg 1995). In this study, I showed that males that possessed
longer feathers in relation to their body size also produced brighter plumage. It is not unlikely
then, that plumage brightness in females may be, in some way, related to their ability to
forage. Future studies on female plumage characteristics should investigate the relationship
between these two attributes.
It is also possible that I did not detect differences in female plumage brightness with
age because there is less selective pressure for females, compared to males, to develop bright
plumage. Plumage brightness of males was found to influence female extra-pair mating
strategy (Chapter 4), suggesting that this trait may be sexually selected in males. Since extrapair fertilization success can be a large component of realized reproductive success
(Kempenaers et al. 2001), males might be expected to invest as much as possible in bright

69

plumage. In contrast, females may not need to display bright plumage to mate with quality
individuals even though I found evidence for assortative mating based on plumage brightness
in this population. As argued in Chapter 3, it is possible that assortative mating in this
population arose through male and female intra-sexual competition for nest sites and/or
mutual mate choice. In many other species, it has been argued that female plumage attributes
are the result of male genetic effects, and would therefore not be adaptive or under the
influence of selective pressures (Amundsen 2000). However, these arguments do not hold for
tree swallows. Females in their first year of breeding display dull brown feathers on their
dorsal surface, strongly implying that females could maintain their non-conspicuous plumage
throughout their life if the costs of producing bright feathers outweighed the benefits of
displaying iridescent plumage. The main hypothesis formulated to explain delayed plumage
maturation in female tree swallows proposes that colour of older females signals their
competitive advantage over dull brown second-year females in competitive interactions for
nest sites (Robertson et al. 1992). Regardless, it is possible that plumage characteristics in
mature females are involved in other intra-or inter-sexual interactions. Future studies on
female plumage attributes should consider manipulating feather colour or brightness to
investigate the influence of these traits on behavioural interactions between conspecifics.
Manipulating colour and brightness should also be conducted on males since the
signalling value of plumage characteristics is somewhat uncertain. Although the results
presented in Chapter 2 show that both of these plumage attributes have the potential of being
honest indicators of quality in tree swallows, the respective role of colour and brightness as
inter- or intra-sexual signals should be investigated in more depth. In Chapter 2,1 showed
that plumage brightness was age-related in males, and that these differences were most likely

70

due to within-individual changes. Older individuals, perhaps because they are better foragers,
would be in better condition at the time of moult. In addition, since plumage brightness was
correlated among individuals between the first capture and subsequent recapture, this
plumage attribute may also signal inherent qualities of the individual. Supporting these ideas,
plumage brightness was shown in Chapter 4 to influence female mate choice, independently
of age. There are potential benefits for females that assess male plumage attributes (Hill
2006b) because in this population, extra-pair young perform better while still in the nest
under certain conditions (O'Brien and Dawson 2007). Therefore, brighter individuals may be
of greater quality and pass on good genes to their offspring.
The signalling content and the role of differences in plumage colour is more
uncertain. While older individuals were bluer than relatively younger ones, which could be
indicative of quality, it is difficult to comprehend how colour could be an honest signal of
condition since this plumage attribute seems mostly inherent to the individual. Indeed,
individual colour was strongly correlated between the first year of capture as an adult and the
subsequent year of capture (Chapter 2). Since this characteristic would not be influenced by
conditions that normally drive the trade-offs between producing elaborate secondary sexual
ornaments and investing in other costly metabolic pathways, plumage colour should not
honestly reflect individual quality. Furthermore, plumage hue and ultraviolet chroma, which
are both highly correlated and contributed the most to PCI in all analyses, were different in
2002 and 2003 (Table 2.1). Yet, plumage colour was the only significant predictor of the
probability of returning from one year to the next, suggesting that this attribute may be
related to the ability of individuals to survive overwinter. In certain species, specific morphs
are over-represented in the adult population when compared to the frequency of the morphs

71

found in juveniles, and these discrepancies may be attributed to differential survival (HaagWackernagell et al. 2006). If certain phenotypic traits are associated with particular
behaviours (Roulin 2004), plumage attributes may be indirectly associated with survival
(Brommer et al. 2005). Since optimal behavioural strategies will be influenced by
environmental conditions (Roulin et al. 2004), stochastic fluctuations in the environment may
lead to differential survival of individuals bearing certain attributes in different years. How
colour-biased survival may function in a species displaying continuous variation in plumage
characteristics is unknown, and needs to be investigated.

5.2 Plumage attributes in relation to past and current reproductive investment
Life-history theory suggests that organisms have limited time and energy, so that investment
in one activity or trait will necessarily lead to a decrease in others (Williams 1966). Such
trade-offs are often observed in wild animal populations and have the potential to influence
individual reproductive strategies. For example, the cost of current reproductive efforts may
limit the potential for future investment in reproduction (Partridge and Harvey 1985). If the
production of elaborate plumage traits is costly, allocation of energy to reproductive efforts
may also reduce the ability of individuals to invest in quality ornaments (Kokko 1998). The
limitation to investment in feathers may be more noticeable in species of birds for which
moult takes place just after the fledging of young. A trade-off between reproductive effort
and future ornamentation may be particularly costly if social pairs mate assortatively by
quality, and use elaborate ornaments as mate choice criteria. Investing heavily in
reproduction one year, for example, could force individuals to produce poor-quality plumage,

72

which in turn, may make it difficult to find a quality mate the subsequent year (Siefferman
and Hill 2005c).
In Chapter 3,1 presented results lending support to the idea that there is no cost of
reproduction to the development of quality plumage for either male or female tree swallows.
Neither plumage brightness or plumage colour of either sex were influenced by primary
investment in reproduction or by the number of nestlings fledged. Although perhaps
surprising because tree swallows will start moulting while still raising young (Robertson et
al. 1992), these results are in accordance with other studies of tree swallows concluding that
there appears to be little or no cost of reproduction to these birds (Shutler et al. 2006; Murphy
et al. 2000). Although the median clutch size for this species is six eggs, tree swallows are
capable of raising much larger broods (Robertson et al. 1992). It is possible then that these
birds lay clutches that would not impose a cost to self maintenance or future reproduction. To
test this hypothesis more stringently, it would be necessary to manipulate brood sizes to the
extremes of natural variation and examine the influence of enlarged and reduced broods on
various health parameters and on ornamental plumage. Such experimental protocol has been
shown to influence subsequent investment in plumage by male eastern bluebirds (Sialia
sialis; Siefferman and Hill 2005c).
Because failing to produce quality plumage may be costly in terms of finding a
quality mate during the subsequent breeding attempt, male and female tree swallows could
also be prioritizing the allocation of energy to the production of feathers, rather than to other
metabolic processes. Trade-offs between allocating colourful carotenoid pigments to
secondary sexual traits or the immune system, for example, have been found in a number of
species (e.g., Peters et al. 2004), and experimental studies have shown a relationship between

73

structural plumage characteristics and infection by parasites (Doucet and Montgomerie
2003b). However, these trade-offs may not be detectable after birds have overwintered, and
could explain why I did not find a cost of reproduction to plumage attributes. Perhaps
measuring health parameters of breeding adults just after moult could identify consequences,
at least in the short term, associated with producing quality plumage.
While there was no cost of reproduction with respect to the development of quality
feathers, female plumage attributes did reflect the potential for investment in reproduction
and fledging success. Plumage brightness was correlated with mean egg mass, and the
number of nestlings fledged from a nest was strongly related to the colour of the attending
females in both years of study (Fig 3.3). Since plumage colour is age-related in female tree
swallows (Fig 3.1), these results, at first, may seem to directly imply that older females are
better at raising young than relatively young females but this inference may be too
conservative. When testing for a relationship between plumage attributes and fledging
success, female age was included in the model. If age by itself was the most important factor
in determining the number of nestling fledged by females, this variable would have been a
significant predictor of reproductive success; that was not the case. This finding suggests that
plumage colour, in part independently of age, is an honest indicator of female quality and
reflects the ability to care for young. Future studies on female plumage colour in tree
swallows should investigate individual characteristics normally associated with reproductive
success, such as incubation and feeding rates, to understand how bluer females differ from
greener ones.
In many species, female investment in reproduction has been shown to be influenced
by male characteristics (e.g., Rutstein et al. 2004). By increasing their investment when

74

mated to attractive and high-quality males, females would maximize their chance of
successfully raising offspring that potentially carry good genes (Maynard Smith 1980). In
this study, I did not find any evidence of male characteristics influencing female reproductive
effort (Chapter 3). The most likely explanation is that, since most females readily engage in
extra-pair copulations, investment in reproduction is more a function of female quality rather
than social mate quality. It would be interesting to determine if investment in specific
nestlings is influenced by the relative attractiveness of the male that fertilized the embryo.
Such a study would be quite demanding, as it would require the matching of nestlings to their
respective eggs.

5.3 Plumage attributes, social pairing, and extra-pair mating strategies
Inter- and intra-sexual competition for access to quality mates (i.e., sexual selection) is,
without doubt, one of the strongest evolutionary forces. It is responsible for the development
of weapons used by males to fight one another for access to females, the evolution of
elaborate secondary sexual ornaments, and complex mating rituals and behaviours. Mate
choice, for example, can have direct fitness consequences; individuals paired with poorquality mates may produce fewer offspring than individuals paired with good-quality mates.
Yet, the underlying mechanisms behind social pairing are not always well understood.
In Chapter 3,1 presented results showing that tree swallows pair assortatively by
plumage brightness, a trait associated with individual quality in both males and females.
These results are in accordance with work done by Robertson and Rendell (2001) who found
assortative mating by age in a long-term study of their population. As argued earlier, this
type of social pairing is most likely caused by intra-sexual competition for nest sites, mutual

75

mate choice, or a combination of both. High-quality males are usually the first to arrive on
the breeding ground in a number of species, including tree swallows (e.g., Lozano 1994), and
can take possession of the best nest sites (Siefferman and Hill 2005d). Females, which arrive
later, are known to visit many potential nest cavities before choosing or settling for a specific
site. Female-female competition for specific nest boxes is fierce, often leading to injury or
death of some birds (Lombardo 1986). Such intra-specific competition could lead to the
pairing of individuals of similar quality, which would be reflected by their plumage
attributes. Females, in addition, may be choosing sites based on the quality of the attending
males. Concurrently, choosy males would chase away unwanted females, presumably
identifying quality individuals by specific plumage attributes. In this scenario, mutual mate
choice for plumage brightness would be driving the observed pattern of assortative mating.
Manipulative experiments could determine whether intra-specific competition for nest sites,
mutual mate choice, or both are responsible for social pairing in this species.
The possibility that females choose high-quality males as mates was further supported
by the finding, in Chapter 4, that males bearing bright plumage have greater probability of
obtaining extra-pair fertilizations. Although it is possible that brighter males are more
persistent in their attempts to obtain extra-pair copulations, it would not be surprising if
females preferentially mated with males displaying an honest signal of quality. By mating
with high-quality individuals, females would potentially obtain good genes for their offspring
(O'Brien and Dawson 2007). When engaging in extra-pair copulations, females did not seem
to consider the quality of their social mates. The proportion of extra-pair young in a nest was
not influenced by the social mate's phenotypic characteristics, and males that were cuckolded
were no different than the female's extra-pair mate(s) in pairwise comparisons. Therefore,

76

females that were mated to attractive males still engaged in extra-pair copulations. This
behaviour may be explained by the genetic diversity hypothesis, which states that females
will try to maximize the genetic variation of their offspring, a potential benefit in
unpredictable environments (Williams 1975).

5.4 Conclusions
With this research, I demonstrated that aspects of the iridescent dorsal plumage of male and
female tree swallows signal individual quality and age; older birds display brighter, bluer
feathers. In females, these displays can provide information about their potential to invest in
reproduction, and their ability to raise young. Furthermore, these secondary sexual traits
influence inter-sexual interactions. As such, males that display brighter plumage have greater
reproductive success and may themselves select bright females as mates.
My results are important because it is the first time that the signalling value of tree
swallow plumage characteristics has been investigated. In addition to enhancing our
knowledge about structural plumage signals in general, this work opens the door to a greater
understanding of social interactions based on plumage signals in this species.

77

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Appendix I
Table Al. Summary of leukocyte statistics for adult tree swallows captured in 2003 and
2004.

Leukocytes
Males (n = 86) .
Heterophils (%)
Eosinophils (%)
Basophils (%)
Lymphocytes (%)
Heterophil to lymphocyte ratio
Females (n = 57)
Heterophils (%)
Eosinophils (%)
Basophils (%)
Lymphocytes (%)
Heterophil to lymphocyte ratio

Minimum

Maximum

Mean

SD

1.0
0.0
0.0
27.4
1.2

36.2
53.8
36.7
96.2
95.0

11.0
12.3
12.6
64.0
20.4

7.5
8.1
7.8
15.0
18.1

0.9
0.0
0.0
25.2
1.1

43.8
54.3
37.0
95.2
104.5

12.4
15.8
13.7
58.1
25.3

9.0
10.9
9.3
15.7
22.5

91