THE EFFECTS OF HABITAT DISTURBANCE ON THE REPRODUCTIVE BEHAVIOUR OF THE BLACKrCAPPED CHICKADEE (POEC/lEATR/CAf/LLA) by Kevin T. Fort B.A. York University, 1992 M.A. University of British Columbia, 1996 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in NATURAL RESOURCES AND ENVIRONMENTAL STUDIES © Kevin T. Fort, 2002 THE UNIVERSITY OF NORTHERN BRITISH COLUMBIA December 2002 All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. 1*1 National Library of Canada Bibliothèque nationale du Canada Acquisitions and Bibliographic Services Acquisitions et services bibliographiques 395 Wellington street Ottawa ON K1A0N4 Canada 395. rue Wellington Ottawa ON K1A0N4 Canada YourfUe Votre référence OurSte Notre The author has granted a non­ exclusive licence allowing the National Library o f Canada to reproduce, loan, distribute or sell copies o f this thesis in microform, paper or electronic formats. L’auteur a accordé une hcence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la forme de microfiche/fUm, de reproduction sur papier ou sur format électronique. The author retains ownership o f the copyright in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author’s permission. L’auteur conserve la propriété du droit d’auteur qui protège cette thèse. N i la thèse ni des extraits substantiels de ceUe-ci ne doivent être imprimés ou autrement reproduits sans son autorisation. 0-612-80673-1 Canada APPROVAL Name: Kevin T. Fort Degree: Master of Science Thesis Title: THE EFFECTS OF HABITAT DISTURBANCE ON THE REPRODUCTIVE BEHAVIOUR OF THE BLACK-CAPPED CHICKADEE fPOECZLE ATR/CAf7LLA) Examining Committee: Chair: Dr. Robert W. Tait Dean of Graduate Studies UNBC Supervisor: Dr. Kenneth Otter Assistant Professor, Biology Program UNBC Committee Member: Dr. Russ Dawson Assistant Professor, Biology Program UNBC Committee Member: Dr. B. Staffan Lindgren Associate Professor, Forestry Program UNBC Committee Member: Dr. Roger Wheate Associate Professor, Geography Program UNBC » External Examkfer: Dr. Coll'een Caésady St. Clair Assistant Professor, Behavioural Ecology Department of Biological Sciences University of Alberta Date Approved: Abstract This thesis examines the effect of habitat disturbance on reproductive behaviour in the black-capped chickadee (fo ecik afncapifZa), a resident cavity-nesting songbird known to breed disturbed habitats. I investigated whether reproductive success was lower in disturbed habitats, how habitat quality affected the intensity of territorial behaviour, and the extent to which chickadees exhibited consistent preferences for habitat types associated with increased reproductive success. Nest success was lower in the disturbed habitat than in the undisturbed habitat. Abandonment was the most common cause of nest failure. A within-habitat comparison of the social rank of birds revealed that low ranking birds had lower nest success than high ranking birds in the disturbed, but not the undisturbed, habitat. Breeding pairs occupying the disturbed site were subject to higher amounts of territorial overlap than pairs in the undisturbed mature woodlands. Birds in disturbed habitat had larger territories, intruded more often into neighbouring territories than those in undisturbed habitat, and their intrusions were more extensive. There was no evidence that chickadees preferred or avoided specific habitat types in my study area. However, birds breeding in territories containing high proportions of disturbed habitat experienced lower reproductive success. Thus, birds breeding in disturbed habitat may be altering their reproductive strategies to compensate for poor habitat quality. Nevertheless, evidence for maladaptive habitat selection and differential reproductive success suggest that disturbed habitats may be functioning as population sinks. Table of Contents Abstract................................ ii Table of Contents............................................................... :............................................iii List of Tables.... .................................................... iv List of Figures.................................................................................................................... v Acknowledgement............................................................................................................vi 1. General Introduction......................................................................... 1 1 1.1. Behavioural Ecology and Conservation Biology................. 1.2. Reproductive Decisions, Territoriality, and Habitat Quality..........................3 1.3. Sources, Sinks, and Habitat Disturbance............................ .....5 1.4. Limits of Traditional Habitat Sensitivity Protocols and Assumptions 7 1.5. Study Site............................................................................................................. 9 1.6. Study Species............................................................................ 10 1.7. Thesis Outline................................................................................................... 12 2.Area sensitivity in an "area-insensitive" songbird: the impact of habitat disturbance on reproduction of chickadees................................................................ 15 2.1. Abstract.............................................................................................................. 15 2.2. Introduction................. 16 2.3. Methodology......................................................................................................18 2.4. R esults...............................................................................................................25 2.5. D iscussion...................................................................................................... ..29 3.Territorial breakdown of black-capped chickadees (Poecile atricapilla) in 41 disturbed habitats.......................................................................... 3.1. Abstract..............................................................................................................41 3.2. Introduction ............ 42 45 3.3. Methodology............................... 3.4. Results...............................................................................................................52 3.5. D iscussion.............................................................................................. 55 4.Lack of habitat preference may prove maladaptive in disturbed habitats.............. 64 4.1. Abstract..............................................................................................................64 4.2. Introduction..................................... 64 4.3. Methodology.................................... 67 4.4. R esults...............................................................................................................74 4.5. D iscussion........................................................................................................ 77 5. General Discussion................................................................................................ ....94 5.1. Habitat Quality, Abandonment, and Reproductive Decisions..................... 94 5.2. Settlement Bias and Patterns of Nest Success.............................................. 96 5.3. The Effect of Year on Reproductive Success............................................... 98 5.4. Source-Sink Dynamics.................................................................................... 99 6.Literature Cited .................................................................................................. 103 List of Tables Table 2.1. 2-way ANOVA comparing effects of Habitat and Rank for nest data variables, Spring 2001 and 2002. Values are means ± SE. Sample sizes are in 35 parentheses. None of these differences were significant at P < 0.05........... Table 2.2. Poisson regressions comparing effects of Habitat, Rank and Year on nest data variables, spring 2001 and 2002......................................................................... 36 Table 2.3. Results of Backwards Stepwise Multiple Logistic Regression using cavity tree variables as predictors of nest success.....................................................37 Table 2.4. Results of Backwards Stepwise Multiple Logistic Regression using nest plot vegetation variables as predictors of nest success............................................. 38 Table 3.1. A comparison of average intrusion behavioural data collected during formal telemetry trials. Spring 2000 and 2001, for pairs that intruded at least once during a set of trials. Only one experimental subject in undisturbed habitat actually engaged in intrusion behaviour, so n = 5 in disturbed vs. n = 1 in undisturbed. Means are given ± SB...................................................................................................60 Table 4.1. 10 zone habitat classification system. Spring 2000...................................84 Table 4.3. Compositional Analysis Ranking Matrix of t-values for 34 birds in undisturbed habitat, Spring 2000 and 2001. Statistically significant departures from random use are in bold, indicating that the habitat type indexed by the row is more preferred (positive value) or less preferred (negative value) than the habitat type indexed by the column. Ranks can be determined by the count of positive values in each row of the table. Rank indicates the degree of preference, from ‘least preferred’ to ‘most preferred’............................................ 86 Table 4.4. Compositional Analysis Ranking Matrix of t-values for 26 birds in disturbed (D l) habitat. Spring 2000 and 2001.................. .87 Table 4.5. Compositional Analysis Ranking Matrix of t-values for 5 birds in smaller disturbed (D2) habitat, Spring 2000 and 2001...........................................................88 Table 4.6. Compositional Analysis results, flock level analysis, for alpha pairs of 25 flocks. Spring 2000 and 2001......................................................................................89 List of Figures Fig. 1.1. Aerial photo of the study site, showing areas of disturbed and undisturbed 14 habitat........................................................................................................... Fig. 2.1. Nest Success in disturbed and undisturbed habitats by rank for 29 and 24 pairs breeding in 2000 and 2001, respectively ........... 39 Fig. 2.2. Snag Decay Class Distributions for 69 nest plots in disturbed and undisturbed habitats. Despite an apparent shift in the distributions between habitats, the effect is not significant.................. 40 Fig. 3.1. Map of study site showing breeding territories for birds breeding in undisturbed (solid lines) and disturbed (broken lines) habitat. Spring 2000..... 61 Fig. 3.2. A typical intrusion telemetry trial in disturbed habitat. Spring 2000. Solid lines indicate territory boundaries. The central territory in this figure belongs to the focal bird. The movement polygon is almost entirely within the territory defended by the neighbouring pair to the east......................................................62 Fig. 4.1. a) Mean ± SE LogArea of territories (n=61) in disturbed and undisturbed habitats, b) Mean ± SE LogArea of territories (n=61) in 2000 and 2001......... 90 Fig. 4.2. a) Mean ± SE Intemest Distance of territories (n=61) in disturbed and undisturbed habitats, b) Mean ± SE Intemest Distance of territories (n=61) in 2000 and 2001.......................................................................................................... 91 Fig. 4.3. 6-Cluster dendrogram produced by Hierarchical Cluster Analysis of 10 habitat zones. Vegetation data collected Spring 2000. Scale at bottom refers to Euclidean distance............................................................................... 92 Fig. 4.4. a) Mean ± SE proportion of VRPINE cluster in Failed vs. Successful breeding territories. Spring 2000 and 2001. b) Mean ± SE proportion of DECMATURE cluster in Failed vs. Successful nests. Spring 2000 and 2001. 93 Acknowledgement First, I would like to thank.my supervisor, Ken Otter, for his unparalleled mentorship and encouragement. There is no question that, through his dedication to the highest standards of quality and tireless attention to detail, he was able to extract from me the best work that I have done in any discipline. Committee members Russ Dawson, Staffan Lindgren, and Roger Wheate must be thanked for their guidance and support throughout. For their tireless slogging in the field, ability to withstand weeks of sleep deprivation and mosquito predation, as well as the good humour to put up with the odd stress attack from their fearless ‘leader’, I would like to thank Ken, Carmen Holschuh, Zoe McDonnell, Kara Litwinow, and Ben Burkholder. Jocelyn Campbell offered her awe-inspiring tree-climbing services, providing me with data points that otherwise would have been lost. Marcel Marcullo, Kathleen Lawrence, Sally Taylor, Dani Thompson, Mark Bidwell, Heather Swystun, and Dave Leman all kindly volunteered their time and enthusiasm. Carmen, Harry Van Oort, and Tania Tripp comprise our small, but dynamic, lab. Of special note, Harry generously made unpublished data from our study site freely available to me, and was a truly inspiring source of brilliant ideas, enthusiasm, and spaghetti. In addition to field assistance, Sally provided invaluable literature searching services and advice, displayed an uncanny knack for asking just the right questions, and offered a ton of emotional support. Dieter Ayers put up with my incessant appeals for statistical advice and consultation, as well as my conspicuous lack of grace on his climbing wall. Moshi Chamell was always more than willing to engage in wild ecological philosophizing, ridiculous hikes, and impromptu hack sessions. Susan Shirley gave me my first job in ecology in 1997, allowing a ‘recovering’ philosophy grad student with a few bird identification skills the chance to experience firsthand both the joys and tribulations of work in the field. I would also like to thank Roger, Robert Legg, Scott Emmons and Ping Bai for tolerating the depth of my ignorance of all things geographical (north goes at the TOP of the map, right Roger?). Special mention has to go to Aubrey Sicotte, without whom I never would have emerged from the GIS lab with my sanity intact. Access to land was provided by the City of Prince George and UNBC. Funding for the study was provided to me by a NSERC PGS-A, a Canfor Scholarship, and a NSERC Research Grant and C. F. I. awarded to Ken Otter. UNBC and the Northern Land Use Institute also provide monetary support. Comments on various drafts of the thesis were provided by Russ Dawson, Staffan Lindgren, and Roger Wheate. Finally, I would like to thank my parents for their unqualified support, patience, and advice over the years and especially for letting me move back in (something they could hardly have anticipated 32 years ago!) for the critical last few months of the write-up stage. VI Chapter 1 General Introduction 1. General Introduction 1.1. Behavioural Ecology and Conservation Biology The potential contribution of behavioural ecology to landscape-level processes, population biology and conservation biology has, until recently, been largely ignored (Caro 1999; Sutherland and Gosling 2000). However, over the past decade, behavioural ecologists have started to take a more applied approach to their discipline; for example, researchers have begun to investigate the extent to which information about individual behavioural responses to differing environmental conditions might increase the predictive power of large-scale population models used in conservation planning. This can be especially useful when anthropogenic change creates environmental conditions significantly different from those forming the empirical basis for statistical population models (Pettifor et al. 2000). Behavioural approaches have advantages over statistical approaches because assumptions of optimality in behavioural models allow organisms to respond to environmental changes in ways that will maximize their fitness. However, the habitat conditions in some anthropogenically-disturbed environments may result in maladaptive behaviours, as organisms which have evolved in undisturbed habitat conditions may apply decision rules inappropriate to the novel environment (Lima and Zollner 1996). Still, an understanding of how such organisms behave in both disturbed and undisturbed environments may lead to insights about decision rules Chapter 1 General Introduction being used, and the extent to which the maladaptive use of these rules will affect survival, reproductive success and, ultimately, population dynamics. One area in which behavioural ecologists have taken strides to bridge the gap between large-scale patterns and individual behavioural decisions is in the effects of landscape fragmentation on movement patterns in birds. Desrochers et al. (1999) recently reviewed the empirical evidence for disruption of normal movement patterns in fragmented habitats and ways in which this information can be used to drive simple, testable landscape-level predictions. For instance, a number of studies (Desrochers and Hannon 1997, Rail et al. 1997, St. Clair et al. 1998) have shown a reluctance of some songbird species to cross habitat gaps. Consequently, one would expect to see a negative relationship between isolation and species abundance in habitat patches. Alternatively, information from empirical studies on the behavioural responses of organisms to, for example, habitat edges can be incorporated into spatially explicit behaviour-based models. Such models can be used to predict movement patterns in fragmented landscapes. For example, individual-based models have been developed which assess the utility of habitat corridors between suitable patches by including behavioural responses to edges as important parameters (Tischendorf and Wissel 1997, Haddad 1999). Although behavioural research has increased our understanding of how species may react and adapt to landscape alteration, it is only one facet of the growing body of work investigating the interaction between behavioural ecology and conservation Chapter 1 General Introduction research. It has long been realized in behavioural research that variation in the natural habitat structure can lead to differences in reproductive strategies and reproductive success (Krebs 1971, Perrins 1979). It is likely then that anthropogenic disturbance may alter behavioural responses of animals through changes in habitat quality. This will be the predominant theme of this thesis. 1.2. Reproductive Decisions, Territoriality, and Habitat Quality The term habitat quality is used to refer to the characteristics of the environment that allow birds inhabiting a particular patch to maximize their fitness. Thus, features such as food resource availability (for adults and for nestlings), access to suitable safe nesting sites and predation risk are all factors that contribute to habitat quality. Habitat quality has sometimes also been used to refer simply to the reproductive output of birds breeding in a particular patch or territory (e.g. Pulliam 1988, Muller et al. 1997). Unless otherwise noted, however, 1 will be using this term to denote the former meaning throughout this study. Habitat quality is known to influence reproductive decisions in birds. Hogstedt (1980) argued that flexibility in clutch size in birds was adaptive and that variation in territorial quality was the most important factor in determining optimal clutch size. In an experimental study, Siikamaki (1995) found that female pied flycatchers (Ficedula hypoleuca) relocated to poor quality territories laid smaller clutches and were more likely to break the pair bond with their mate than those relocated to good quality territories. Habitat quality also has been shown to influence dispersal decisions; female pied flycatchers were more likely to disperse to other habitat Chapter 1 General Introduction patches if either they had previously experienced poor reproductive success in that patch or if the overall reproductive success of the patch was low (Doligez et al. 1999). Disturbed environments or small habitat fragments assumed to be of lower habitat quality often contain higher proportions of young and inexperienced males (Hatchwell et al. 1996, Zanette 2001), suggesting that habitat quality also drives intraspecific competitive interactions. Habitat quality also is known to affect territorial behaviour in birds. Gill and Wolf (1975a) found that nectivorous sunbirds {Nectarinia rechenowi) adopt territorial defence of a patch of flowers if the patch resource levels were sufficiently high, but refrain from active defence when resource levels are low. Carpenter et al. (1983) found that migrating rufous hummingbirds (Selasphorus rufus) alter feeding territory size to maximize daily rate of weight gain. Further, optimality models predict that organisms should adjust the size of their feeding territories based on changes in local resource levels (although the relationship between size adjustment and resource availability depends crucially on the shape of cost and benefit curvesSchoener 1983). These studies suggest that even minor variation in habitat quality can have large impacts on the behavioural responses and, ultimately, reproductive success of birds. As many of these same species are resilient enough to anthropogenic disturbances to continue breeding in these areas, it is pertinent to determine whether such alterations to the landscape are having similar effects on the remnant populations. Chapter 1 13. General Introduction Sources, Sinks, and Habitat Disturbance. Restricted movement of animals due to avoidance of habitat gaps, and changes that leave intervening 'matrix' habitats of such poor quality that they remain unoccupied can lead to fragmentation of avian populations. Such metapopulations can be defined as a set of local populations within some larger area, where typically dispersal from one local population to at least some other patches is possible (Hanski and Simberloff 1997). One component of the metapopulation model is the source/sink system formalized by Pulliam (1988). As with other versions of the model, regional metapopulations are divided into local populations or compartments. Source populations are characterized by birth rates in excess of death rates, and emigration rates in excess of immigration rates. Thus, they are net exporters of surplus individuals. Conversely, sink populations are characterized by death rates in excess of birth rates, and immigration rates in excess of emigration rates. As sink populations suffer from negative local recruitment, such populations would not persist in the absence of an influx of immigrants from local sources. Theoretically, for metapopulations in dynamic equilibrium (i.e. when population size is constant in all compartments, and there is no net population change in the assemblage of compartments), large sinks can be maintained by relatively small source patches. In such circumstances, removal of source patches or restriction of inter-patch dispersal rates may result in the decline and eventual extinction of sink populations as well as a general decline of the metapopulation. Chapter 1 General Introduction Anthropogenic habitat disturbance has the potential to impact metapopulations in at least two ways. First, the disturbance may take the form of a matrix of unsuitable habitat, creating isolated fragments or 'islands' of usable habitat that are no longer connected by inter-patch migration. If certain habitat islands consist of sub-optimal habitats acting as population sinks, local recruitment cannot be supplemented with immigration and the population will decline to extinction. Alternatively, disturbed habitats (such as early serai habitat regenerating after logging activity) may themselves represent sink habitats if organisms breeding in such sub-optimal habitats experience lower reproductive output or survival rates (Blondel et al. 1994). Individuals may settle in these areas as a result of interference competition stemming from overcrowding in source habitats (Sutherland 1998, Caro 1999) or due to an inability to recognize their sub-optimality (Pulliam and Danielson 1991, Remes 2000, Delibes et al. 2001). Regional resource extraction activities may alter the proportion of the landscape in source and sink habitats to such an extent that existing sources will be unable to restock sink populations and the metapopulation will decline. Such a scenario may be difficult to predict in organisms whose patch population dynamics are not well understood, yet the ramifications of failing to account for this could potentially be high. Chapter 1 1.4. General Introduction Limits of Traditional Habitat Sensitivity Protocols and Assumptions From an evolutionary perspective, if a particular habitat patch does not meet the life-history needs of a particular organism as well as other available patches, the organism should avoid that environment. Also, population density has commonly been used as a proxy for reproductive success or resource levels in a particular patch, as low densities would presumably be an indicator of the decreased productivity of the local breeding population. Thus, the sensitivity of bird species to habitat disturbance has traditionally been assessed using presence/absence and species abundance census methods (e.g. point counts, line transects, spot-mapping). However, a number of studies have questioned the utility of using density as an indicator of reproductive success or habitat quality. Van Home (1982) found that deer mouse (Peromyscus maniculatus) density was highest in sub-optimal habitat, and argued that intraspecific competitive interactions explained this result. Thus, Van Home contended (1983) that population density was an unreliable measure of habitat quality. Vickery et al. (1992) concurred with this assessment, in a study that showed no correlation between territory density and reproductive success in three emberizine sparrows. Roberts and Norment (1999) found that density did not differ, although reproductive success did, between populations of breeding scarlet tanagers {Piranga olivacea) in habitat fragments of varying size. In a recently published long-term study of productivity in a wood thrush (Hylocichla mustelina) population. Chapter 1 General Introduction Underwood and Roth (2002) determined that density was a poor predictor of nest success. These results indicate that a deeper understanding of the mechanisms controlling population density and habitat-specific reproductive success will be required if we are to determine the extent to which animals are affected by habitat disturbance. A greater emphasis on determining behavioural responses of individuals breeding in both disturbed and undisturbed habitats will contribute to greater accuracy in predictions of population responses to habitat disturbance. The aim of my thesis is to investigate the impacts of habitat disturbance on the reproductive and territorial behaviour of black-capped chickadees {Poecile atricapilla). This species is commonly found in mixed woodlands, but also breeds in a variety of disturbed habitats including urban settings and early successional forests. It, therefore, serves as a perfect model to investigate how habitat alteration can impact retained species. In addition, a large body of work exists for this species where it breeds in undisturbed habitats, and so many aspects of its social structure, territorial behaviour and natural life history are known. I studied two adjacent local populations occupying differing habitats, a mature mixed sub-boreal woodland (undisturbed) and a forest in regeneration following logging/land clearing (disturbed). My goal was to determine whether habitat altered reproductive success in the species and to what extent habitat disturbance had cascading effects on territoriality and habitat selection in the species. Chapter 1 15. General Introduction Study Site The study location was immediately west of the University of Northern British Columbia, Prince George, BC (53°E 55’ N, 122°E 50’W, and 850 m elevation), within the Sub-boreal Spruce (SBS) biogeoclimatic zone. The study area was composed of two at^acent habitat types: 1) an 85 hectare block of mature forest and 2) two sites (total area: 65 hectares) which have been disturbed as a result of forest management practices (Fig. 1.1). The undisturbed habitat is a continuous forested area composed of patches of various mature forest types. Canopy species represented in this area are trembling aspen (Populus tremuloides), paper birch (Betula papyrifera), black cottonwood {Populus balsamifera ssp trichocarpa), hybrid spruce (Picgo gZawca %Picea lodgepole pine (Pmw.; conforfa), Douglas-fir (Pseudotsuga menziesii) and subalpine fir {Abies lasiocarpa). Canopy height is 25-30 m. The understory stratum is dominated by green alder {Alnus crispa), willow {Salix sp.), prickly rose {Rosa acicularis), low-bush cranberry {Viburnum edule), and twinberry {Lonicera involucrata). The primary disturbed site (~ 75 hectares) was logged in 1962 and cleared to agricultural standards for the purposes of horse and cattle pasturing. The site was designated a model forest in 1985, and many areas were cleared and replanted with lodgepole pine and other conifers from 1986-89. Other sites regenerated naturally, and still others were never harvested. Consequently, the disturbed habitat was characterized by a mosaic of different habitat types, ranging from young managed lodgepole pine stands, somewhat older aspen/birch/willow stands, and isolated Chapter 1 General Introduction patches of mature forest. Although species composition was similar to that of the undisturbed site, canopy height was lower (5-15 m), there were fewer large trees, and there was a much larger understory component. Where small patches of mature forest existed, they were similar in composition and structure to the undisturbed site. However, these exist as isolated patches of 1-4 hectares in the surrounding landscape. None of the birds classified as settling in disturbed habitat were able to establish territories exclusively in these patches. In all cases, the majority of the territory of any bird classified as breeding in disturbed habitat consisted of various early serai habitat types. The smaller disturbed site (~9 hectares) was a stand of mature birch that had been subjected to selective harvesting practices, in which many trees had been left standing. As a result, canopy height was similar to that found in the undisturbed site, but canopy cover is drastically reduced and there is a more pronounced understory component. 1.6. Study Species The black-capped chickadee {Poecile atricapilla) is a small (~ 11 g) resident songbird. Chickadees are territorial during the breeding season (mid-April to early July locally), but forage and travel in small flocks consisting of 2-5 mated pairs during most of the non-breeding season. During most of the year, chickadees consume a mixed diet of seeds, berries, and invertebrates, but switch to a completely insectivorous diet during the breeding season (Smith 1991). A weak cavity excavator, chickadees nest in hardwood snags, dead limbs or knotholes of live trees. Thus, they are dependent on significant densities of trees or 10 Chapter 1 General Introduction snags with advanced decay, and have evolved primarily in mature forests of North America. However, this species is known to breed in fragmented and otherwise disturbed habitats (Smith 1991) and preliminary investigations revealed that population densities in disturbed and undisturbed portions of my study site are roughly equivalent. Nest sites are chosen in late April, at which time both pair members excavate the cavity. The bottom of the cavity is then lined with a nest cup, and the female begins egg-laying (in my study site, egg-laying commenced during the first or second week of May). One egg is laid daily until the clutch is complete (average clutch size is 6 eggs in my study area). Incubation begins on the day prior to the laying of the last e g g , and lasts for a period of 12-13 days (Smith 1991). Only the female incubates the eggs, although the male will devote considerable effort to feeding the female at the nest during this phase. Once the eggs hatch, both male and female will deliver food to the nestlings, although the female will also spend much of her time in the nest cavity, especially when fledglings are young and unable to thermoregulate effectively. Fledging typically takes place 16 days after hatch, although disturbance at the nest after Day 13 will likely trigger an early fledge. In my study area, most nests fledged in mid- to late June, although a few nests did not fledge until early July. Post-fledge, juveniles will remain with and continue to be fed by their parents for a period of 2-4 weeks, and then disperse in random directions, usually settling a few kilometres from the nest site as low-ranking members of winter flocks (Smith 1991). 11 Chapter 1 General Introduction Chickadees maintain a rigid social hierarchy in winter flocks, which can be used as a measure of male resource holding potential (Ficken et al. 1990). Because this species is resident year-round, dominance rankings of colour-banded birds may be determined in the non-breeding season by means of aggressive interactions at winter feeders (Ficken et at. 1990). 1.7. Thesis Outline 1.7.1. Area-Sensitivity, Reproductive Success, and Habitat Although present in densities similar to those found in undisturbed habitat, chickadees breeding in disturbed habitats may nevertheless be experiencing lower reproductive success. This may be a consequence of lower habitat quality relating to features of the environment local to the nest site. In chapter 2 ,1 investigate whether reproductive success differs between disturbed and undisturbed habitats, and to what extent nest tree and nest site variables are predictive of fledge success. 1.7.2. Does Habitat Disturbance Influence Territorial Behaviour? Most songbird species defend exclusive territories during the breeding season. If resources levels are low, benefits associated with exclusive access to resources necessary for reproduction may no longer outweigh energetic expenditures associated with territory defence. In chapter 3 ,1 investigate whether birds breeding in disturbed habitats alter their territorial behaviour by comparing the frequency of anomalous territorial behaviour in disturbed and undisturbed habitats. Two methodologies are used to accomplish this goal; 1) a radio-telemetry study and 2) a comparison of territory intrusion rates observed during daily territorial surveys. 12 Chapter 1 General Introduction 1.7.3. Territory Size, Habitat Selection, and Reproductive Success In chapter 2 ,1 looked at the extent to which habitat features in and around the nest site are predictive of nest success. However, territorial habitat quality may also be an important factor determining songbird reproductive success. Certain available habitat types will likely offer more of the resources critical to nest success than others. Consequently, birds should seek to maximize their fitness by including these habitats in their territories in greater proportion to their availability in the landscape. If birds are prevented from utilizing favoured habitat types, they may respond by increasing territory area to encompass enough low-quality habitat to meet their reproductive requirements. In chapter 4,1 investigate whether territory size differs for birds breeding in disturbed and undisturbed habitats, if chickadees show clear preferences for certain habitat types, and if there are any relationships between habitat type and nest success. 13 Chapter 1 General Introduction 500 m Fig. 1.1. Aerial photo of the study site, showing areas of disturbed and undisturbed habitat. 14 Chapter 2 Habitat Disturbance and Reproductive Success 2. Area sensitivity in an "area-insensitive" songbird: the impact of habitat disturbance on reproduction of chickadees. 2.1. Abstract Avkzn apgcigf f a r e fAowgAf fo unaffected by disturbance, yet there is growing evidence that altered environments may negaftve/y rgprntfwcfzve oW ngff 7 comparée^ chickadee nest success in two adjacent habitats, a mature mixed wood forest (undisturbed) versus a forest regenerating post-logging (disturbed). Despite similar breeding densities, nest success was lower in the disturbed habitat than in the undisturbed habitat. Abandonment was the most common causé o f nest failure. A within-habitat comparison o f the social rank o f birds revealed that low-ranking birds had lower nest success than high-ranking birds in the disturbed, but not the undisturbed, habitat. However, clutch size, brood size, and total fledgling productivity did not differ significantly between habitats. Nests situated in snags wffA Zowgr tverg more fwccgfj/wZ. ogffg were ako Zocofgtf m gtfg^y wzf/i AzgAgr canopy AgzgA/, fow wnckr^fory tfgnj;(y Zg.yf fAan 7 m, and higher understory density between 2 and 3 m. This study provides evidence that disturbed habitats may potentially function as habitat sinks, despite their ability fo rgfam apgcfgj af normoZ dgnjif/g^y. TTtgr^rg, .yggmmg/y ffaZ?Zg mgfapppMZofionj 15 Chapter 2 Habitat Disturbance and Reproductive Success may gxpgrfgncg ropitf popwWfo/! fowrce o/^mofwrg ybrggf become wocommo» ocroa^f fZig Zo/ifkct^g. 2.2. Introduction Research on habitat disturbance and its effects on the reproductive success of forest songbirds typically focuses on community-level effects and is primarily determined by presence/absence census methods (Schmiegelow et al. 1997). Also, studies of focal-species are often restricted to species deemed “area sensitive” {i.e. species no longer present following habitat disturbance) (Gibbs and Faaborg 1990). However, recent single-species studies suggest that altered environments negatively affect various aspects of reproductive behaviour (Chase 2(X)2, Ruiz et al. 2(X)2, Zanette 2001). There are potential dangers of assessing the degree to which a species is affected by habitat disturbance based solely on presence/absence methods. Specifically, reproductive output could be diminished in disturbed habitats in comparison to undisturbed habitats, despite similar breeding densities. This could arise as a result of reproductive decisions made by animals breeding under sub-optimal and stressful conditions. A number of studies have shown that birds breeding in poor-quality territories will compensate for the lowered resources by adjusting clutch size downward (Dhondt et al. 1992, Dhondt et al. 1990, Slagsvold and Lifjeld 1990). However, birds experiencing extremely stressful conditions may opt to forgo breeding altogether if the perceived survivorship risk is too high relative to the 16 Chapter 2 Habitat Disturbance and Reproductive Success potential fitness benefit of a successful nest. Such conditions might arise either naturally (as a result of stand-level disturbance such as fire or insect outbreak, in which birds begin to re-colonize the area of disturbance from ac^acent undisturbed areas) or as a result of anthropogenic disturbance (such as when birds re-colonize regenerating clearcuts). From a landscape perspective, subpopulations of birds breeding in disturbed habitats may represent population sinks that are dependent on adjacent sources (relatively undisturbed patches) for their continued persistence (Pulliam 1988). The entire metapopulation will persist as long as population sources can export individuals to nearby sinks. Once disturbance levels are too high across the landscape, the number individuals emigrating from source subpopulations may be inadequate to maintain the large number of sink populations, resulting in a largescale population collapse. This could also happen if certain patches become inaccessible to colonizers, due to behavioural avoidance of intervening habitat. Black-capped chickadees {Poecile atricapilla), for example, are known to avoid crossing habitat gaps such as clearcuts (St. Clair et al. 1998). The black-capped chickadee, a resident cavity-nesting songbird, is known to breed in fragmented and otherwise disturbed habitats (Smith 1991). While its breeding behaviour in pristine woodland habitats has been well studied (Otter and Ratcliffe 1996, Otter et al. 1998), the effects of breeding in disturbed habitats are not well understood. Despite the presence of black-capped chickadees in disturbed habitats, these populations could experience reduced reproductive success as a result 17 Chapter 2 Habitat Disturbance and Reproductive Success of habitat alteration. This may be due to effects traditionally considered to impact species in disturbed habitats, such as increased predation rates, lower food availability, and a decrease in appropriate nesting sites. It may also stem from more subtle impacts; reproductive strategies of birds based on social ranks (Otter et al. 1998, Otter et al. 1999a) may interact with habitat effects to impact overall reproductive success of populations. For instance, breeding in sub-optimal habitats may differentially impact high-ranking and low-ranking birds if competitively superior high-ranking birds are able to secure better breeding territories. The purpose of this study is to determine whether chickadees do in fact experience lower reproductive success in disturbed habitats than in undisturbed habitats, and to what extent this can be attributed to specific characteristics of that habitat. 23. Methodology 23.1. Winter Banding and Dominance Assessment Adult chickadees were captured at established feeding stations using box (Potter) traps mounted on platform feeders and banded during December through February of both years. The banding protocol consisted of applying one numbered aluminum band (under Canadian Wildlife Services license) and three colour plastic bands. Each bird was given a unique colour combination, allowing individuals to be identified from a distance. At the time of banding, body measurements were taken (length of rectrices, flattened wing chord, and mass). Sex of the bird can be determined with 90% accuracy at time of banding using a combination of these three 18 Chapter 2 Habitat Disturbance and Reproductive Success measures (Desrochers 1990), and this was confirmed by behavioural observations during the breeding season. The age of the bird was determined by examining the shape of the rectrices (Meigs et al. 1983). Birds were classified as either second-year (SY) or after-second-year (ASY). SY birds are entering their second calendar year, and are therefore approaching their first breeding season. ASY birds are any birds entering their third or higher calendar year (i.e. second or higher breeding season). Once the birds were banded, dominance ranks were assessed by monitoring aggressive interactions between birds at winter feeding stations. A bird was considered dominant to another if it "won" the majority of dyadic interactions. Three behaviours were used to assess dominance. If a focal bird 1) supplants or chases away its opponent, 2) gives a display which elicits a submissive posture in an opponent, or 3) the opponent waits for the bird to leave before approaching a feeder (Ficken et al. 1990, Otter et al. 1998), that bird was considered dominant to its opponent. Flock membership was determined by observing patterns of feeder use and by tracking foraging activities throughout the flock range. These data were collected using a voice-activated recorder (Optimus CTR-116) at a distance of not less than 10 m from the station to minimize the risk of influencing feeding behaviour. A linear dominance matrix was determined for each flock. Birds were classified either as low, mid, or high rank, depending on their position within the flock. As female rank is known to be correlated with rank of their social mate (Otter et al. 1999a, Smith 1991), I concentrated on determining relative rank of males 19 Chapter 2 Habitat Disturbance and Reproductive Success within flocks. In flocks consisting of three pairs, the mid-rank was applied to the male submissive to the alpha male but dominant over the low-ranking male. No flocks consisting of greater than three mated pairs were observed in my study area over the course of the two-year study period. In flocks consisting of two mated pairs (the most common flock size in my study area), the dominant male was assigned the high rank while the other male was considered low-ranking. This relative ranking system is likely a more biologically accurate measure than absolute ranks, because high-ranking birds from one flock tend to dominate low-ranking individuals from other flocks. As interactions between birds from each habitat type were relatively rare (K. Fort unpubl. data), it was not feasible to assess whether birds from one habitat type were consistently dominant to birds from the other habitat type (i.e. evidence for a habitat-induced settling bias, such that high-quality birds competitively exclude low-quality birds from undisturbed habitat). In early spring (prior to flock breakup) of the first year, a 50 m by 50 m grid system was created in the undisturbed habitat, and grid points were marked with flagging tape. All grid points were recorded using a Trimble Geoexplorer HI (Trimble, Sunnyvale, CA) handheld GPS unit. Thus, the location of bird observations and territory boundaries in relation to aerial photos of the study site could later be determined with a high degree of accuracy. By also marking locations of specific landmarks in either habitat, the GIS images could be superimposed onto satellite images of the area to give a high resolution of accuracy in marking animal movements. It was unnecessary to establish a grid system in the 20 Chapter 2 Habitat Disturbance and Reproductive Success disturbed habitat, as existing trails and other landmarks were sufficient to determine locations of territory boundaries and nest sites. 23,2. Breeding Season After the breakup of flocks in early spring, three field assistants and I conducted surveys of the study area from 0800-1600 hours daily to determine settling patterns, territorial boundaries, and nest locations. Territorial boundaries were determined by recording locations of territorial disputes between neighbouring males, male singing posts, and the geographical extent of foraging bouts by mated pairs. During this period, mated pairs will excavate nest-cavities, and these sites were recorded and monitored to determine when pairs initiated incubation. All nest sites were marked with flagging tape at a random distance (minimum 5 m away) and direction (indicated on the marker flag, to facilitate relocation of the nest) from the actual cavity tree to minimize the risk of attracting potential nest predators. I also maintained a minimum distance of 5 m away from the nest during all monitoring activities. Once a nest-site had been determined, it was monitored every 3-4 days for changes in status (i.e. excavation, nest-lining, egg-laying, incubation, hatch, fledge). Change in nesting status can often be determined (within a range of accuracy of 1-2 days) by noting certain characteristic behaviours. During the nest-lining phase, females will bring nest-material such as animal hair or dried plant material to the cavity. The egg-laying phase is accompanied (a few days prior to onset) by the use of the ‘broken-dee’ call by the female (D. Mennill pers. comm.). Once incubation 21 Chapter 2 Habitat Disturbance and Reproductive Success begins, the female spends the majority of her time within the cavity, and the male feeds the female at the nest entrance. After the eggs have hatched, both male and female feed the young, although the female still spends much of her time brooding within the cavity. However, when the male arrives with food, the female will often leave the cavity to allow the male to enter, feed the young, and remove any fecal sacs. All accessible nests were visited on or around day 7 post-hatch for the purposes of banding nestlings. Nests were accessed in one of three ways. The mzyoiity of nests were accessed using a 10 m extension ladder, a tree-climbing belt, or with the help of an experienced tree-climber. Inaccessible nests could still be monitored to determine whether a successful fledge took place. Once at the cavity, a small saw was used to cut a square portal in the side of the tree several cm above the level of the nest cup. Whenever possible, chicks were removed in two stages in order to minimize the risk of nest abandonment (no nests were abandoned as a result of my activities). Fledglings were enumerated and the nest cup was examined for unhatched eggs. Once the fledglings were returned to the nest, the portal was re-inserted and held in place with duct tape. Using this methodology, clutch size (# hatched + # unhatched eggs is a valid measure, as chickadees are not known to remove unhatched eggs or dead nestlings - Otter et al. 1999a), brood size, and proportion hatched (# hatched/ clutch size) could be determined. A successful nest was defined as a nest that was still active at day 14 post-hatch; although fledging does not normally take place until day 15 or 16, any 22 Chapter 2 Habitat Disturbance and Reproductive Success disturbance in the vicinity of the nest at or beyond day 14 will trigger fledging. Failed nests were classified according to the cause of failure (abandonment, nest predation, weather event) whenever possible. Nest predation events could be determined easily, as local nest predators (red squirrels and, in one instance, a young black bear) leave signs of forced entry in and around the cavity entrance. Abandoned nests were further classified according to the nesting phase (pre­ incubation, incubation, or nestling) at which abandonment occurred. 23 Vegetation Sampling Protocol Nest-site habitat characteristics were assessed using, at each established nest site, 0.04 ha (11.3 m radius) circular plots centred on the cavity tree. Vegetation sampling took place within two weeks after fledging had occurred. As the vegetation is fully developed well before the time of fledging, my vegetation plots should be an accurate reflection of habitat conditions at the nest during the nestling phase. Characteristics of the cavity tree itself as well as the surrounding habitat were recorded. With respect to the cavity tree, species, diameter at breast height (dbh), tree height (using a clinometer), cavity height, and cavity type (top or side entrance, knothole, or branch) was recorded. Within the plot, species and dbh (in six size classes) of each tree was recorded. The height, species, and dbh of a representative canopy tree were also recorded. Canopy cover was measured using a convex densiometer at the edge of the plot in the four cardinal directions. For all snags within the plot, species, dbh size class, height, and decay class was recorded. The understory component was assessed by estimating the overall percent cover (in 23 Chapter 2 Habitat Disturbance and Reproductive Success seven cover classes) of all shrub species (including young trees) at four vertical height classes (0-1 m, 1-2 m, 2-3 m, 3-4 m). 2.3.4. Statistical Analyses I used G-tests to determine whether nest success differed between disturbed and undisturbed nests, between high- and low-ranking birds, and to assess whether birds responded differentially by rank within each habitat type. When cell frequencies were less than five, I used Fisher Exact tests. As rank is known to influence reproductive output (Otter et al. 1999a), I included rank as an additional factor in analyses of nest data. I also included year as a factor in ANOVA models. If annual variation was detected, I standardized the data by determining the average value of the variable for each year and then expressed the data as a deviation from the yearly average. Two-factor ANOVA was used where assumptions were met. Poisson multiple regressions were used for count variables. Year was included as a factor in these models. As Incubation Date (commencement of incubation of the clutch) was also not distributed normally and is known to be highly correlated with rank (Smith 1991), a nonparametric comparison of high-ranking birds only was used to control for this factor. I employed backward stepwise multiple logistic regression to determine which, if any, habitat variables were predictive of nest success, both with respect to cavitytree and nest plot-level characteristics, irrespective of overall habitat type. This analysis allows differentiation of success based on microhabitat, within larger landscape categories. Data were collected from 69 nest plots in 2000 and 2001. The 24 Chapter 2 Habitat Disturbance and Reproductive Success following cavity tree variables were entered into the cavity tree model; tree height, tree diameter at breast height, cavity height, decay class at Cavity, number of cavities in the cavity tree. Decay class was assessed using the Wood Classification system outlined in the Field Manual for Describing Terrestrial Ecosystems (Ministry of Forests 1999). The nest plot vegetation variables that I entered into the model were canopy height (distance from ground to the top of the canopy layer), canopy cover, understory cover (in four 1 m vertical classes), basal area of all trees, snag density, density of large hardwoods. A Kolmogorov-Smimov test was used to determine whether the distribution of snags in each decay class differed between the disturbed and undisturbed sites. Also, I used a t-test to determine whether the ratio of cavity height to canopy height differed significantly between disturbed and undisturbed habitats. Non-parametric tests were used when distributions were not normal. All statistical analyses were performed using SYSTAT 9.0 (SPSS Inc. Chicago, IL). 2.4. Results 2.4.1. Overall Reproductive Success between Habitats I collected nest success data for 68 breeding pairs over the two-year study period. Birds breeding in disturbed habitat had significantly lower nest success than did those in undisturbed habitat (G-test, P = 0.02), and this pattern did not differ between years (G-test, P = 0.14). For the 52 breeding pairs where dominance rank was known, high-ranking birds were significantly more successful than low-ranking 25 Chapter 2 Habitat Disturbance and Reproductive Success birds (G-test, P = 0.02). For clarity, this analysis also excluded the small number of mid-ranked birds. In order to look for a possible interaction between habitat and rank, I examined the ratio of successful to failed nests in each habitat separately by rank (Fig. 2.1). Rank influenced patterns of nest success to a much greater extent in disturbed habitat (G-test, P = 0.05) than in undisturbed habitat (Fisher Exact test, P = 0.26) in that the majority of successful nests in disturbed habitat were attributable to high-ranking birds. Breeding densities were not appreciably different between habitats or years. There were 0.25 pairs per hectare breeding in the disturbed habitat averaged over two years, compared with 0.33 pairs per hectare in the undisturbed habitat. However, the density of successful pairs in the undisturbed habitat was 0.26 pairs per hectare, twice the density of 0.13 in the disturbed habitat. 2.4.2. Comparisons of Nesting Chronology and Reproductive Output between Habitats Nest failure due to predation was a relatively rare event (less than 5% of all nests were depredated) and does not appear to differ between habitats. The majority of nest failure occurred through abandonment. High-ranking birds nesting in disturbed habitat started incubating earlier than those in undisturbed habitat (Mann-Whitney U-test, U = 10.5, P = 0.01, n = 11 undisturbed vs. 7 disturbed nests). However, hatch date, incubation period, and fledge date did not differ between habitats or ranks (Table 2.1). Decreasing sample sizes in these analyses are due to nest failures and instances of abandonment accumulating over the breeding season. 26 Chapter 2 Habitat Disturbance and Reproductive Success Clutch size and brood size did not differ between habitats (Table 2.2). For this analysis, nests that failed pre-incubation were excluded (N= 14), as were failed nests where males abandoned (N= 2) and a single nest where behavioural and genetic evidence implicated conspecifics brood parasitism (Otter et al. in prep.). 2.43. Overall Productivity in Each Habitat To compare productivity between disturbed and undisturbed areas, I calculated the average number of fledglings per pair over two years in both habitats. In this analysis, I did not consider pairs for which the number of fledglings was not known (i.e. inaccessible nests), but did include all nests where pairs initiated a clutch and abandoned either pre-hatch or post-hatch. In undisturbed habitat, 3.33 ± 0.49 fledglings were produced per pair (or 1.67 fledglings per breeding individual), whereas only 2.30 ± 0.56 fledglings per pair (1.15 fledglings per breeding individual) were produced in the disturbed site over the same period. These estimates did not differ statistically (Mann-Whitney U-test, U= 323, P = 0.17, n = 30 undisturbed and 27 disturbed). Note, that I have no information on rates of postfledging juvenile survivorship. As total area of each habitat type was known, and the reproductive output of nearly all pairs within the study area was also known, I was able to calculate the productivity in each habitat in terms of the number of fledglings produced per hectare. In this analysis, I inserted average values for number of fledglings produced per successful nest for those successful nests (N=10) for which brood size 27 Chapter 2 Habitat Disturbance and Reproductive Success was unknown. In undisturbed habitat, 0.92 fledglings per hectare were produced compared to 0.53 fledglings per hectare in the disturbed habitat. 2.4.4. Nest Success and Habitat The cavity tree multiple logistic regression model was significant (Chi-square = 8.447, df = 1, P < 0.01), although only Cavity Decay was retained after the stepwise analysis. Successful nests were those that were in nest sites with lower decay (Table 2.3). The nest plot vegetation model was also significant (Chi-square = 9.665, df = 3, P= 0.02). Canopy Height, Understory Aspen>Lodgepole>Conifer> V ari able Retention>Willow-Alder>Marsh> Riparian>Birch. Of these. Mix is preferred significantly over Willow-Alder, Riparian, and Birch habitats. Aspen, Lodgepole, Conifer, Variable Retention, Willow-Alder, and Marsh are all preferred significantly over Riparian and Birch habitats, while being interchangeable in rank with each other. Riparian and Birch habitats are interchangeable in rank (Table 4.3). The compositional analysis ranking of disturbed (D l) habitats was in the following order: Willow-Alder>Lodgepole>Marsh>Remnant>Riparian>Conifer> Mix. Of these, Willow-Alder is preferred significantly to all other habitats. Lodgepole and Marsh are preferred significantly over Conifer and Mix, but are interchangeable with each other as well as with Remnant and Riparian. Remnant and 75 Chapter 4 Maladaptive Preferences in Disturbed Habitat? Riparian habitats are preferred over the Mix zone only (Table 4.4). D2 site compositional analysis produced the following ranking: Variable Retention>Conifer>Marsh>Birch. Variable Retention is preferred over Marsh only (but not Birch, despite the ranking). All other rankings are interchangeable (Table 4.5). 4.4.4. Compositional Analysis- Flock Level Analyses Compositional Analysis with reference to the six habitat clusters was performed on territories of alpha pairs for 25 flocks. Although an average ranking matrix could not be performed in this analysis, the proportion of times each habitat cluster, when available, was the most preferred and least preferred habitat was calculated. No strong patterns of preference or avoidance were detected for any habitat using this method, although there is some weak evidence for preference of WETMATURE and DECMATURE habitats and avoidance of VRPINE habitats. Each habitat assessed was ranked both as ‘most preferred’ and ‘least preferred’ by different pairs. No one habitat ranked consistently high or low (i.e. always in the top two most preferred or least preferred habitats). Rather, habitats were scattered from highest to lowest rank among birds (Table 4.6). 4.4.5. Territory Composition and Nest Success Birds experiencing nest failure had higher proportions of VRPINE (MannWhitney U-test, U = 547, P = 0.048, n = 37 successful and 23 failed; Fig. 4.4a), while those experiencing nest success showed a trend towards higher proportions of DECMATURE habitat in their territories (Mann-Whitney U-test, U = 316, P = 76 Chapter 4 Maladaptive Preferences in Disturbed Habitat? 0.085, n = 37 successful and 23 failed; Fig. 4.4b). Despite this, the amount of VRPINE incorporated into territories in the disturbed site did not differ between high and low ranking pairs (Mann-Whitney U-test, U = 72.5, P = 0.554, n = 14 high ranking and 12 low ranking). 4.5. Discussion 4.5.1. Territory Size and Inter-nest Distances Territories were larger and inter-nest distances were greater in disturbed than undisturbed habitats. Such larger territory sizes have been linked to poor habitat quality and lower reproductive success in other studies (Krebs 1971, Conner et al. 1986, Smith and Shugart 1987, Hunt 1996, Roberts and Norment 1998, Jones et al. 2001). This may arise because birds should defend a territory that provides sufficient food and nesting resources for successful reproduction, while minimizing energetic expenditure (Carpenter et al. 1983, Hixon et al. 1983). Increased territory size will amplify energetic costs associated with defence, as well as foraging and delivery of food to the nestlings, and thus larger territory size in disturbed habitats suggests that birds are experiencing lower resource levels in comparison to those in undisturbed habitats. 4 .5 .2 . C o m p o s itio n a l A n a ly s is Compositional analysis at the treatment level produced few significant preferences, counter-intuitive results, and a lack of consistency in rankings between habitats. This suggests that chickadees do not exhibit strong habitat preferences in 77 Chapter 4 Maladaptive Preferences in Disturbed Habitat? my study area. Alternately, these site-level analyses may be uninformative due to inherent flaws in large-scale analyses when working with chickadees. Two assumptions are implicit in these kinds of analyses: 1 ) the definition of habitat availability at the level of the treatment accurately reflects habitat type options and 2 ) intraspecific competitive interactions do not influence settlement patterns. Violation of either of these assumptions will impact results of the analysis in unpredictable ways. The life history patterns of chickadees could lead to violations of both assumptions. First, uncommon habitat types were present in certain territories at proportions much higher than their availability in the treatment overall, due to their clumped spatial distribution in my study area. Unless these birds sampled the entire treatment block, habitat rankings for such birds would tend to result in an artificial ‘preference’ for these rare habitats, whereas birds settling in territories distant from such habitat types would appear to be ‘avoiding’ them. Chickadees are known to establish breeding territories within the home range of the winter flock with which they were associated (Smith 1991, personal observations). As the flock ranges are much smaller than treatment blocks, definition of availability at the treatment level will be inaccurate when habitat types are not evenly distributed across the landscape. Second, as black-capped chickadees have a well-defined hierarchical social structure (Smith 1991), intraspecific interactions may influence settlement patterns. Lowranking birds forced into sub-optimal habitat types in greater proportion to 78 Chapter 4 Maladaptive Preferences in Disturbed Habitat? availability of those types will appear to ‘prefer’ them and, conversely, to ‘avoid’ optimal habitats from which they are excluded. The flock level compositional analysis is a far better test of habitat selection in this species, as it involves a more biologically accurate determination of habitat availability and considers only high-ranking birds, thus eliminating potential biases associated with intraspecific competition. Unfortunately, average matrices could no longer be calculated to look for significant preferences across all birds. Therefore, I looked for strong patterns of preference or avoidance of each habitat type, for all birds containing that habitat type. No strong patterns emerged in this analysis, so I still conclude that there is little evidence for territory-level habitat selection in my population of chickadees. Alternatively, differential response to habitat preferences in disturbed and undisturbed habitats may compromise my ability to detect strong habitat preferences. This may occur if organisms breeding in each type: 1) have evolved adaptations to the local environment, or 2 ) as a result of behavioural plasticity in habitat selection. An evolutionary response to local habitat conditions can only take place under conditions of restricted gene flow (Blondel and Dias 1994). Given the dispersal mechanisms of chickadees (Smith 1991) and the local spatial distribution of disturbed and undisturbed sites, gene flow between habitat types is likely to be unrestricted. Thus, there is no strong evidence for a genetic basis for differing habitat preferences between birds breeding in different disturbed and undisturbed habitats. 79 Chapter 4 Maladaptive Preferences in Disturbed Habitat? Behavioural plasticity in habitat preferences has been inferred in other studies in the context of response to natural disturbances. Jones et al. (2001) showed that cerulean warblers (Dendroica cerulea) demonstrated a significant shift in nest-site location patterns following a large-scale natural habitat disturbance. Pellech and Hannon (1995) hypothesized that black-capped chickadees may shift foraging strategies to spend more time in the tmderstory following severe tent caterpillar (Malacosoma disstrid) outbreaks, as canopy-level food abundance decreased drastically. Mysterud and Ims (1998) formalized this phenomenon in a model of functional response in habitat use in which habitat preference is conditional on habitat availability such that birds might ‘switch’ to preferring certain habitats, if their availability is very high. However, disturbed habitats, which are characterized by low canopy heights, contain a much smaller foraging volume per hectare than a mature habitat. It is unlikely that birds in disturbed areas would prefer early serai habitats, as these habitats are likely to have lower food abundances. Thus, factors other than habitat selection must determine territory composition in chickadees. Many studies have shown consistent habitat preferences for migratory songbirds (Oliamyk 1996, Stoate et al. 1998, Esely and Bollinger 2001). However, site tenacity may affect territory composition in resident songbirds, which spend the entire year in the breeding habitat, as the benefits of familiarity with habitat features in the territory may outweigh benefits of obtaining higher-quality, but less familiar, areas (Krebs 1971, Stamps 1987). For example, familiarity with a territory may decrease search times in foraging bouts, as birds may already be familiar with areas 80 Chapter 4 Maladaptive Preferences in Disturbed Habitat? of high arthropod abundance. There is some evidence that pairs breeding in both years of my study tend to locate their territories in the same general area, despite a concomitant upward shift in dominance rank (K. Fort unpubl. data). Altematively, breeding chickadees may focus only on obtaining an appropriate nest site, and defend a territory that encompasses it regardless of habitat composition. Nest-site selection in birds has been demonstrated in other studies (Clark and Shutler 1999, Chase 2002). If appropriate nest sites are limiting, and undisturbed chickadee habitat is reasonably homogeneous with respect to food abundance and predation risk, such a strategy may be adaptive. Although I did not test for selection for certain nest site characteristics, I found that certain variables associated with the cavity tree and the habitat immediately surrounding the nest site predicted nest success (chapter 2 ). 4,5.3. Territory Composition and Nest Success Unsuccessful birds had higher proportions of VRPINE habitat and tended to have lower proportions of DECMATURE habitat than successful birds. Chickadees breeding in disturbed habitat also had lower fledge success than those in undisturbed habitat (Chapter 2). Together, these results suggest that, although chickadees show no strong habitat preferences, birds breeding in territories containing high proportions of disturbed habitat experience lower reproductive success. In this context, their lack of strong preference or avoidance of particular habitat types may be seen as maladaptive. That is, birds are not responding to environmental features that lower their fitness. Such a scenario is unlikely to persist in evolutionary 81 Chapter 4 Maladaptive Preferences in Disturbed Habitat? timescales, as organisms will tend to develop more adaptive habitat preferences (‘niche conservatism’ sensu Holt 1995) or, altematively, adapt to habitat conditions in their new environments (Holt 1996). However, at ecological timescales, the phenomenon of maladaptive habitat selection may be quite common (Remes 2000, Delibes et al. 2001), especially when anthropogenic influences are considered. Blondel and Dias (1994) concluded that gene flow between populations of blue tits living in optimal (source) and suboptimal (sink) habitats explained maladaptive timing of breeding in the sink habitat (see above). Recapture data in my study site show that juvenile birds fledged in one habitat have settled in the other (K. Fort unpubl. data). As dispersal mechanisms in chickadees also allow free gene flow between disturbed and undisturbed habitats, and there is no evidence of settling bias based on condition in natal habitat (H. van Oort unpubl. data), maladaptive habitat selection behaviour may persist indefinitely. In general, resident species may be more at risk with respect to maladaptive behaviours, as evolutionary processes may have resulted in selection for site tenacity or nest-site preferences over specific territory-level habitat preferences in certain environments. If the environment is altered by anthropogenic disturbance, the advantages of territory familiarity may no longer outweigh the costs associated with breeding in sub-optimal habitat. If a maladaptive lack of habitat preference exists, it has population-level implications. Delibes et al. (2001), using a modelling approach, showed that hypothetical metapopulations that fail to avoid ‘sink’ habitats due to a lack of habitat 82 Chapter 4 Maladaptive Preferences in Disturbed Habitat? preferences experience steadily declining growth rates. Such metapopulations will eventually decline to extinction, especially if sink habitats increase in abundance across a landscape. Similarly, Pulliam and Davidson (1991) argue that the extent to which the presence of habitat sinks is damaging to metapopulation size depends critically on the selectivity of the organism. As anthropogenic disturbance continues to alter natural landscapes, the inability of organisms to respond to these changes may have serious conservation implications. 83 Chapter 4 Maladaptive Preferences in Disturbed Habitat? Table 4.1. 10 zone habitat classification system, Spring 2000. Zone Aspen Conifer Mix Marsh Birch Willow-Alder Lodgepole Variable Retention Mature Remnant Riparian_________ Dominant Canopy Species and Site Description Mature trembling aspen. Moderate understory. Mature hybrid spruce, subalpine fir, lodgepole pine. Sparse under story. Mature deciduous/coniferous mix. Moderate understory. Mature black cottonwood, senescent willow. Dense understory. Mature paper birch. Moderate understory. Early serai mix of willow, green alder, young aspen and conifers. Low canopy height. Early serai monoculture, lodgepole pine plantation. Low canopy height. Mature birch and aspen, low stem density due to partial harvesting. Dense understory. Mature douglas fir, lodgepole pine, birch. Sparse understory. Mature birch, senescent willow. Dense understory._____________________________ 84 Chapter 4 Maladaptive Preferences in Disturbed Habitat? Table 4.2. Cluster membership and mean values ± SE for 8 habitat variables for six clusters. Cluster sample sizes are reported in parentheses. Vegetation data collected Summer 2000. Cluster Conifer (3) DecMature (9) Remnant (3) WetMature (6) VRPine (6) Early Serai (3) Willow Density (stems/ha) Large Conifer Density (stems/ha) 1043.3 ± 433^ 0.0 ± 0.0 550.0 ± 75.0 169.4 ± 44.4 769.4 ± 95.9 0.0 ± 0.0 105.6 ± 58.6 4.0 ± 0.3 50 ± 0.0 375.0 ± 75.0 0.0 ± 0.0 275.0 ± 75.0 5.0 ± 0.4 3.4 ± 0.2 130 ± 47.0 230.0 ± 41.4 20.0 ± 9.4 45.0 ± 33.9 22.7 ± 9.7 5.6 ± 0.3 4.0 ± 0.4 20.8 ± 16.4 275.0 ± 98.3 0.0 ± 0.0 108.3 ± 80.0 37.8 ± 13.3 5.6 ± 0.3 4.3 ± 0.5 66.7 ± 44.1 75 ± 38.2 75.0 ± 62.9 58.3 ± 36.3 Canopy Height (m) Canopy Cover (%) Shrub Cover