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Electronic Theses and Dissertations Theses, Dissertations, and Major Papers

2010

The influence of visual environment and sympatry on plumage coloration in New World warblers

Allison Flora Mistakidis University of Windsor

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Recommended Citation Mistakidis, Allison Flora, "The influence of visual environment and sympatry on plumage coloration in New World warblers" (2010). Electronic Theses and Dissertations. 8283. https://scholar.uwindsor.ca/etd/8283

This online database contains the full-text of PhD dissertations and Masters’ theses of University of Windsor students from 1954 forward. These documents are made available for personal study and research purposes only, in accordance with the Canadian Copyright Act and the Creative Commons license—CC BY-NC-ND (Attribution, Non-Commercial, No Derivative Works). Under this license, works must always be attributed to the copyright holder (original author), cannot be used for any commercial purposes, and may not be altered. Any other use would require the permission of the copyright holder. Students may inquire about withdrawing their dissertation and/or thesis from this database. For additional inquiries, please contact the repository administrator via email ([email protected]) or by telephone at 519-253-3000ext. 3208. THE INFLUENCE OF VISUAL ENVIRONMENT AND SYMPATRY ON PLUMAGE COLORATION IN NEW WORLD WARBLERS

BY

ALLISON FLORA MISTAKIDIS

A Thesis Submitted to the Faculty of Graduate Studies through Biological Sciences in Partial Fulfillment of the Requirements for the Degree of Master of Science at the University of Windsor

Windsor, Ontario, Canada

2010

© Allison Flora Mistakidis 2010 Library and Archives Bibliotheque et 1*1 Canada Archives Canada Published Heritage Direction du Branch Patrimoine de I'edition

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1*1 Canada DECLARATION OF CO-AUTHORSHIP

I hereby declare that this thesis incorporates material that is result of joint

research, as follows: my advisor, Dr. Stephanie Doucet, assisted in project design, data

collection, statistical analyses, and editorial comments in the preparation of each

manuscript. Chapters 2 and 3 have been prepared for submission to American Naturalist

and Proceedings of the Royal Society of London Series B: Biological Sciences,

respectively.

I am aware of the University of Windsor Senate Policy on Authorship and I

certify that I have properly acknowledged the contribution of other researchers to my

thesis, and have obtained written permission from each of the co-author(s) to include the

above material(s) in my thesis.

I declare that, to the best of my knowledge, my thesis does not infringe upon

anyone's copyright nor violate any proprietary rights and that any ideas, techniques, quotations, or any other material from the work of other people included in my thesis, published or otherwise, are fully acknowledged in accordance with the standard referencing practices. Furthermore, to the extent that I have included copyrighted material that surpasses the bounds of fair dealing within the meaning of the Canada

Copyright Act, 1 certify that I have obtained a written permission from the copyright owner(s) to include such material(s) in my thesis.

I declare that this is a true copy of my thesis, including any final revisions, as approved by my thesis committee and the Graduate Studies office, and that this thesis has not been submitted for a higher degree to any other University or Institution.

iii ABSTRACT

Several factors may drive interspecific divergence, and investigating possible mechanisms from multiple angles may be most efficient for understanding the evolutionary history of diverse groups. The main objective of my thesis was to examine mechanisms that may promote plumage divergence in Parulidae warblers. In my first study, I investigated the signalling environments of 17 sympatric species of warblers to test predictions of sensory drive. I determined sexual selection has likely influenced plumage evolution in warblers, plumage and visual environment coloration were correlated, and warblers were not most conspicuous in chosen display environments. In my second study, I tested the relationship between sympatry and plumage divergence across 77 species and found an increase in sympatry is correlated with greater plumage divergence, and that closely related species tended to be highly sympatric. My thesis demonstrates multiple selective pressures have likely shaped the evolution of plumage coloration in this diverse group of birds.

IV ACKNOWLEDGEMENTS

Who knew how much a field course in Costa Rica could change one's life? I would sincerely like to thank Dr. Stephanie Doucet and Dr. Daniel Mennill for teaching me more in the two weeks I spent in the jungle than I'd learned in an entire semester of second year Ecology. Dan, if not for your Ornithology class, I can't say I'd be here right now. Thank you for the enthusiastic introduction to birds, the unforgettable field course in Costa Rica, and for suggesting I ask the "new faculty member" about doing an honours thesis. Steph, I loved being one of the founding members of the Doucet Lab when I started my fourth year. I'm happy I've been around to see your lab grow, and I'm not sure how I'll feel when I don't have to come in anymore, to be quite honest. I thank you for taking me on when you really didn't know all that much about me, and for keeping me around once you knew all I was about. I really appreciate all the support you've given me throughout these past three years, and I don't doubt you've been a positive influence on my life in general. Thank you so much for being a great mentor and friend.

I'd like to thank my graduate committee members, Dr. Melania Cristescu and Dr.

Trevor Pitcher, for their helpful suggestions about my research and for their guidance throughout my tenure as a graduate student.

No person in their right mind can measure 898 specimens at museums without a little help. I'd like to thank Melissa Abdellah, David Bradley, Celia Chui, Karen

Cogliati, Julie Koloff Holly Hennin, and Dave Reis for helping me measure the extensive Parulidae collection at the University of Michigan; it took several trips to

Noodles, but we got it done! I'd also like to thank my Mom for coming along when I took the spectrometer on the road to Toronto and New York; you had a tough job, I

v know. I'd especially like to thank the curators and collection managers at each museum for allowing me to measure the specimens: Dr. Dave Mindell and Janet Hinshaw at the

University of Michigan Museum of Zoology, Dr. Joel Cracraft and Peg Hart at the

American Museum of Natural History, and Mark Peck at the Royal Ontario Museum.

For help with field work, I'd like to thank Stephanie Doucet for being the superb birder that she is; if I had to spot all of those warblers on my own, my sample size probably would have been five. I'd also like to thank the staff at Queen's University

Biological Station, especially Floyd Connor for his assistance with my project, as well as station manager Frank Phelan.

I'd like to thank Paul Grzeszczak for GIS help and for preparing the degree of sympatry matrix comprising all possible species pairs of warblers. I would still be working on this part currently if not for his expertise and generous donation of personal time and energy.

I would like to thank the gracious photographers who allowed me to use their photos. Credits are as follows: Raymond J. Barlow: blackbumian warbler (Fig. Lie) and black-throated blue warbler (Figs 1. If and 3.2e). Brandon Holden: cerulean warbler (Fig.

Lib). Chuck Musitano: yellow warbler (Fig. 1.1a), chestnut-sided warbler (Fig. Lie), and black-throated green warbler (Fig 3.2b). Nick Saunders: blackpoll warbler (Lid).

Liz Stanley: blackbumian warbler (Fig. 3. Id). Ron Wolf: Townsend's warbler (Fig.

3.1a).

To all of the members of the Doucet Lab, both past and present, thanks for your help with my presentations and the preparation of these manuscripts, but most of all thanks for making my time here enjoyable: Melissa Abdellah, Irina Bobeica, Michelle

vi Bondy, Fabio Castelli, Celia Chui, Karen Cogliati, Jessica Cuthbert, Jillian Faraci, Daniel

Hanley, Dave Reis, Katie Rieveley, Summer Ross and Kara-Anne Ward. I've really loved getting to know all of you, and can only hope we remain friends for a very long time. Keep the kettle clean.

To my very best friends - Hoa, Ivana, Karen, Laura, Sarah and Silvia - thanks for pushing me when 1 needed it, for helping me keep my sanity, and inspiring me in so many ways. I would have enjoyed this time in my life far less if it weren't for all of you being a part of it.

To my family - my gigantic, ever expanding family - thank you for understanding when I was too busy to deal with you, for encouraging me to do my best, and for reminding me what's important in life. Just know that I love you, and I'm looking forward to this thing called 'free time' in which I will be able to find more of what I've been missing. Also, to my Grandpa, for constantly asking when I'm going to get a job (bientot, esperons-le!), and Tiayia (Yiayia), for demanding I find a husband and start a family (ta Ppouue evav KC^.6 dv0pco7ro, unv avncruxeiTE!) — thanks for the laughs and support. Mom, I appreciate how you are always proud of me, no matter what

I do. I can't wait for us to go on a vacation that doesn't involve work! And Dad, thanks for never questioning where I'm headed in life, and for understanding me better than I understand myself. I'm grateful for all of the opportunities you've given me.

Time for the next step.

vn TABLE OF CONTENTS

DECLARATION OF CO-AUTHORSHIP iii ABSTRACT iv ACKNOWLEDGEMENTS v LIST OF TABLES x LIST OF FIGURES xi CHAPTER 1 1 EXPLAINING DIVERSITY 2 SEXUAL SELECTION 4 INTERSPECIFIC DIVERGENCE 7 Environmental pressures 7 Influence of sympatry 9 STUDY SYSTEM 11 TECHNIQUES AND CHALLENGES 15 Quantifying and analyzing colour 15 Use of museum specimens in comparative analyses 16 LITERATURE CITED 19 CHAPTER 2 28 SYNOPSIS 29 INTRODUCTION 29 METHODS 34 Plumage reflectance spectra 34 Field methods 35 Irradiance and background spectra 35 Avian colour space modelling 36 PCA of plumage reflectance spectra 38 PCA of ambient light and background reflectance spectra 39 RESULTS 40 Sex differences in conspicuousness 40 Interspecific differences in display environments 42 Correlation between plumage colour and signalling environment 43 Do species-specific display environments increase conspicuousness? 44 DISCUSSION 44 Conspicuousness and sexual dichromatism in warbler plumage 45 Warblers exhibit species-specific differences in display environment 47 Plumage coloration is correlated with the visual environment 49 Plumage is not maximally conspicuous in preferred visual environments 51 CONCLUSIONS AND IMPLICATIONS 53 LITERATURE CITED 55 CHAPTER 3 67 SYNOPSIS 68 INTRODUCTION 68 METHODS 73 Plumage reflectance 73 Principal components analysis of plumage reflectance spectra 74

Vlll Plumage divergence matrix 74 Degree of sympatry matrix 75 Genetic distance matrix 75 Testing correlation between distance matrices 76 RESULTS 77 Plumage divergence vs. degree of sympatry 77 Plumage divergence vs. genetic distance 78 Genetic distance vs. degree of sympatry 79 DISCUSSION 79 LITERATURE CITED 85 CHAPTER 4 95 THESIS SUMMARY AND DISCUSSION 96 FUTURE DIRECTIONS 99 Visual modelling 99 Influence of sympatry 101 SUMMARY AND SIGNIFICANCE 101 LITERATURE CITED 103 VITAAUCTORIS 106

IX LIST OF TABLES

Table 1.1. Average linear measurements for six representative species of warbler in the genus Dendroica 26

Table 2.1. Average values of ecological variables of display sites for 17 species of wood warblers 59

Table 2.2. Average values of visual environment variables of display sites for 17 sympatric species of wood warblers 60

Table 2.3. Generalized linear regression analyses of the influence of background and irradiance variables on plumage 61

x LIST OF FIGURES

Figure 1.1. Representative photos of six warblers in the genus Dendroica 27

Figure 2.1. Factor loadings for PCA analyses on plumage, and background reflectance and irradiance, with example spectra 63

Figure 2.2. Mean contrast against the background for males and females and mean dichromatism across body region 65

Figure 2.3. Relationship between chromatic contrast against the background and sexual dichromatism for both sexes 66

Figure 3.1. Factor loadings for PCA analyses conducted on plumage data and representative reflectance curves 92

Figure 3.2. Breeding ranges and representative photos of both an allopatric and sympatric pair of species in the genus Dendroica 93

XI CHAPTER 1: GENERAL INTRODUCTION EXPLAINING DIVERSITY

"Seeing this gradation and diversity of structures in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends ... it at once struck me that under these circumstances, favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be the formation of new species. " (Darwin 1845)

In the 150 years since the publication of On the Origin of Species, biologists have often relied on Darwin's (1859) theory of natural selection to explain the diversity of life on Earth. Natural selection outlines how favourable traits - ones that enhance a species' survival - will be passed on to the next generation (Darwin 1859). If isolated populations of a species adapt to divergent local environments, they can potentially acquire enough distinctive traits in allopatry that they will be reproductively isolated from one another when they come into contact, and then continue to evolve as separate species (Darwin

1859). Darwin's most fundamental theory was heavily shaped by his encounter with a diverse group of ground finches on the island archipelago of the Galapagos; the different species diverged in size and shape to such an extent that Darwin had originally misclassified them as wrens, blackbirds, grosbeaks and finches. These birds are so intricately linked with his work that they are now simply referred to as 'Darwin's finches'. Through adaptation to different food resources, Darwin's finches evolved divergent body size and beak morphology, which in turn promoted reproductive isolation amongst populations based on different beak sizes and subsequent song divergence

2 (Grant and Grant 2008). The Galapagos finches remain one of the best examples of

speciation promoted by divergent natural selection. However, natural selection is not the

sole force promoting the evolution of diverse and varied forms of species. Sexual

selection may also exert an important influence on speciation, as it can drive divergence

in characteristics that are also used as indicators of species identity (Andersson 1994).

For example, beak morphology also affects song production in Darwin's finches through

physiological constraints, such that species with large beaks produce less complex songs

with slower trills (Podos 2001). Female preference for conspecific male song maintains

reproductive isolation among closely related sympatric species (Podos 2001; Grant and

Grant 2002; Podos and Nowicki 2004). In this case, natural selection for divergent beak

size also promoted divergence in song, a sexually selected trait that influences female

mate preference and, consequently, reproductive isolation.

Alternatively, sexual selection can be the primary agent of change, through the

action of female preferences for divergent male traits (Panhuis et al. 2001). For example,

sexual selection has been established as a driving force in the speciation of African

haplochromine cichlids in Lake Victoria. Their interspecific divergence in coloration is

astounding, and female choice for coloration of conspecific males has been recognized as

the sole isolating mechanism among sympatric species (Seehausen and van Alphen 1998;

Seehausen et al. 2008). Likewise, behavioural isolation between populations of

Amazonian frogs (Physalaemus petersi) is driven by female choice for divergent male

song, which also supports sexual selection promoting speciation (Boul et al. 2007).

Overall, sexual selection can influence speciation either directly through the creation of behavioural isolation between populations, or indirectly through an increase in

3 divergence of sexually selected traits in isolated species (Panhuis et al. 2001). Often in examples of adaptive radiations, closely related species are effectively isolated from one another based on divergence in a single trait (Grant and Grant 2008). Thus, how interspecific divergence arises and is fixed within populations has become a fundamental aspect of speciation research.

Several factors may influence the evolution of interspecific diversity, and investigating multiple components in tandem may paint a broader picture of a group's evolutionary history. In the sections below, I first describe the theory of sexual selection, and explain how female preference for extravagant male traits may have played an important role in producing the diversity of species we observe today. Then, I focus on specific factors, such as the external environment or amount of range overlap between species, which may drive differences in sexually selected traits and in turn promote reproductive isolation. Finally, I introduce my study system, the Pamlidae warblers, and explain how their extraordinary divergence renders them a compelling group within which to investigate evolutionary forces promoting diversity.

SEXUAL SELECTION

Elaborate ornamental traits in males, which appear to contradict evolution by natural selection, inspired Darwin's theory of sexual selection (Darwin 1871). Unlike traits promoted through the stmggle for survival, sexually selected traits arise either due to competition between males for access to females, or female selection on male traits.

Sexual selection is defined by the existence of variable reproductive success among individuals; males who express the trait most desired by females will amass the greatest

4 reproductive success (Andersson 1994). Extensive research on the topic of sexual selection has focused on how females choose their mates, and has also attempted to explain the existence of conspicuous and seemingly detrimental secondary sex traits

(Searcy 1982; Kirkpatrick and Ryan 1991; Maynard Smith 1991).

Several non-mutually exclusive theories have been proposed to explain why females prefer and choose elaborately ornamented males (Kokko et al. 2002; Andersson and Simmons 2006). For example, females may prefer males that display conspicuous traits because they indicate the general quality of the male, based on his ability to survive while bearing these costly ornaments (Fisher 1915). Such traits may be indicators of an individual's health or parasite resistance, and females choose males based on these heritable benefits for their offspring (Trivers 1972; Zahavi 1975; Zahavi 1977; Andersson

1994). Additionally, females may prefer conspicuous traits because they indicate the male's ability to provide her with direct benefits, such as food resources and contribution to parental care (Hoelzer 1989; Moller and Jennions 2001). Preference for a trait may also arise based on previous affinity in the female sensory system, such as partiality for certain colours based on foraging ecology (Fisher 1930; Basolo and Endler 1995;

Dawkins and Guilford 1996; Endler and Basolo 1998; Ryan 1998). A male ornament will be favoured if it exploits previously established female sensory abilities through sensory bias (Endler and Basolo 1998). Regardless of how female preferences arise, it is possible for the combination of female preferences and elaborate male traits to co-evolve in concert (Fisher 1930; Andersson 1994). Runaway selection describes the rapid co- evolution of female preferences and male traits, wherein the alleles for the preference and for the trait become linked in future generations (Fisher 1915, 1930). Variability in male

5 traits within a population presents the opportunity for females to develop preferences for

different male traits, and for new preferences and traits to become fixed in the population

(Lande 1981, 1982; West-Eberhard 1983). Besides direct selection by females,

ornamental traits are influenced by various other mechanisms which may influence their

evolution. Forces such as natural selection for crypsis, selection for species recognition,

and environmental adaptation are only a few mechanisms which can impact the evolution

of sexually selected traits (Higgie and Blows 2007; Stuart-Fox et al. 2007; Hegyi et al.

2008).

Because sexually selected traits tend to be especially divergent across species,

some researchers have proposed that they may also serve as important indicators of

species identity (West-Eberhard 1983; Price 2008). Therefore, divergence in secondary

sex traits between species might assist in recognition of conspecifics (Wallace 1889;

Fisher 1930; Maynard Smith 1978). Accelerated divergence among populations in sexually selected traits, and associated recognition of those traits, may result in the generation of premating reproductive isolation (Lande 1981; West-Eberhard 1983; Price

1998). Often, closely related species show extensive divergence in sexually selected characteristics; this has encouraged researchers to suggest a role for sexual selection in speciation (West-Eberhard 1983; Price 1998). To test this hypothesis, several studies have investigated whether indices of sexual selection intensity, such as sexual dichromatism, were correlated with species richness in a particular group; a positive correlation suggests that sexual selection is at least partly responsible for increased proliferation of species (Barraclough et al. 1995; Moller and Cuervo 1998; Owens et al.

1999; Panhuis et al. 2001; Stuart-Fox and Owens 2003). However, more recent studies

6 did not find evidence of sexual selection promoting speciation (Gage et al. 2002; Morrow

et al. 2003). Additionally, sexual selection has also been proposed to drive extinction,

and estimating speciation rates from only extant species presents a conundmm (Morrow

and Pitcher 2003). Although these studies explore the direct relationship between sexual

selection and speciation, the correlation is rarely explicit. In fact, sexual ornaments are

often highly labile signals, which are shaped by many selective factors (Kusmierski et al.

1997; Omland and Lanyon 2000; Stuart-Fox and Moussalli 2008). One such factor is the

environment and location in which sexual ornaments are displayed. Adaptation to

variation in the signalling environment could potentially drive divergence in sexual

signals, and thus indirectly promote reproductive isolation between species.

INTERSPECIFIC DIVERGENCE

Environmental pressures

The signalling environment presents multiple selective factors which may promote divergence in ornamental traits. Firstly, displaying a conspicuous trait in any

environment can increase the chance of predation. A seminal example in guppies

(Poecilia reticulata) demonstrated how predation risk can drive sexual ornaments to be

less conspicuous (Endler 1980). In both natural and artificial stream experiments, male guppies originally from low predation areas that were placed under situations of high predation showed a dramatic reduction in ornamental coloration, with fewer and smaller spots on each fish in subsequent generations (Endler 1980). In general, flashy ornaments favoured by sexual selection are disfavoured by natural selection because they attract predators as well as mates (e.g. Stuart-Fox et al. 2003; Kemp et al. 2008). Isolated

7 populations subject to variable predation pressure could potentially diverge in ornamental

traits, and subsequently become reproductively isolated from one another.

Additionally, how and where a signal is transmitted will influence how it is

perceived (Endler 1992). There are several ways in which the signalling environment

may influence interspecific divergence in ornamental traits. Firstly, it is possible for

species to colonize and adapt to novel environments. For example, the Grand Cayman

anole (Anolis conspersus), which directly descended from A. grahami of Jamaica, has a

bright blue and UV-reflective dewlap in contrast to the orange and yellow dewlap of its

ancestor (Macedonia 2001). Colonization by the ancestral A. grahami to the new

environment on Grand Cayman, which had different ambient light conditions than

Jamaica due to vegetation structure, likely promoted the divergence observed in dewlap

and body coloration (Macedonia 2001). Many studies in various taxa have demonstrated

how the environment can shape which types of signals are used, as certain signals are

favoured under certain environmental conditions (Bradbury and Vehrencamp 1998;

Fuller 2002; Leal and Fleishman 2004; Uy and Endler 2004; Seddon 2005; Schultz et al.

2008; Barker and Mennill 2009). While sexual signals have been investigated in multiple modalities and in a diversity of habitats, a significant amount of research has been devoted to investigating how visual signals are influenced by the terrestrial environment.

Since visual signals are surrounded by elements of the visual background in any particular environment, the coloration of the signal and the background are often highly correlated (Endler 1992). Whereas naturally selected coloration tends to match the background to enhance crypsis (Endler 1992), sexually selected ornamental coloration is expected to contrast maximally against the background to effectively attract mates

8 (Andersson and Simmons 2006). Because environments differ in background coloration, the ideal coloration for optimizing conspicuousness will vary in dissimilar environments

(Endler 1992, 1993a, b). As a consequence, adaptation to different signalling environments can promote divergence in colourful traits. For example, the intensity of red nuptial coloration in male sticklebacks (Gasterosteus spp) is negatively correlated with the amount of ambient red light in their environment; species inhabiting reddish environments display less red coloration to increase contrast against the background

(Boughman 2001). In isolated populations with variable environments, male coloration is optimized for maximal conspicuousness based on ambient light, and the perceptual ability of females matches the condition of the local environment (Boughman 2001,

2002). The correlated evolution between the environment, the perceptual abilities of females, and male coloration in sticklebacks supports the predictions of sensory drive, which emphasizes the role of the environment in the evolution of signals and sensory systems (Endler 1992). In sticklebacks, female preference for conspecific male coloration is what maintains reproductive isolation between closely related species in sympatry (Boughman 2001, 2002). Clearly, the need for mechanisms of reproductive isolation should be greatest when closely related species coexist in sympatric overlap.

Influence of Sympatry

When two or more related species are in close contact, accurate species recognition is important for maintaining species integrity. Errors in mate choice can be very costly, since mate search efforts are wasted on unsuccessful reproduction or unfit, unviable hybrid offspring (Coyne and Orr 2004). Effective premating isolation

9 mechanisms can ensure correct species recognition and prevent maladaptive

hybridization. Character displacement occurs when selection for minimized competition between overlapping species directly drives divergence in reproductive, morphological,

or physiological traits (Brown and Wilson 1956; Pfennig and Pfennig 2009). When

closely related species are in sympatric overlap, they are expected to diverge in traits used in species recognition to ensure accurate mate choice among conspecifics (Saetre

2000). Thus, sympatry could potentially drive interspecific divergence in traits to enhance species recognition through natural selection against unfit hybrids (Dobzhansky

1941; Coyne and Orr 2004). In European Ficedula flycatchers, sexually selected plumage coloration is an important premating isolation barrier between closely related species (Saetre et al. 1997). When in allopatry, male pied flycatchers (F. hypoleuca) are very similar in coloration to collared flycatchers (F. albicollis); both species are mostly black with white patches on the forehead and neck. However, sympatric populations of these species have evolved divergent plumage coloration compared to their allopatric counterparts: the pied flycatcher is dull brown, and the collared flycatcher is black, with a much larger white neck ring (Saetre et al. 1997; Saetre 2000). Mate choice trials reaffirm that divergent coloration in sympatry facilitates species recognition and accurate female choice for conspecific males (Saetre et al. 1997). A number of studies have found patterns of sympatric divergence in several taxa, including Drosophila (Noor 1995;

Coyne and Orr 1997; Noor 1997), crickets (Honda-Sumi 2005), sticklebacks (Rundle and

Schluter 1998), frogs (Gerhardt 1994; Hobel and Gerhardt 2003), and birds (Kirschel et al. 2009). Most of these studies have investigated the influence of sympatry on a small scale, by investigating divergence between a pair of species with populations in sympatry

10 and allopatry. It may be interesting to investigate whether sympatry drives divergence when there are varying degrees of overlap between several closely related species.

STUDY SYSTEM

Wood warblers (Family: Pamlidae) comprise some of the most intriguing avian species in the Western Hemisphere, with bright and bold plumages, diverse songs, and dramatic migrations - often on an intercontinental scale. Parulid warblers are small, primarily insectivorous, and socially monogamous songbirds that often display marked sexual dichromatism in plumage coloration (Curson et al. 1994). While they are largely socially monogamous, some warblers have been reported to engage in extra-pair copulations (e.g., Stutchbury et al. 1997; Chuang et al. 1999; Thusius et al. 2001;

Yezerinac and Weatherhead 1997). A high incidence of extra-pair mating often increases variability in male mating success, and, consequently, the intensity of sexual selection

(Amqvist and Kirkpatrick 2005). Although different warbler species diverge extensively in sexually selected ornaments such as plumage and song (Shutler and Weatherhead

1990), they are often quite similar with respect to morphology (Lovette and Bermingham

1999, 2002). The genus Dendroica is considered the most diverse of all parulid genera, with most species displaying several contrasting plumage patches, including eye stripes, throat patches, wing bars and streaked tails (Fig 1.1; Lovette and Bermingham 1999;

Dunn and Garrett 1997). Some examples of these dramatically ornamented species are the blackbumian warbler (D.fusca; Fig Lie), which has bright white wing patches, a yellow breast, and jet black cheeks surrounded by a brilliant orange throat patch and eyebrows, and the black-throated blue warbler (D. caerulescens; Fig 1.If), which has a white belly and undersides, a black throat, and dark blue upperparts from head to tail

11 (Dunn and Garrett 1997). These differences are even more remarkable when we consider that these plumage ornaments are produced by entirely different mechanisms including carotenoid pigmentation, melanin pigmentation, incoherent scattering, and structural coloration (Hill and McGraw 2006). While these species differ remarkably in plumage coloration (Fig. 1.1), they are very similar in morphology (Table 1.1). These patterns of exceptional plumage divergence and relatively fixed morphology lend support for sexual selection driving such diversity (Lovette and Bermingham 1999).

The two above-mentioned species, along with several others, are often found in highly sympatric distributions. Warblers' ability to coexist in sympatry with limited interbreeding and hybridization has long fascinated researchers, and a few studies have postulated that ecological niche differentiation plays a role in successful isolation

(MacArthur 1958; Lovette and Hochachka 2006). However, the exceptional divergence in plumage coloration between species in this group suggests a role for sexual selection in either promoting speciation or maintaining reproductive isolation between species upon secondary contact (Price, 2008; Price, 1998).

Several hypotheses have been put forth to explain how Pamlidae warblers evolved. Warblers reach their greatest diversity in Central America, and it is has been proposed that the evolution of warblers began in this area (Mayr 1946). The genera

Basileuterus and Myioborus most likely entered South America when the continents were still separated, and subsequently diverged independently of North American genera

(Curson et al. 1994; Mayr, 1946). Researchers have speculated that most speciation in

North American warblers occurred in the Pliocene period, due to range expansion and colonization during interglacial periods (Morse 1989; Price et al. 2000; Lovette and

12 Bermingham 2002; Lovette 2005). Geological features formed during glacial retreats presumably presented isolating barriers between warbler populations, which then diverged from one another in allopatry (Bermingham et al. 1992). Indeed, most avian populations are considered to have diverged in allopatry, and subsequent expansion into each others' ranges likely produced the high sympatric overlap observed today

(Phillimore et al. 2008; Price 2008).

Latitudinal variation in pamlid warblers is quite obvious, as temperate species are highly sexually dichromatic long-distance migrants, whereas tropical species are typically monochromatic and sedentary (Curson et al. 1994). The unique pressures of the temperate zone, including less time to establish territories and find mates, most likely promoted intensified sexual selection and possibly drove the evolution of dramatic sexual dichromatism observed in temperate species (Chek et al. 2003; Weir and Schluter 2007).

In addition, pamlid species in the tropics appear to exhibit much less intrageneric sympatry with one another compared to the extensive overlap observed between species in the temperate zone, even within a single genus (Mistakidis and Doucet, pers. obs.).

Dramatic shifts in breeding ranges and range expansions during interglacial periods probably enabled more frequent secondary contact between temperate species when they shifted into each others' ranges, which may have promoted interspecific phenotypic divergence (Martin et al. 2009). This potential for rapid adaptation on an intercontinental scale highlights Pamlidae as an ideal family within which to investigate the various evolutionary forces driving divergence in ornamental traits.

In general terms, the main objective of my thesis was to investigate some of the mechanisms that might be responsible for promoting or enhancing plumage divergence in

13 Pamlidae warblers. In Chapter 2, we characterized the signalling environments of 17

sympatric species of pamlid warblers, and used avian colour space models to assess

visual conspicuousness. We sought to determine whether sexual selection has influenced

plumage evolution relative to the signalling environment, whether warblers displayed in

species-specific microhabitats and used different visual environments for display,

whether differences in habitat actually correlate with differences in plumage coloration,

and finally whether plumage signals are adapted to be optimally conspicuous in chosen display environments. We predicted that sexual selection has influenced plumage evolution, and that as a result male plumage ornaments would contrast more against the background and sexual dichromatism would be positively correlated with increased background contrast. We also predicted that warblers would utilize different microhabitats for display, that plumage coloration would be correlated with the coloration of the visual background, and that species would be most conspicuous in the environments in which they chose to display to enhance recognition by conspecifics. In

Chapter 3,1 explored the impact of sympatry by testing the relationship between sympatric overlap and plumage divergence across species pairs for a total of 77 warbler species. I predicted that plumage coloration would increase in divergence with increasing degree of sympatry. My thesis demonstrates that multiple selective pressures have likely shaped the evolution of plumage coloration in this diverse family of birds, and also supports a role for plumage divergence in maintaining reproductive isolation between species.

14 TECHNIQUES AND CHALLENGES

Quantifying and analyzing colour

The advent of reflectance spectrometry for colour quantification was a major advancement in colour research, which had its beginnings with subjective comparisons to paint chips, colour charts, and human-biased assessments of colour rank (Montgomerie

2006). Reflectance spectrometry allows researchers to objectively quantify coloration across the entire visible spectmm of most bird species, from 300 to 700 nm (Bennett and

Cuthill 1994; Vorobyev et al. 1998), which extends beyond the range of human visual acuity. Over the past decade, reflectance spectrometry has become the method of choice for collecting coloration data, although several methods exist for interpreting and explaining variation in colour (Andersson and Prager 2006; Montgomerie 2006). These include calculating tristimulus colour variables (hue, saturation, and brightness), summarizing spectral data using multivariate statistics, or analyzing spectra using segment classification and visual modelling (Endler 1990; Montgomerie 2006). Certain methods of analysis may be more appropriate depending on the type of question being addressed. For example, when the aim is to compare coloration across several species and multiple patches of colour have been measured, multivariate approaches such as principal components analysis (PCA) can successfully summarize the variation in reflectance data (Montgomerie 2006). PCA generates orthogonal variables from raw spectra without making assumptions about the source of the variation, and generates principal component scores for use in further analyses comparing coloration among individuals or species; its ability to summarize large amounts of highly variable colour

15 data has made it one of the most commonly used multivariate methods of analysis in avian colour research (Montgomerie 2006).

One method of colour analysis that actually takes into account how a colour may be perceived in the environment is referred to as avian visual modelling (Vorobyev et al.

1998; Osorio et al. 1999; Montgomerie 2006). These models evaluate the colour of a patch while taking into account the spectral sensitivity of the visual receiver and data from the display environment, such as the irradiance spectra of ambient light and background reflectance spectra, to evaluate how the patch is perceived in that particular environment (Vorobyev et al. 1998). Cone-type photoreceptor sensitivities have been evaluated in detail for only a relatively small number of species as this technique requires highly specialized equipment and also necessitates killing the birds under study (Odeen and Hastad 2003). However, it has been established that most oscine passerines have very similar cone types, which allows spectral sensitivity data from other related species to be used instead (Hart et al. 2000; Hart 2001; Hart and Vorobyev 2005). Using avian colour space modelling not only allows us to evaluate how a plumage ornament is perceived, but also how perception can be altered based on the location and behaviour of the signaler within the environment.

Use of museum specimens in comparative analyses

Specimen collections from natural history museums are an invaluable tool for investigating the taxonomic history and the compelling diversity of earth's creatures

(Winker 2004). In addition, the vast availability of specimens enables compilation of large amounts of data in a relatively short amount of time, which would pose extreme logistical challenges, and would perhaps be unnecessarily invasive, if live animals were

16 used instead. Museum specimens have been used to investigate a large number of research questions related to plumage coloration, such as the evolution of sexual dichromatism (Mahler and Kempenaers 2002; Eaton 2005), the relative ubiquity of ultraviolet plumage reflectance in birds (Eaton and Lanyon 2003), and patterns of geographic variation (Chui and Doucet 2009). In addition, museum specimens have been used in several studies evaluating the contrast and conspicuousness of colour patches in present day light environments (Endler and Thery 1996; McNaught and Owens 2002;

Doucet et al. 2007).

The potential disadvantage of using museum specimens is the risk that specimen coloration is not representative of coloration in wild birds. Previously, Endler and Thery

(1996) noted that the amount of fading in the plumage of museum specimens used in their study did not significantly affect the interpretation of their results. Although possible feather degradation or fading is an important factor to consider, the variability of coloration observed within a species due to fading should be much smaller than variability observed between species (Doucet and Hill 2009). A recent study investigated the effect of specimen age on plumage coloration in a large diversity of species and found that there was very little difference between live birds and specimens that were younger than 50 years old (Armenta et al. 2008). Other studies found only modest (Doucet and

Hill 2009) or no effects of fading (Chui and Doucet 2009) for specimens in good condition. It is important, then, to account for potential individual variation of specimens by measuring several individuals for each species. It is also important to choose specimens that appear in good condition, that are relatively young and/or of similar age, and that are from similar geographic locations to minimize possible sources of variation.

17 We preferentially chose young specimens in good condition from similar geographic locations to minimize the potential impact of specimen degradation and geographic variation on our dataset. Our methodology thereby minimized the potential variability introduced in our measurements through the use of museum specimens, and maximized the possibility of resolving questions about plumage evolution in a fascinating avian family, which may have been impossible to address using alternate means.

18 LITERATURE CITED

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25 Table 1.1: Average linear measurements for six representative species of warbler in the genus Dendroica. Values are given as mean length in millimetres or as a range in millimetres where means were unavailable. See Figure 1.1 for photographs of each warbler. Latin Common Wing Tail Tarsus Bill Name Name (mm) (mm) (mm) (mm) References D.caerulescens black-throated 65.2 51.1 18.7 9.4 Holmes et al. blue warbler 2005 D. cerulea cemlean 64.5 42.4 15.7 9.6 Hamel 2000 warbler D. fusca blackbumian 67.8 48.3 17.5 9-10.5 Curson et al. warbler 1994; Morse 2004 D. pensylvanica chestnut-sided 63.2 50.3 18.3 9.9 Richardson and warbler Brauning 1995 D. petechia yellow 64.2 45.1 17-20 8.0 Curson et al. warbler 1994; Lowther etal. 1999 D. striata blackpoll 74.2 51.3 19.1 9.9 Hunt and Bonita warbler 1999

26 Figure 1.1: Representative photos of six warblers in the genus Dendroica. A) yellow warbler, D.petechia, B) cemlean warbler, D. cerulea, C) chestnut-sided warbler, D. pensylvanica, D) blackpoll warbler, D. striata, E) blackbumian warbler, D. fusca, F) black-throated blue warbler, D. caerulescens. All photos copyrighted by respective photographers and used with permission; see Acknowledgements for photo credits. CHAPTER 2: SYMPATRIC WOOD WARBLERS UTILIZE DIFFERENT SUBSETS OF THE FOREST FOR DISPLAY! A TEST OF SENSORY DRIVE SYNOPSIS

Environmental pressures can potentially drive divergence in ornamental traits, and

many researchers have found strong correlations between the mating signal and the

environment in which it is displayed. We characterized the visual signalling environments for 17 species of warbler living in sympatric overlap in Northern Ontario,

and used avian visual colour modelling techniques to assess plumage conspicuousness in their chosen environments. We found male plumage ornaments to be more conspicuous against the visual background and found a positive correlation between male contrast and sexual dichromatism, indicating that sexual selection may have driven male ornaments to be highly conspicuous in the environment. We also found species subdivided the habitat for display, and as a result the composition of the visual background was significantly different between species. Contrary to expected results, however, warbler plumage ornaments do not appear to be adapted for maximal conspicuousness in their species- specific display environments. Possibly, species may be under more pressure to differ in coloration from sympatric species sharing similar environments than to be maximally conspicuous. Overall, our findings suggest that sensory drive has influenced plumage coloration in warblers.

INTRODUCTION

The incredible diversity of signals that animals use to communicate has stimulated extensive research on how such signals evolve. Since signals are transmitted through the environment to a receiver, multiple aspects of the environment can influence or alter the signal and, ultimately, how that signal is perceived. The behaviour of the

29 signaler, the signalling environment, the content of the signal as well as the perceptual capacity of the receiver are all potential factors driving the evolution of animal signals, and the interaction between these processes has been termed 'sensory drive' (Endler

1992). According to this hypothesis, a species could develop a tendency to display in environmental conditions that are favorable for optimal signal transmission. Likewise, a species could evolve a signal that is optimally transmitted in its native habitat. The sensory system of the receiver should also evolve to enhance signal reception (Endler

1992). In the context of sexual selection, sensory drive predicts that evolution will favor the production of conspicuous signals that are easily detected by conspecifics and potential mates (Endler and Basolo 1998).

Support for the influence of sensory drive on sexually selected traits has been demonstrated in a number of different taxa and across multiple signalling modalities

(Losos and Chu 1998; Fuller 2002; McNaught and Owens 2002; Smith et al. 2004;

Seehausen et al. 2008). Several studies have documented general correlations between signals used in visual communication and the habitat in which they are perceived. In

Phylloscopus warblers, for example, species breeding in dark habitats have more numerous bright colour patches than those breeding in open habitats (Marchetti 1993).

Since light environments and visual backgrounds are often temporally and spatially heterogeneous, males may also evolve preferences for displaying in environmental conditions that enhance their conspicuousness (Endler 1993). Accordingly, a recent study on homed beetles Coprophanaeus lancifer found that body coloration was more conspicuous in crepuscular light environments than other diurnal environments, corresponding with the time of day this species is most active (Thery et al. 2008).

30 Since sensory drive supports the potential for adaptation to different signalling

environments and divergence in sexually selected traits, it is often implicated in

speciation (Boughman 2002). A study of the Crested anole Anolis cristatellus determined

that two populations occupying distinct light habitats exhibited habitat-specific divergent

dewlap coloration, such that signal detectability was enhanced in each population's native habitat, and reduced when evaluated in another population's habitat (Leal and Fleishman

2004). Divergence of dewlap coloration between allopatric populations could promote reproductive isolation, thereby implicating sensory drive as a potential mechanism driving speciation in anoline lizards (Leal and Fleishman 2004). One of the most comprehensive examples of speciation by sensory drive has been documented through extensive research on African cichlids (Maan et al. 2004; Seehausen and Schluter 2004;

Maan et al. 2006; Terai et al. 2006; Seehausen et al. 2008; Magalhaes et al. 2009).

Collectively, these studies have demonstrated that male coloration is correlated with the light environment in each species' habitat, and that females exhibit preferences for conspecific males that are mediated in part by differences in visual sensitivities. Alleles for visual pigments representing divergent sensitivities were found to be strongly correlated with male nuptial coloration among sympatric cichlid species, such that females of each species are spectrally tuned to the nuptial coloration of conspecific males

(Seehausen et al. 2008). Thus, sexual selection and sensory drive appear to drive the co- evolution of visual signals and receiver sensitivities in this system.

The majority of studies investigating adaptation to visual environments in terrestrial systems have focused on birds. Several studies have found that plumage coloration is correlated with ambient light environments (Marchetti 1993; McNaught and

31 Owens 2002; Gomez and Thery 2004), and that birds display in specific subsets of the

environment that maximize the conspicuousness of their displays (Endler and Thery

1996; Heindl and Winkler 2003b; Doucet et al. 2007). Forests are particularly

structurally complex, creating an environment that is heterogeneous with respect to the

type and colour of ambient light and backgrounds available for display (Endler 1993).

We investigated how several sympatric species of warblers differentially used display microhabitats within a forest, and whether their chosen display environments enhanced

the conspicuousness of their plumage ornaments.

Wood warblers (family Pamlidae) are a diverse group of songbirds with a large distribution comprising most of North and South America (Lovette and Bermingham

2002). These birds exhibit strikingly diverse plumage ornamentation, and closely related species often differ considerably in coloration. Among warblers, the genus Dendroica represents one of the most well-documented adaptive radiations in birds (Lovette and

Bermingham 1999). In contrast with other avian groups wherein closely related species are separated by geographical barriers, Dendroica warblers are commonly found in high levels of local sympatry (Lovette and Bermingham 1999). Sympatric bird species usually exhibit divergent morphological characteristics, such as bill shape or size, to facilitate coexistence through occupation of different ecological niches (Grant 1986). The high level of sympatry among Dendroica warblers is particularly perplexing since these birds lack obvious morphological differentiation. MacArthur's (1958) seminal work suggests that Dendroica warblers can coexist in sympatry because they utilize different niches within the forest. Through observations of five sympatric Dendroica species, MacArthur

(1958) determined that they occupied species-specific microhabitats, such that each

32 species subdivided resources within a coniferous tree. This work on Dendroica warblers is now widely recognized as a classic example of competitive exclusion (MacArthur

1958). Although such microhabitat specialization surely facilitates species coexistence within Dendroica by reducing competition for resources, multiple mechanisms are likely involved in maintaining reproductive isolation between these closely related species.

Here, we investigate whether signalling microhabitat specialization may have influenced plumage divergence in pamlid warblers through sensory drive.

We characterized the signalling environments of 17 sympatric species of pamlid warblers and used avian colour space models to assess visual conspicuousness with specific aims to answer the following questions. Firstly, has sexual selection influenced plumage evolution relative to the signalling environment in warblers? If so, we predict that male plumage will be more conspicuous than female plumage, and that male contrast against the background will increase with degree of sexual dichromatism across species.

Secondly, do these warblers display in species-specific microhabitats? If so, we predict that species will display in different locations within the forest, and that those differences will translate into the use of different visual environments for display. Thirdly, do differences in microhabitats correlate with differences in plumage coloration? If so, we predict that species-specific plumage coloration will correlate with components of the signalling environment. Finally, are plumage signals adapted to be optimally conspicuous in chosen display environments? If so, we predict that plumage ornaments should be more conspicuous in their corresponding, species-specific signalling environment rather than the signalling environments of other species.

33 METHODS

Plumage Reflectance Spectra

We measured the plumage reflectance of museum specimens at the University of

Michigan Museum of Zoology in 2007 and 2008. We measured five male and five

female specimens for each of the 17 species for which we collected visual environment

data. Previous researchers used similar sample sizes per species when evaluating the

relationship between visual environment and plumage coloration (McNaught and Owens

2002; Doucet et al. 2007), and measuring five representatives per sex potentially controls

for intraspecific variability within species when specimens are from similar geographic

locations. We measured spectral reflectance using a USB4000 Spectrometer and a PX-2

pulsed xenon lamp (Ocean Optics, Dunedin, FL). A bifurcated fiber-optic cable was used

to deliver light from the light source to the specimen and from the specimen to the

computer (R-400-UV-VIS, Ocean Optics). The reflectance probe was mounted in a matte black mbber holder that excluded all external light and maintained the probe at a

fixed distance from (~5 mm), and perpendicular to, the measured surface. All reflectance spectra were expressed as the percentage of reflectance relative to a white standard (WS-

1, 97-98% incident light; Ocean Optics). We measured eight body regions on each museum specimen and collected five readings per region. We used the means of these five readings in our analyses. When selecting specimens, we chose birds captured in breeding plumage and during the breeding season, which excluded birds collected during molt. We recorded relevant information from the specimen voucher, including sex, capture date, and capture location.

34 Field Methods

We measured the display environments of singing warblers at Queen's University

Biological Station in Elgin, Ontario, Canada in May of 2008. In total, we measured

characteristics of the visual environment for 119 individual warblers representing 17

sympatric species present at this location. As warblers sing to attract mates, we

considered a male's singing location to be its preferred environment for attracting a

female. We located each warbler by ear and recorded specific attributes of its singing

location and visual environment, namely: date, time, type of light at song perch (sun,

shade), tree type and species, perch height, tree height, composition of the visual

background (percentage represented by vegetation, bark and sky), percent canopy cover

at display perch, and percent cloud cover; percentages were estimated visually to

approximately 5 %.

Irradiance and Background Spectra

At the display location for each warbler, we measured ambient light spectra using

a cosine-corrected fiber-optic probe (P400-10-UV-VIS, Ocean Optics) with a measuring

surface of 6 mm in diameter (CC-3-UV, Ocean Optics) and an angle of acceptance of

180°. The spectrometer (USB4000, Ocean Optics) was calibrated using a calibration light

source of known colour temperature (LS-l-CAL, range 300-1050 nm, Ocean Optics) prior to each ambient light measurement. To measure the ambient light at the specific perch location, we used one or two extension poles (model #6618, 1.8-5.4 m, Mr.

LongArm, Butler, MO) to raise the probe to the appropriate height. We collected 20 ambient light spectra at the height of the display perch for each individual, as well as 20 spectra at the same height but two metres away to serve as a random control. We

35 assigned the control measurement's location using a compass and increasing the bearings by ten degrees for each successive location. We measured ambient light between the hours of 0556 and 1225 (EST), and all light measurements were collected at the display site immediately following the observation of a warbler singing from that specific location.

At each display perch, we also collected samples of the vegetation and bark comprising the visual background of the singing bird for reflectance analysis. We measured spectral reflectance of these samples using the same methods described above for collecting plumage reflectance from museum specimens. We collected five reflectance measurements per vegetation or bark sample and used means in our analyses.

Avian Colour Space Modelling

To approximate how different patches of colour are perceived by a typical passerine bird, we used a previously developed model of avian colour space (Vorobyev and Osorio 1998; Vorobyev et al. 1998). This model incorporates the sensitivities of avian photoreceptors and transmission of ocular media (Hart et al. 2000), photoreceptor noise (Maier 1992), and the ambient light environment and visual background (Endler

1990; Thery 2006). Diurnal birds have four types of single-cone photoreceptors: long- wavelength sensitive (LWS), medium-wavelength sensitive (MWS), short-wavelength sensitive (SWS), and ultraviolet/violet sensitive (UVS/VS). With the exception of the

VS/UVS cone, the wavelength of maximum sensitivity appears generally conserved across avian species within each cone type (Hart et al. 2000). While some birds have VS type cones, which have a peak absorbance near 410 nm, most oscine passerines have

UVS type cones with a peak absorbance near 370 nm (Bowmaker et al. 1997; Hart et al.

36 2000; Hart 2001). Since spectral sensitivities have not been measured in warblers, we

used spectral sensitivity data from the blue tit Parus caeruleus, an oscine passerine with

UVS cones, to estimate photoreceptor quantum catches for each species.

We calculated the photoreceptor quantum catches for eight plumage patches

across the 17 species of warblers, separating them by sex. The photoreceptor quantum

catch (Q,), which is the proportion of total quantum catch for each of the four avian

photoreceptors, is calculated as:

_ /i?!(A)5(A)/(A)Oa)dA Ql ~ /. Rt (X)dX

where X represents wavelength, R,(k) represents the spectral sensitivity of receptor type /,

S(X) is the reflectance of the colour patch, I(k) is the irradiance of the illuminant, and

0(X) is the transmission of the ocular media. We normalized all data for spectral

sensitivities, irradiance, and ocular transmission to I. These values spanned the range

from 300 to 700 nm. We calculated the photoreceptor quantum catches for each of the

four avian single cone types (UVS, SWS, MWS, and LWS). We then calculated colour

distance, AS, which describes the discriminability between two colours:

2 2 2 AS = [(^K) CA/4- sf3y + (chchy(\fA- sf2y + O-^-CVJ- A/2) 2 2 2 + (^^mA-A/i) * (ra2«4) (A/3-A/1) 2 2 2 2 2 + (a3o)4) (Af2- A/1) ]/[(

space, A/,' is the log ratio of the quantum catches for each of the four avian receptor types, and co, indicates the noise-to-signal ratio (Weber fraction). The Weber fraction is

modeled as follows under bright viewing conditions:

37 CO, = —=

v Vi

where v represents the noise-to-signal ratio in a single photoreceptor of type /' and 7 is a

scaling factor which accounts for the relative number of photoreceptors of type /. The

noise-to-signal ratios were estimated from the red-billed Leiothrix Leiothrix lutea (Maier

1992), and we used data from the blue tit to estimate the relative abundance of receptor

types (Hart et al. 2000).

Principal Components Analysis of Plumage Reflectance Spectra

We performed Principal Components Analysis (PCA) to summarize variation in

reflectance spectra (Endler 1990). We began by summarizing mean reflectance values

into 10-nm-wide bins using CLR v. 1.05 (Montgomerie 2008) and then ran a PCA on the

entire dataset, including all 17 species and eight body regions for each species. We

retained the three principal components with eigenvalues larger than one for our analyses,

which together explained 99% of the variation in plumage reflectance. The first principal

component (PCI) explained 88% of the variation in reflectance, PC2 explained 8%, and

PC3 explained 3% (Fig 2.1a). PCI, with loadings that were moderate and positive across

the visual spectmm, represents plumage brightness; high PC 1 scores would indicate a bright plumage patch. The factor loadings for PC2 were positive at shorter wavelengths and negative at longer wavelengths; thus, high positive scores for PC2 would indicate proportionally high reflectance at UV or blue wavelengths and lower reflectance at longer wavelengths. PC3 had factor loadings that were high and positive at ultraviolet wavelengths, negative at blue/green wavelengths, and moderately positive at longer wavelengths. Thus, a spectmm with positive values for PC3 would reflect proportionally

38 more at ultraviolet and red wavelengths than in the blue/green region of the spectmm.

Example plumage reflectance spectra are given in Fig. 2.1b.

Principal Components Analysis of Ambient Light and Background Reflectance Spectra

We also performed PCA on the ambient light spectra, obtaining 38 mean irradiance values (in 10-nm-wide bins) from 330 to 700 nm for each spectmm. We omitted wavelengths below 330 nm because the low intensity of our calibration source at those wavelengths resulted in artificially high ambient light spectra for this range when working in open habitats. We obtained three principal components that explained 99% of the variation in irradiance, with PCI explaining 93%, PC2 explaining 5%, and PC3 explaining 1% (Fig. 2.1c). As PCI loadings were moderate and positive across all wavelengths, we again interpret this as a brightness axis, where high PCI scores would indicate a bright irradiance spectmm. PC2 had positive values at short wavelengths (330

~ 450 nm) and negative values for long wavelengths, hence a positive score for PC2 would indicate an ambient light spectmm with a high proportion of short wavelength irradiance. PC3 was positive at very short wavelengths (330-400 nm) as well as in the green/yellow region of the spectmm (500-600 nm), which means that a positive score for

PC3 would indicate a spectmm with proportionally high UV and green/yellow irradiance.

The mean irradiance spectra for three types of light environments we encountered in the field (sunny, overcast and forest shade) are shown in Fig. 2. Id.

To summarize the variation in reflectance of background coloration, we ran PCA on the entire background coloration dataset, including vegetation and bark samples for all species. We retained three principal components that explained 98% of the variation in background reflectance, with PC 1 explaining 88%, PC2 explaining 6%, and PC3

39 explaining 4% (Fig. 2.1e). PCI represents the brightness of background reflectance, as the factor loadings were again moderate and positive across all wavelengths. PC2 had factor loadings that were negative at short wavelengths and showed a clear peak in the green region of the spectmm at 550nm, which indicates that high PC2 scores represent green background reflectance spectra. The factor loadings for PC3 were high and positive at low wavelengths (300-400nm) and show a similar moderately positive peak in the green region of the spectmm. High scores for PC3 would thus indicate a background spectrum which reflects highly in UV and green wavelengths. Mean reflectance spectra for green vegetation and bark are shown in Fig. 2.If.

We did not control for phylogenetic relatedness in our analyses because we were interested in the influence of the environment on plumage coloration irrespective of the genetic relationship between these species. Moreover, sympatric groups of warblers tend to be closely related, and their plumage coloration diverges readily even among closely related species pairs (Chapter 3). This lack of congmence between plumage divergence and genetic relatedness suggests that plumage coloration is unlikely to be constrained by phylogeny. All statistical analyses were carried out in JMP v. 6.0 (SAS Institute Inc).

RESULTS

Sex Differences in Conspicuousness

We used AS to compare differences in avian perceptual colour space between plumage colour and the visual background - plumage patches with higher AS values should be more conspicuous to warblers as they should contrast against the background.

We used each species' mean green vegetation background reflectance for this comparison, since green vegetation encompassed the highest proportion of the visual

40 background for all species (Table 2.2). We calculated chromatic contrast separately for

males and females as well as separately across body regions, and we averaged across

plumage regions to obtain species means. Contrast against the background varied

significantly by sex and body region, with breast, crown, throat, and mmp contrasting the

most against the background (Fig 2.2a; two-way ANOVA, F= 3.95, df = 15, P < .0001;

sex: F = 19.64, P< .0001; region: F = 4.45, P< .0001; sex x region: F= 1.21, P= .30), and males exhibited higher mean contrast against the background than females (paired t- test:/ = -3.96,n= 17,P=.001).

To quantify sexual dichromatism, we evaluated the distance in perceptual colour space between analogous plumage patches in males and females and averaged these values to obtain mean chromatic dichromatism values for each species. We found that the degree of sexual dichromatism was not evenly distributed across body regions, with throat and crown exhibiting the highest dichromatism scores (Fig 2.2b; ANOVA, F =

6.78, df = 7, P < .0001). There was also a significant positive relationship between sexual dichromatism and mean chromatic contrast against the background in males (Fig 2.3a; r =

0.66,F = 11.43, df= 1, 16, P = . 004) but not females (fig 2.3b; r = 0.04, F = 0.02, df= 1,

16, P = .89). When we considered each body region separately, there was a significant positive correlation between sexual dichromatism and chromatic contrast in male plumage for the breast (r = 0.57, F= 7.28, df = 1, 16, P = .02), crown (r = 0.73, F =

17.58, df = 1, 16, F = .0008), flanks (r = 0.63, F= 9.98, df= 1, 16, P= .007), mmp (r =

0.59, F= 8.05, df= 1, 16, P= .01), throat (r = 0.73, F= 17.28, df= 1, 16, P= .0008), and upper wing (r = 0.66, F = 11.69, df = 1, 16, P = .004), but not the mantle (r = 0.06, F =

0.06, df= 1, 16,F=.81)oroutertail(r = 0.31 ,F= 1.61, df= 1, 16, P = 0.22). There

41 was no significant relationship between dichromatism and chromatic contrast in females for any region (all P > .42).

Interspecific Differences in Display Environments

Warbler species displayed at different locations within our study site.

Specifically, there were significant interspecific differences in the height of the trees in which they sang, their song perch heights, and the percentage of canopy cover at their song perches (Table 2.1). Warblers also displayed more often in deciduous trees rather than coniferous trees and shmbs (Table 2.1). Similarly, warblers exhibited interspecific variation in visual signalling environments. In particular, there were significant differences between species in the percentage of the visual background comprised of bark, vegetation or sky (Table 2.2). Although cloud cover could also influence the amount and colour of light available in which to display (Endler 1993), there were no interspecific differences in percent cloud cover at the time birds sang (Table 2.2). The ambient light present in chosen display locations was significantly different between species for PCI scores, but not for PC2 or PC3 (Table 2.2). When we compared the mean ambient light measurements recorded for males of each species to mean control measurements taken two metres away from the song perch, we found that the control spectra were significantly higher than chosen ambient light spectra for PC 1 and PC3

(paired /-tests, n= 17; PCI: t = 2.24, P = .04; PC3: / = 2.58, P = .02), whereas PC2 scores were not significantly different between chosen and control spectra (PC2: t = 0.24, P =

.81).

42 Correlation Between Plumage Colour and Signalling Environment

To determine whether plumage coloration was correlated with components of the visual environment, we ran multiple regressions with plumage colour PC scores as the dependent variables, and bark, green vegetation, and ambient light PC scores as predictor variables. We found that plumage coloration was significantly correlated with several components of the visual environment, with each plumage PC score showing differential correlations with background and ambient light PC scores (Table 2.3). In males, plumage

PCI was significantly predicted by green vegetation PC3, as well as body region. Male plumage PC2 was predicted by bark (PC 1 and PC2), green vegetation (PC2 and PC3), as well as irradiance (PCI). Finally, male plumage PC3 was significantly correlated with bark (PCI and PC2) as well as irradiance (PC3) and body region. In females, plumage

PC 1 was significantly predicted by green vegetation (PC3) as well as body region.

Female plumage PC2 was only significantly predicted by body region, PC3 was significantly correlated with all predictors with the exception of irradiance PC2.

When we evaluated each body region separately, we found similar correlations between plumage scores and the signalling environment. In males, flank plumage PCI was negatively correlated with ambient light PC2 (p'= -0.97, df = 1, 9, F= 6.45, P = .04), and mantle plumage PC2 was positively correlated with bark PC2 (P'=1.67, df = 1, 9, F =

6.04, P = .04). Also, male flank plumage PC3 was positively correlated with bark PC2

(P'= 1.77, df = 1, 9, F= 5.65, P= .05), and mmp plumage PC3 was negatively correlated with bark PCI (p"= -2.52, df = 1, 9, F = 6.33, P = .04). In females, mantle plumage PC2 was positively correlated with ambient light PC3 (p"= 0.95, df = 1, 9, F= 5.68, P = .05);

43 no other female plumage PC scores were predicted by components of the signalling environment.

Do Species-Specific Display Environments Increase Conspicuousness?

To determine whether species were more conspicuous in their own display environments, we used a random number generator to evaluate each species' contrast against the background in a randomly selected species' display environment. There were no significant differences between average species-specific and random background contrasts (paired /-tests: males: t = 1.37, n = 17, P = .19; females: / = 1.77, n = 17, P =

.10). Although these differences were not significant, more than half of the species

(12/17) exhibited greater chromatic contrast in their original environment (binomial test,

P = 0.09). We also tested for differences separately by body region and again found that species-specific contrasts did not differ from random contrasts (paired /-tests: all P >

0.10).

DISCUSSION

Sensory drive is thought to enhance communication through co-evolutionary interactions among signals, signalling environments, and perceptual systems. We used avian colour space modelling to test predictions generated by models of sensory drive in relation to visual communication in wood warblers. Our findings suggest that sensory drive may have played an important role in the evolution of colourful sexual ornaments in warblers. First, our data revealed that male plumage ornaments were more conspicuous than female plumage ornaments as demonstrated by higher chromatic contrast against the background. There was also a positive correlation between sexual dichromatism and

44 chromatic contrast against the background in males but not females, suggesting an

influence of sexual selection on male plumage conspicuousness. In general, different

species of warblers exhibited significant differences in both singing locations within the

forest as well as the composition of their visual environments, supporting our hypothesis

that species are utilizing different subsets of the habitat for their displays. Moreover,

plumage coloration was correlated with components of the visual environment, as

predicted by sensory drive. Contrary to our predictions, however, species were not more

conspicuous in their species-specific display environments.

Conspicuousness and Sexual Dichromatism in Warbler Plumage

In the context of sexual selection, sensory drive should favor the evolution of

courtship signals that are easy to detect and therefore conspicuous (Andersson 1994;

Endler and Basolo 1998). In wood warblers, females should be the choosy sex due to greater investment in offspring (Bent 1963; Kokko and Monaghan 2001), whereas males should be the competing sex due to variation in male mating success, including extrapair matings (Andersson and Iwasa 1996). Thus, if plumage traits are important in sexual selection, sensory drive should favor the evolution of conspicuous male plumage in warblers. As predicted, male plumage coloration had higher chromatic contrast against the background than female plumage. Although sexual dichromatism is commonly thought to result from the evolution of elaborate ornaments in males (Darwin 1871;

Andersson 1994), it is also possible that natural selection is promoting crypsis in female plumage (Wallace 1889; Irwin 1994). Female warblers are solely responsible for incubation and may be under greater pressure to be cryptic than males, who only participate in feeding the young (Martin and Badyaev 1996; Dunn 1997). Either

45 mechanism supports the influence of the environment, and thus the role of sensory drive.

Since female conspicuousness was evaluated using male display environments, it is possible that female plumage conspicuousness is even lower than what we have reported here.

If sexual selection favors signals that are easily detected by conspecifics, then we anticipate a positive correlation between the degree of sexual dichromatism and the degree of chromatic contrast against the background. In Neotropical manakins, the degree of male contrast against the background was significantly correlated with the degree of sexual dichromatism, whereas female contrast did not show this pattern

(Doucet et al. 2007). Thus, increased sexual selection on male plumage to exhibit greater contrast against the background is likely partly responsible for evolution of the extreme dichromatism observed in manakins (Doucet et al. 2007). Similarly, we also documented a positive association between sexual dichromatism and male contrast against the background in warblers for all plumage regions with the exception of the mantle and outer tail. Sexual selection should not only promote colourful ornaments in males, but also drive male contrast against the background. If the degree of sexual dichromatism can serve as a proxy for the intensity of sexual selection (Barraclough et al. 1995; Mailer and Cuervo 1998; Owens and Hartley 1998; Dunn et al. 2001; Morrow et al. 2003), this suggests that sexual selection has driven warbler plumage ornaments to be highly conspicuous in the species-specific display environments used by males. The fact that some male plumage regions did not show this partem could indicate that natural selection for crypsis may limit overall conspicuousness (Endler 1978; Endler 1991), or that some body regions are more important as sexual ornaments, an idea supported by variation in

46 the extent of sexual dichromatism across body regions in warblers. A negative

correlation between chromatic contrast against the background and sexual dichromatism

in females might indicate dichromatism evolved partially due to strong natural selection

for crypsis (Martin and Badyaev 1996); however, our findings did not show this pattern.

Thus, dichromatism in warbler plumage coloration appears to have evolved primarily due

to strong sexual selection for conspicuous male plumage ornaments.

Warblers Exhibit Species-Specific Differences in Display Location and Visual Environment

Sexually-selected traits are thought to be important in maintaining reproductive

isolation in closely related species (Saetre et al. 1997; Boughman 2001, 2002; Price

2007). As wood warblers exhibit an unusually high degree of sympatry (Lovette and

Bermingham 1999; Lovette and Hochachka 2006), the use of different visual

environments for displays could promote reproductive isolation among species. Indeed,

warblers exhibited species-specific differences in signalling location with respect to the

height of their display trees, the height of their song perches, and the composition of their

visual backgrounds. Perhaps competition for display environments caused sympatric

species to display in different signalling environments to prevent competitive exclusion

(Gause 1935), whereby warblers partitioned the habitat to prevent overlap of signalling

localities between species. Alternatively, differences in the typical niches occupied by

these species during non-display activities could have facilitated divergence in signalling environments. Most species, however, perform displays and broadcast songs from atypical subsets of their habitat (Dunn 1997). Variation in display location could be due, in part, to selection for specific song transmission characteristics (Slabbekoom et al.

47 2007). A non-mutually exclusive possibility is that birds chose to display in different locations to take advantage of different types of light environments.

Interspecific differences in display location translated into interspecific differences in visual environments used by wood warblers. In particular, warbler display sites differed in the composition of their visual backgrounds, the amount of cloud cover present during their displays, whether the ambient light consisted primarily of sun or shade, as well as the relative brightness of the display location. Variation in visual environments influences the conspicuousness of visual signals (Endler 1992), and males of each species may seek out certain light environments for display. Forests, in particular, offer a variety of different light environments to exploit for signalling purposes (Endler 1993). For example, a previous study showed that three species of manakins displayed at different heights in the same forest and that differences in display environments at these different strata maximized the conspicuousness of each species' plumage displays (Heindl and Winkler 2003b). In general, more blue and green signals should be favored at greater heights in the forest, whereas red and orange signals should be favored in the forest understory (Endler 1993). Cemlean warblers Dendroica cerulea, which are largely blue in colour, displayed on exposed branches high in the forest canopy as expected. Similarly, ovenbirds Seiurus aurocapillus, which are mostly brown with orange crowns, displayed in the forest understory. Factors other than display height are also known to influence visual environments in forests. For instance, signalling environments with extensive green vegetation provide effective contrast for long wavelength signals, such as oranges or reds (Endler 1993). Accordingly, among the species we sampled, American redstarts Setophaga ruticilla, which are strikingly

48 patterned in orange and black, had the highest percentage of green vegetation comprising their visual background. Some Neotropical lekking birds preferentially display with a portion of their bodies illuminated by sunny gaps penetrating the canopy. For example, the white-throated manakin Corapipo gutturalis, displays with its white throat and chest in part of a sunny gap, while its black back and sides are in forest shade. This display behavior enhances the brightness of the white throat and chest, and increases its contrast against the dark body coloration (Endler and Thery 1996). Similarly, some warblers appeared to preferentially display in sunny gaps, such as blackbumian warblers D. fusca, or out in the open on low vegetation in sunny conditions, such as yellow warblers D. petechia and common yellowthroats G. trichas.

Warbler Plumage Coloration is Correlated with the Visual Environment

Sensory drive should select for signals that either match or contrast against the visual background, depending on whether the signal function is crypsis or conspicuousness, and this process should lead to a correlation between visual signals and the signalling environment (Endler 1992; Endler and Basolo 1998). In chameleons, for example, variation in display coloration is predicted by habitat structure, and males in closed habitats exhibit both higher relative UV reflectance and higher contrast against the background (Stuart-Fox et al. 2007). In warblers, we found a number of correlations between plumage coloration and visual display environments. In general, plumage brightness (PCI) for both sexes was significantly predicted by green vegetation PC3, which is a measure of the extent of UV and green wavelengths reflected by the background vegetation. This result implies that plumage brightness in warblers is correlated with the green background of the forests in which they display. In contrast to

49 PCI, the other principal components for plumage coloration were differentially predicted

by environmental variables between the sexes. Plumage PC2 scores differentiate short

versus long wavelength colours; high PC2 scores correspond to plumage ornaments with

UV or blue wavelengths, whereas low PC2 scores correspond to orange or red plumage.

Male plumage PC2 was predicted by the coloration of bark (PCI, PC2), green vegetation

(PC2, PC3) and ambient light (PCI) at display sites. Plumage PC2 scores represent

colours frequently found in warbler plumage ornaments, suggesting the evolution of

chromatic components of male plumage is influenced by multiple aspects of the display

environment.

When we evaluated the relationships by body region, male mantle PC2 colour

was positively correlated with bark PC2 colour. Most species' visual backgrounds were

dominated by either green vegetation or sky, with bark often comprising the lowest

percentage of the visual background. It is possible the positive correlation between male

plumage and bark coloration may aid in male crypsis during non-display activities, such

as when foraging or engaging in activities at the nest. Female plumage PC2, in contrast,

does not appear to be influenced by the environment as it was not predicted by any

environmental variables. Since females do not typically display ornamental coloration in

the region of the spectmm represented by PC2, or if they do it is significantly reduced

from the corresponding male coloration, female PC2 scores were not expected to be

correlated with the visual environment. The third principal component for plumage

coloration, PC3, differentiates reflectance between short and medium wavelength regions of the spectmm; high PC3 scores are associated with plumage ornaments with UV reflectance, whereas low PC3 scores correspond to green or blue plumage ornaments.

50 Male plumage PC3 was significantly predicted by only a few environmental variables, namely bark (PCI, PC2) and irradiance (PC3). Female plumage PC3, on the other hand, was significantly predicted by nearly all of the environmental variables, with the exception of irradiance PC2. If female warbler plumage is selected to be cryptic, then we would expect this high correlation between plumage PC3 and background coloration which is often rich in blue/green wavelengths (Endler 1993; Endler 2000). This correlation of warbler plumage coloration and signalling environments, in combination with interspecific differences in display environments, implies the potential for warblers to be adapted for display in species-specific display environments.

Warbler Plumage is Not Maximally Conspicuous in Preferred Visual Environments

When several closely-related species display in different visual environments, sensory drive predicts that species will show divergent colour signals to enhance optimal conspicuousness in their respective environments (Endler 1992). For example, five of six species of congeneric pond damselflies showed maximal contrast against the visual background during the time of day when each species was most active, thus temporally subdividing the shared pond habitat (Schultz et al. 2008). If warbler plumage ornaments are adapted for optimal conspicuousness in species-specific environments, plumage coloration could potentially be most conspicuous in each species' respective signalling environment. Our findings suggest that while warbler species display in distinct visual environments, they do not appear to be maximally conspicuous in their chosen environments. There are a number of potential explanations for these findings. First, temperate forests are highly variable with respect to weather and leaf phenology, and as a result experience tremendous temporal variation in light environment, both diurnally and

51 throughout the breeding season of most birds (Chiao et al. 2000; Moyen et al. 2006). In

contrast, some tropical animals can consistently exploit distinct, established light

environments for signalling purposes during the breeding season. In some lek-mating

species, for example, lekking location and timing are chosen based on the ambient light

available for display to maximize contrast against the background (Endler and Thery

1996; Heindl and Winkler 2003b; Uy and Endler 2004; Doucet et al. 2007). The

increased variability of temperate signalling environments may prohibit warblers from establishing the tight associations with specific display locations observed in the tropics.

Another possibility is that the signalling microhabitats used by each species are inherently similar as they are subdivisions of the same habitat, and there may only be a

limited number of ways for each species to increase its conspicuousness in the same environment without converging on the signals used by other species. In the woodland shade environment used by most warblers we measured, a signaler is predicted to use blue, green, and UV signals to maximize brightness or red and orange signals to maximize chromatic contrast (Endler 1993). Many of the warbler species we measured do have plumage ornaments that satisfy Endler's (1993) predictions, although some species do not fit this pattern, including blackpoll warblers D. striata and black-and-white warblers M. varia, both of which are black and white in colour. Since plumage signals are important in species recognition (Saetre et al. 1997; Price 1998; McNaught and

Owens 2002), sympatry may drive divergence in plumage ornaments to reinforce reproductive isolation. Thus, it is possible that selection for species recognition conflicts with selection for optimal conspicuousness, and this may be especially important in groups with such a high incidence of sympatry (Schluter and Price 1993). Indeed, we

52 found evidence for the strong influence of sympatry on warbler plumage in a large

comparative study of 77 warbler species (Chapter 3). Species recognition is also

undoubtedly facilitated by the complex songs of warblers, which have distinct features

that enable effective discrimination between species (Lemon et al. 1983). It is possible

that strong sexual selection in this highly sympatric group has favoured the evolution of

complex song and plumage ornaments in tandem to provide conspecifics with multiple

species recognition cues.

Alternatively, warblers may be unable to use maximally conspicuous plumage

signals due to predation pressure (Shutler and Weatherhead 1990; Haskell 1999; Morton

2005). Natural selection for crypsis will reduce a species' ability to be conspicuous when

predation pressure is high (Endler 1987; Endler 2000). Several species have established

signalling behaviour which enables them to be conspicuous while displaying for potential

mates, but remain cryptic at other times to avoid predation (Endler 1991; Stuart-Fox and

Ord 2004; Gomez and Thery 2007). Thus, plumage ornaments in warblers potentially

represent a balance between naturally-selected cryptic coloration and sexually-selected

conspicuous signals.

CONCLUSIONS AND IMPLICATIONS

In summary, signalling environment appears to have had an important influence

on the evolution of plumage coloration in warblers. Sexual selection has promoted greater chromatic contrast against the background in males, and heterospecific males

exhibit significant differences in the location and visual environment of their displays.

Significant associations between plumage coloration and environmental variables in

53 warblers indicate that plumage and environment are inextricably linked in both sexes.

Although several studies have investigated the influence of signalling environment on plumage coloration (Marchetti 1993; McNaught and Owens 2002; Heindl and Winkler

2003a; Uy and Endler 2004; Doucet et al. 2007; Gomez and Thery 2007), our study represents the first to test predictions across several closely-related sympatric species.

Since temperate forests exhibit an abundance of potential signalling microhabitats, much more research on microhabitat use during signalling is needed. Recently, shorter diversification rates between species were found at higher latitudes, indicating that the most recent speciation events have occurred in the temperate zone (Weir and Schluter

2007). The subdivision and utilization of divergent habitats for sexual signalling may be one avenue for divergence among closely related species. Sensory drive may thus impart a strong influence on trait evolution and potentially speciation, as the adaptation to divergent signalling environments could either promote speciation or maintain reproductive isolation between recently diverged species.

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58 Table 2.1: Average values of ecological variables of display sites for 17 sympatric species of wood warblers measured in Southeastern Ontario. Values are given as mean ± SE. Species Name N Most Tree Perch Canopy Common Tree Height (m) Height (m) Cover (%) Type Dendroica 5 Deciduous 15.4 ± 1.8 11.6± 1.1 30.0 ±8.9 caerulescens Dendroica cerulea 10 Deciduous 15.8 ± 1.3 11.3 ±0.8 33.8 ±6.3 Dendroica 5 Deciduous 13.5 ± 1.8 8.0 ±1.1 36.0 ±8.9 coronata Dendroica fusca 6 Coniferous 16.8 ± 1.6 12.1 ±1.0 34.2 ± 8.2 Dendroica 2 Both 6.0 ±2.8 4.5 ±1.8 10.0± 14.1 magnolia Dendroica 3 Deciduous 6.2 ±2.3 4.9 ± 1.5 48.3 ±11.5 pensylvanica Dendroica 12 Deciduous 5.6 ± 1.2 3.9 ±0.7 6.8 ±5.8 petechia Dendroica pinus 3 Deciduous 19.3 ±2.3 12.7 ± 1.5 28.3 ±11.5 Dendroica striata 3 Deciduous 16.0 ±2.3 12.7 ± 1.5 53.3 ±11.5 Dendroica virens 4 Deciduous 16.6 ±2.0 9.6 ± 1.3 48.8 ±10.0 Geothlypis trichas 11 Both 3.8 ± 1.2 2.6 ±0.8 16.6 ±6.0 Mniotilta varia 9 Deciduous 14.1 ± 1.3 9.9 ±0.9 33.7 ±6.7 Seiurus 13 Deciduous 10.3 ± 1.1 6.7 ±0.7 40.0 ±5.5 aurocapillus Seiurus 2 Deciduous 13.3 ±2.8 11.8 ± 1.8 7.5 ±14.1 noveboracensis Setophaga ruticilla 15 Deciduous 12.9 ± 1.0 8.0 ±0.7 76.3 ± 5.2 Vermivora 13 Deciduous 8.1 ±1.1 6.6 ±0.7 12.2 ±5.5 chrysoptera Vermivora 3 Deciduous 11.1 ± 2.3 8.5 ± 1.5 28.3 ± 11.5 ruficapilla XorF 37.78 8.92 10.5 8.2 P 0.0016 < .0001 < .0001 < .0001 Note: Values are given as mean ± SE. Significance tests are from ANOVAs or Chi- Square tests. Significant F-values are indicated in boldface (P < .05). Percent canopy values were arcsine transformed for analyses.

59 Table 2.2: Average values of visual environment variables of display sites for 17 sympatric species of warblers measured in Southeastern Ontario. Values are given as mean ± SE. Species N Preferred Ambient Ambient Ambient Cloud Bark Vegetation Sky Perch Light PCI Light PC2 Light PC3 Cover (%) (%) (%) (%) Light D. caerulescens 4 Shade -3.7 ±2.6 0.2 ±0.6 -0.1 ±0.3 23.0 ±17.9 6.8 ±5.5 49.6 ±11.2 43.6 ±11.8 D. cerulea 10 Shade -3.1 ± 1.7 0.1 ±0.4 0.1 ±0.2 32.5 ±12.6 5.6 ±3.9 46.6 ±7.9 47.8 ±8.3 D. coronata 5 Shade 0.5 ±2.4 -1.0 ±0.6 -0.04 ±0.3 0.4 ±17.9 31.6±5.5 40.0 ±11.2 28.4 ±11.8 D. fusca 6 Both 0.8 ±2.2 -1.0±0.5 0.02 ±0.3 27.5 ±16.3 18.3 ±5.0 43.8 ± 10.3 37.8 ± 10.7 D. magnolia 2 Both 5.8 ±3.7 -0.1 ±0.9 0.1 ±0.5 12.5 ±28.3 3.5 ±8.7 49.0 ± 17.8 47.5 ±18.6 D. pensylvanica 3 Both 5.7 ±3.0 0.2 ±0.7 -0.9 ±0.4 20.0 ±23.1 5.7 ±7.1 61.0 ± 14.5 33.3 ± 15.2 D. petechia 12 Sun 1.6 ± 1.5 0.02 ± 0.4 -0.3 ± 0.2 27.9 ±11.5 4.8 ±3.6 26.6 ±7.3 68.6 ±7.6 D. pinus 3 Both 5.7 ±3.0 2.1 ±0.7 0.9 ± 0.4 8.3 ±23.1 6.7 ±7.1 35.0 ± 14.5 58.3 ± 15.2 D. striata 3 Shade -2.9 ±3.0 -0.3 ± 0.7 0.1 ±0.4 3.3 ±23.0 20.0 ±7.1 56.7 ±14.5 23.3 ±15.2 D. virens 5 Shade 0.7 ±2.4 0.5 ± 0.6 -0.2 ± 0.3 30.0 ±19.9 22.5 ± 6.2 60.0 ±12.6 15.0± 13.1 G. trichas 11 Shade -0.3 ±1.6 -0.6 ± 0.4 -0.02 ± 0.2 42.3 ± 12.1 11.8 ± 3.7 44.1 ±7.6 45.0 ±7.9 M. varia 9 Both 0.1 ±1.8 -0.7 ± 0.4 -0.1 ±0.2 17.8 ±13.3 23.9 ±4.1 39.9 ±8.4 36.2 ±8.8 S. aurocapillus 13 Shade -4.2 ±1.5 0.4 ±0.4 -0.1 ±0.2 26.6 ± 11.5 10.9 ±3.4 44.5 ± 6.9 44.7 ± 7.3 S. noveboracensis 2 Both 5.0 ±3.7 -0.5 ±0.9 0.1 ±0.5 87.5 ±28.3 10.0 ±8.7 15.0± 17.8 75.0 ± 18.6 S. ruticil/a 15 Shade -4.9 ± 1.4 0.4 ±0.3 -0.3 ± 0.2 38.6 ± 10.3 8.3 ±3.2 82.3 ±6.5 9.4 ±6.8 V. chrysoptera 13 Shade 3.1 ±1.5 0.4 ± 0.4 -0.1 ±0.2 25.8± 11.1 5.9 ±3.4 19.2 ±6.9 74.9 ±7.3 V. ruficapilla 3 Both 1.5 ±3.0 -0.4 ±0.7 -0.1 ±0.4 36.7 ±23.1 5.7 ±7.1 36.0 ± 14.5 58.3 ±15.2 XorF 74.1 2.8 1.63 1.03 0.7 2.5 4.2 4.2 P < .0001 0.001 0.07 0.43 0.8 0.003 < .0001 < .0001 Note: Values are given as mean ± SE. Significance tests are from ANOVAs or Chi-Square tests. Significant F-values are indicated in boldface (P < .05). Analyses were mn on arcsine transformed data where appropriate.

60 Table 2.3: Generalized linear regression analyses of the influence of background and irradiance variables on plumage principal components, separated by sex. See text for interpretation of PC scores. Response Sex Model Effects R1 F Df P Plumage PCI Male Whole Model 0.53 8.33 136 <.000l Bark PCI 0.81 0.37 Bark PC2 0.68 0.41 Bark PC3 1.55 0.22 Green Vegetation PC I 1.80 0.18 Green Vegetation PC2 0.13 0.72 Green Vegetation PC3 4.97 0.03 Irradiance PCI 0.27 0.61 Irradiance PC2 2.63 0.11 Irradiance PC3 0.22 0.64 Region 17.39 7 <.0001 Female Whole Model 0.57 9.84 136 <.0001 Bark PCI 1.78 0.18 Bark PC2 1.70 0.19 Bark PC3 3.15 0.08 Green Vegetation PC 1 3.13 0.08 Green Vegetation PC2 0.12 0.73 Green Vegetation PC3 7.97 0.006 Irradiance PCI 0.65 0.42 Irradiance PC2 3.51 0.06 Irradiance PC3 0.14 0.71 Region 19.80 <.0001 Plumage PC2 Male Whole Model 0.22 2.09 136 0.013 Bark PCI 7.41 0.007 Bark PC2 5.00 0.03 Bark PC3 3.32 0.071 Green Vegetation PC 1 1.81 0.18 Green Vegetation PC2 8.49 0.004 Green Vegetation PC3 5.07 0.03 Irradiance PCI 13.49 0.0004 Irradiance PC2 0.14 0.71 Irradiance PC3 1.42 0.24 Region 2.00 0.06 Female Whole Model 0.21 1.98 136 0.02 Bark PCI 0.97 0.33 Bark PC2 2.02 0.16 Bark PC3 0.47 0.49 Green Vegetation PC 1 0.03 0.86 Green Vegetation PC2 2.83 0.10 Green Vegetation PC3 2.07 0.15 Irradiance PCI 2.30 0.13 Irradiance PC2 0.33 0.57 Irradiance PC3 2.79 1 0.09 Region 2.76 7 0.01 Plumage PC3 Male Whole Model 0.28 2.86 136 0.0006 Bark PCI 4.67 I 0.03 Bark PC2 6.03 1 0.02 Bark PC3 3.90 1 0.05 Green Vegetation PC 1 0.52 1 0.47 Green Vegetation PC2 0.62 I 0.43 Green Vegetation PC3 0.65 1 0.42 Irradiance PCI 2.84 1 0.09 Irradiance PC2 1.20 1 0.28 Irradiance PC3 4.00 1 0.048 Region 4.73 7 0.0001 Female Whole Model 0.39 4.82 136 <.0001 Bark PCI 7.55 1 0.007 Bark PC2 6.86 1 0.01 Bark PC3 4.18 1 0.04 Green Vegetation PC 1 11.55 1 0.0009 Green Vegetation PC2 5.41 1 0.02 Green Vegetation PC3 15.46 1 0.0001 Irradiance PCI 10.57 1 0.002 Irradiance PC2 3.72 1 0.06 Irradiance PC3 7.78 1 0.006

62 Factor Loadings Factor L oadings Factor L.oading s i o o o o o O o o O o o o o o © h-> *» <7\ *> Nj O K> *. NJ O SJ -c O-i

<

-J UI TO §

o o

o © Relative Irradiance % Reflectance % Reflectance ^ O t- Kj UI

o ' • I I I I o 00

<

^* o

o-. o o

o o Figure 2.1: A) Factor loadings for principal components analysis (PCA) conducted on plumage reflectance in 17 species of warbler. PCI, PC2 and PC3 are represented by blue, red and yellow lines, respectively. B) Plumage reflectance spectra representative of a few of the coloured plumage patches in the warbler species we measured. Each spectmm is the mean (± SE) plumage reflectance across five males of the given warbler species. The coloured lines represent reflectance values for each region as follows: blue - mmp (D. caerulescens), brown - flank (D. pensylvanica), orange - throat (D. fusca), yellow - breast (D. petechia). C) Factor loadings from PCA of ambient light spectra recorded in the field for 118 individual warblers. PC 1, PC2 and PC3 are represented by blue, red and yellow lines, respectively. D) Average ambient light spectra for two types of light environments: sun (yellow line, n = 23), and shade (green line, n = 66). Bars indicate ± SE. E) Factor loadings from PCA of background reflectance data. PCI, PC2 and PC3 are represented by blue, red and yellow lines, respectively. F) Average reflectance spectra for bark (brown line, n = 17) and vegetation (green line, n = 26) collected from the visual environment of displaying warblers. Bars indicate ± SE.

64 25

breast crown flanks mantle outer tail rump throat upper wing 12

10

1 3 o JyC Q 6 c

4 -

2 -

breast crown flanks mantle outer tail tump throat upper wing Body Region

Figure 2.2: A) Mean contrast against the background for males (blue bars) and females (red bars) across body regions. B) Mean dichromatism values across body regions. Vertical bars indicate standard errors.

65 Z-\J - B • 1/1 l_ 15 - •*-» • c • • uo • • • a; • • E 10 - • •

5 - 5 10 15 Mean Dichromatism

Figure 2.3: Relationship between chromatic contrast against the background and sexual dichromatism for male (A) and female (B) warblers in their respective signalling environments.

66 CHAPTER 3: THE INFLUENCE OF SYMPATRY ON PLUMAGE DIVERGENCE IN PARULIDAE WARBLERS SYNOPSIS

Closely related species often exhibit dramatic diversity in ornamental traits, and

sexual selection has been suggested as a mechanism promoting such diversity. When

closely related species are in sympatric contact, traits may diverge via selection for

species recognition. Our objective in this study was to determine whether plumage divergence relates to degree of sympatry in pamlid warblers. We calculated pairwise differences in plumage coloration across 77 species of warbler, and compared plumage divergence with degree of sympatric overlap while controlling for genetic divergence.

As we predicted, plumage divergence was significantly positively correlated with degree of sympatry, which indicates that as degree of sympatry increases, so too does plumage divergence between species, presumably to enhance reproductive isolation between closely related species. We also investigated the relationship between degree of sympatry and genetic distance, and found a significant negative correlation, indicating that an increase in sympatry is associated with a decrease in genetic distance between species. Overall, our findings suggest sympatric overlap has driven diversification of plumage ornaments in warblers.

INTRODUCTION

Sexually selected traits often serve as important indicators of species identity

(West-Eberhard 1983; Coyne and Orr 2004), and divergence in such traits could result in the generation of premating reproductive isolation, an important step in speciation

(Panhuis et al 2001). Traditional speciation models assume that traits diverge in isolation, due to ecological adaptation or genetic drift, and then drive reproductive

68 isolation between populations during secondary contact (Dobzhansky 1941; Mayr 1942).

In Darwin's finches, for example, the ecological adaptation of beak size to divergent feeding ecologies has indirectly influenced vocal evolution, as selection for larger or smaller beak sizes based on food resources also influences how the vocal tract can be modulated to produce song (Podos 2001). As a result, species produce different vocal signals, and females mate with males based on appropriate recognition of conspecific vocalizations (Podos 2001; Podos and Nowicki 2004), maintaining reproductive isolation between closely related species. Darwin's finches can produce viable and fertile hybrids, so it is evident that speciation in this group was driven primarily by premating isolation mechanisms (Podos 2001; Podos and Nowicki 2004). In the Galapagos finches, divergent ecological adaptation indirectly promoted divergence in song; however, traits can also diverge under a more direct influence of sexual selection. For example, diversification in the African cichlids of Lake Victoria appears to have been driven by sexual selection for divergent colour traits (Seehausen et al. 1997; Seehausen et al. 1998;

Maan et al. 2004; Seehausen et al. 2008). Species in these lakes are highly variable in coloration, yet very similar in size and shape, leading researchers to propose that reproductive isolation between sympatric species is maintained by differences in coloration (Seehausen and van Alphen 1998; Seehausen et al. 1998). Seehausen et al.

(1997) tested this hypothesis through mate choice trials in monochromatic and full spectmm light. As predicted, females chose conspecific males in normal lighting conditions and were unable to distinguish between conspecifics and heterospecifics under monochromatic light (Seehausen et al. 1997). Reproductive isolation among cichlids

69 thus appears to be maintained by divergent selection for conspecific colour signals, even within highly sympatric assemblages (Seehausen et al. 2008).

Accurate species recognition is especially important when species ranges overlap.

Sympatric species are expected to diverge in traits used in species recognition to maintain reproductive isolation and prevent maladaptive heterospecific matings (Dobzhansky

1941; Brown and Wilson 1956; Coyne and Orr 2004). Several studies have tested this assertion by evaluating whether signals used in mating are more divergent in areas of heterospecific overlap than in allopatry (Howard 1993; Alatalo et al. 1994; McNaught and Owens 2002; Hobel and Gerhardt 2003; Seddon 2005; Kirschel et al. 2009; Lemmon

2009). In antbirds, for example, sympatric pairs of species exhibited greater divergence in song characteristics than did allopatric pairs of species (Seddon 2005). Similarly, sexually selected plumage coloration appears to be an important premating isolation barrier between closely related European Ficedula flycatchers (Saetre et al. 1997). In allopatry, male pied flycatchers (F. hypoleuca) and collared flycatchers (F. albicollis) are both mostly black with white patches on the forehead and neck. In sympatry, however, populations of these species have evolved divergent plumage coloration: the collared flycatcher is black and has a larger white neck ring compared to the allopatric population, whereas the pied flycatcher is dull brown in colour (Saetre et al. 1997; Saetre 2000).

Most studies investigating the association between plumage divergence and sympatry have only assessed divergence between two related species that occur in sympatric and allopatric distributions (Hardy and Dickerman 1965; Saetre et al. 1997;

Rolshausen et al. 2009). While such studies are have established that plumage coloration can be an important isolating mechanism, larger-scale studies are needed to determine

70 whether there could be a broader association between sympatry and plumage divergence.

In one comparative study, McNaught and Owens (2002) compared plumage coloration between sympatric and allopatric pairs of Australian birds and did not find increased

plumage divergence in sympatry (McNaught and Owens 2002). Another recent study

found that greater pattern divergence between species was correlated with degree of

sympatry in seven families of North American passerines, although plumage differences were simply ranked by observers (Martin et al. 2009). Further research on this topic is clearly needed, especially in species that may have gained significant range overlap over a short evolutionary timeframe.

New World warblers (Pamlidae) represent one of the most widespread avian families in the Western Hemisphere, with 115 species distributed from South America through to northern Canada and Alaska (Curson et al. 1994). Pamlid warblers exhibit an exceptional degree of sympatric overlap with heterospecifics, yet effectively maintain species integrity (Lovette and Bermingham 2002). Most warbler speciation is thought to have occurred during the Pliocene period, and the divergence of several genera likely began in allopatry (Morse 1989; Curson et al. 1994). During the interglacial period, when new habitats became available for colonization by new populations, several species began spreading northward; subsequent range expansion by species into each other's breeding ranges may have produced the elevated sympatry observed today (Lovette and

Bermingham 1999; Price et al. 2000; Lovette 2005). While pamlid warblers appear to show little divergence in morphology, they exhibit outstanding diversification in sexually selected characteristics such as plumage ornaments and song (Price 2008). MacArthur

(1958) speculated that warblers' ability to coexist in sympatry results from divergent

71 feeding ecologies that prevent competitive exclusion. While this may be tme from an ecological standpoint, divergence in sexually selected traits may be more important for maintaining reproductive isolation in sympatry in this group (Lovette and Bermingham

1999; Price 2008). Pamlid warblers have been known to produce viable hybrids, even across genera (McCarthy 2006), which highlights the possibility that diversity in this group is maintained by premating isolation mechanisms. The interplay between sympatry and plumage divergence could operate through two non-mutually exclusive mechanisms. First, sexual selection may have driven divergence in plumage ornaments when species were in allopatry, such that existing premating isolation barriers allowed species to come together in sympatry (West-Eberhard 1983; Price 1998). Alternatively, sympatric overlap may have driven divergence in plumage coloration to enhance accurate species recognition through reinforcement of reproductive barriers (Dobzhansky 1940).

Our objective in this study was to determine whether plumage divergence relates to degree of sympatry in pamlid warblers. We calculated pairwise differences in plumage coloration across 77 species of warbler, and compared plumage divergence with degree of sympatric overlap while controlling for genetic divergence. We predicted that plumage divergence should increase with degree of sympatry to enhance reproductive isolation between closely related species. We also investigated the relationship between degree of sympatry and genetic distance, to determine whether closely related warbler species are more often found in sympatry or allopatry. We predicted that sympatric species pairs would tend be more distantly related.

72 METHODS

Plumage Reflectance

We measured the plumage reflectance of museum specimens of pamlid warblers from the University of Michigan Museum of Zoology (n=316), the American Museum of

Natural History (n=29), and the Royal Ontario Museum (n=18) in 2007 and 2008. We measured five males for each species (when available), and collected data for 71 pamlids as well as six Incertae sedis species near Pamlidae (Table 3.1). We measured spectral reflectance using a USB4000 Spectrometer and a PX-2 pulsed xenon lamp (Ocean

Optics, Dunedin, FL). A bifurcated fiber-optic cable was used to deliver light from the light source to the specimen and from the specimen to the computer (R-400-UV-VIS,

Ocean Optics). The reflectance probe was mounted in a matte black mbber holder that excluded all external light and maintained the probe at a fixed distance (ca. 5mm), perpendicular to the measured surface. All reflectance spectra were expressed as the percentage of reflectance relative to a white standard (WS-1, 97-98% incident light;

Ocean Optics). We measured 10 body regions on each museum specimen, collected five readings per region, and used the means of these five readings in our analyses. When selecting specimens, we chose birds captured in breeding plumage and during the breeding season, and excluded birds captured during molt. We recorded relevant information from the specimen voucher, including sex, capture date and capture location.

73 Principal Components Analysis of Plumage Reflectance Spectra

We performed Principal Components Analysis (PCA) to summarize variation in

reflectance spectra. We summarized mean reflectance values into 10-nm-wide bins using

CLR v. 1.05 (Mongomerie 2008) and ran a PCA on the entire data set, including all 77

species and all 10 body regions for each species. We retained the three principal

components with eigenvalues larger thanonefor our analyses, which together explained

99% of the variation in reflectance. The first principal component (PCI) explained 85%

of the variation in reflectance, PC2 explained 11% of the variation, and PC3 explained

3%> of the variation (Fig. 3.1 A). The factor loadings for PCI were moderate and positive

across the visual range, and we therefore interpret PCI as a brightness axis (Endler 1990).

The factor loadings for PC2 were positive at shorter wavelengths and negative at longer

wavelengths, which we can interpret as a UV/blue versus orange/red axis. The factor

loadings for PC3 were high and positive at ultraviolet wavelengths, negative at

blue/green wavelengths, and moderately positive at longer wavelengths. Thus, we can

interpret PC3 as a UV/red versus blue/green axis. Example plumage spectra are provided

in Fig. 3.1b. Statistical analyses were performed using JMP v. 6.0 (SAS Institute, Inc.).

Plumage Divergence Matrix

We used Mantel tests to compare matrices of pairwise differences between

species in plumage coloration, genetic distance, and sympatric overlap (Mantel 1967).

To evaluate plumage divergence between species, we calculated the Euclidean distance between plumage PC scores for all possible pair-wise combinations of species, which

allowed us to assess linear divergence in a three-dimensional space. We constmcted

74 matrices of Euclidean distance in plumage for all species pairs and across each of the 10

body regions, and averaged these matrices to obtain a mean plumage divergence matrix.

Degree of Sympatry Matrix

To calculate the amount of geographic overlap between warbler species pairs, we

first obtained digital distribution maps of breeding ranges from NatureServe, an online

database (Ridgely 2007). Using ArcGIS (version 9.3.1, ERSI, Redlands, CA) we

projected each species' breeding distribution polygon into a cylindrical equal-area

projection to calculate the total area of each species' range. Degree of sympatry between

two pairs of species is defined as the proportional area of the range of the species with a

smaller range which is overlapped by the species with the larger range (Fig. 3.1; Chesser

and Zink 1994; Barraclough and Vogler 2000). This value can range from zero,

indicating no overlap to one, which would indicate complete overlap of the species with

the smaller range by the species with the larger one. We calculated degree of sympatry

for all species pairs and assembled a matrix for comparison with plumage and genetic

divergence matrices. Because degree of sympatry is a proportional measure, the values

in this matrix were square root and arcsine transformed prior to statistical analysis (Sokal

and Rohlf 1981; Johnson and Cicero 2002).

Genetic Distance Matrix

To estimate genetic distance between species pairs, we calculated uncorrected p-

distances for cytochrome b sequences (1140bp) obtained from NCBI GenBank (Table

3.1; http://www.ncbi.nlm.nih.gov/Genbank/). We used one sequence per species, as the availability of more than one sequence per species was quite infrequent within Pamlidae.

75 Additionally, while the use of nuclear DNA may be preferable for some types of

comparisons of genetic relatedness, sequences for nuclear DNA were not available for as

many species as mitochondrial cytochrome b in Pamlidae. Moreover, mitochondrial

DNA has been established as an acceptable measure of phylogenetic relationships in this

group (Lovette and Bermingham 2002; Lovette and Hochachka 2006). We chose to use

uncorrected p-distances because similar comparisons of degree of overlap and genetic relatedness using Mantel tests has been successfully demonstrated in birds (Johnson and

Cicero 2002) and other taxa (Stelkens and Seehausen 2009). Finally, genetic distance calculated using Kimura two-parameter (K2P) distances (Kimura 1980) yielded similar distances and did not change our results. We used Mesquite (version 2.71, Maddison and

Maddison, 2009) to calculate uncorrected p-distances and assembled these into a genetic distance matrix.

Testing Correlation Between Distance Matrices

We used Mantel tests to assess pairwise correlations between species in degree of sympatry, plumage distance, and genetic distance (Mantel 1967). Because we wanted to evaluate the influence of sympatry on plumage divergence while controlling for genetic relatedness, we employed the Partial Mantel test, which evaluates the correlation between two distance matrices while controlling for the effects of a third. This is accomplished by performing permutations on the residuals of a null model (Legendre and Anderson 1999).

For greater precision, we used 100,000 iterations when generating the correlation coefficient between matrices. We conducted three separate Partial Mantel tests: 1) the correlation between plumage distance and degree of sympatry, controlling for genetic distance, 2) the correlation between plumage distance and genetic distance, controlling

76 for degree of sympatry, and 3) the correlation between degree of sympatry and genetic distance, controlling for plumage distance. We performed Mantel tests using zt software

(Bonnet and Van de Peer 2002).

RESULTS

Plumage Divergence vs. Degree of Sympatry

Mean plumage divergence was not significantly correlated with degree of sympatry when controlling for genetic distance (r = 0.07, P = 0.09). When assessing each body region separately, only throat coloration diverged more with increasing sympatry (r = 0.07, P = 0.05), with nape coloration approaching significance (r = 0.08, P

= 0.08). Because PCI accounted for 85% of the variation in reflectance in our plumage data (Fig 3.1a), its influence may have masked chromatic divergence in plumage coloration (PC2 and PC3; Endler 1990). We used two methods to control for the over- influence of PCI. First, we removed PCI and calculated Euclidean distance between

PC2 and PC3 alone as a measure of chromatic plumage divergence. This allowed us to test divergence between the two principal component scores which are more indicative of variation in hue and chroma (Fig 3.1a), while eliminating the influence of achromatic variation (Endler 1990). Second, we standardized PC scores to a mean of zero and a standard deviation of one to reduce the influence of extensive variation in brightness in our calculation of Euclidean distance.

Mean chromatic plumage divergence was positively correlated with degree of sympatry (r = 0.17, P = 0.0008) when controlling for the influence of genetic divergence.

That is, as sympatric overlap increases, chromatic plumage coloration becomes more

77 divergent between species pairs. Mean standardized plumage divergence (Euclidean distance between standardized PCI, PC2 and PC3 scores) was also significantly correlated with degree of sympatry (r = 0.11, P = 0.01).

When we evaluated body regions separately, we found significant positive correlations between chromatic plumage divergence and degree of sympatry for six regions: mantle (r = 0.09, P = 0.03), nape (r = 0.10, F = 0.05), outer tail (r = 0.23, F =

0.00005), mmp (r = 0.13, F = 0.01), undertail (r = 0.11, F = 0.004) and upper wing (r =

0.11, F = 0.03). For standardized plumage divergence, only two body regions became more divergent in sympatry: outer tail (r = 0.13, F = 0.001) and undertail (r = 0.07, F =

0.05). Two other regions approached significance (mmp: r = 0.08, P = 0.07; throat: r =

0.05, P = 0.08).

Plumage Divergence vs. Genetic Distance

Mean unstandardized plumage divergence was significantly positively correlated with genetic distance (r = 0.16, F = 0.01), as was standardized mean plumage divergence

(r = 0.12, F = 0.04), such that plumage divergence increased with increasing genetic distance. We did not find a significant correlation between chromatic plumage divergence and genetic distance (r = 0.06, F = 0.17).

When assessing each body region separately, four body regions showed positive correlations between unstandardized plumage divergence and genetic distance: flanks (r

= 0.27, F = 0.0008), outer tail (r = 0.11, F = 0.004), throat (r = 0.13, F = 0.03), and undertail (r = 0.14, F = 0.02). Similarly, standardized plumage divergence was significantly positively correlated with genetic distance for the flanks (r = 0.23, F =

0.001), throat (r = 0.15, F = 0.005), and undertail (r = 0.15, F = 0.006). Finally, only the

78 flanks (r = 0.15, P = 0.03) exhibited a significant positive correlation between chromatic plumage divergence and genetic distance.

Genetic Distance vs. Degree of Sympatry

We found a significant negative correlation between genetic distance and degree of sympatry (r = -0.18, F = 0.0003), indicating that species pairs with high degrees of sympatry also tend to be more closely related to one another.

DISCUSSION

Closely related taxa often exhibit dramatic diversity in ornamental traits, and sexual selection has been identified as a mechanism promoting this diversity (West-

Eberhard 1983). For example, sexual selection has promoted divergence in male calls and female preferences between neighbouring populations of Physalemus petersi, an

Amazonian frog (Boul et al. 2007). In this system, divergent female preferences are fixed throughout the range, and the result is strong reproductive isolation between populations based on divergent male calls (Boul et al. 2007). Sexual selection may drive divergence directly as in the previous example, or divergence in sexually selected traits may enhance species recognition and maintain reproductive isolation (Questiau 1999).

Indeed, a number of studies have found that divergence in sexually selected traits enhances recognition of conspecifics in sympatry (Saetre et al. 1997; Honda-Sumi 2005;

Seehausen et al. 2008; Kirschel et al. 2009). Sympatric distributions present various pressures on traits involved in species recognition, and selection against unfit hybrids is presumed to favour the production of mating signals that are accurate indicators of species identity (Dobzhansky 1941; Liou and Price 1994).

79 Pamlid warblers exhibit an extensive diversity of plumage coloration, even among species within a single genus (Dunn and Garrett 1997). Our findings show that both mean chromatic plumage divergence and mean standardized plumage divergence were significantly positively correlated with increasing sympatric overlap. In addition, six body regions were significantly more divergent in sympatry for chromatic divergence

(mantle, nape, outer tail, mmp, undertail coverts, and upper wing), whereas two regions were significantly different for standardized plumage divergence (outer tail and undertail). Our findings fall in line with previous research on plumage divergence in sympatry (Saetre et al. 1997; Martin et al. 2009; Uy et al. 2009). Because the relationship between plumage divergence and sympatry was restricted to chromatic plumage differences, however, our findings suggest that differences in colour may be more important for species recognition in warblers than differences in brightness.

Several studies have shown that warbler plumage ornaments can influence male reproductive success (Lemon et al. 1992; Lozano and Lemon 1996; Yezerinac and

Weatherhead 1997; King et al. 2001; Reudink et al. 2009; Dunn et al. 2010). In another recent study (Chapter 2), we documented extensive sexual dichromatism in warblers and found that male plumage exhibited more contrast against the visual background than female plumage. Taken together, these studies suggest that sexual selection has promoted elaborate plumage coloration in warblers. While some of the body regions we found to diverge in sympatry likely function as sexual ornaments, others, such as outer tail coloration and undertail coverts, may not. This pattern highlights the possibility that certain regions have been established as indicators of species identity, and through natural selection, may have diverged to enhance species recognition (Price 1998).

80 Indeed, outer tail coloration is one of the least sexually dichromatic body regions in warblers (Chapter 3), and may therefore be used for species recognition in both sexes.

Thus, species recognition may be accomplished in warblers through divergence in both sexual ornaments and other plumage traits.

We also evaluated the relationship between plumage divergence and genetic distance, and found that both unstandardized and standardized mean plumage divergence were significantly positively correlated with genetic distance. Across separate body regions, unstandardized plumage divergence was positively correlated with genetic distance for flanks, outer tail, throat and undertail, and standardized plumage divergence showed positive correlations with genetic distance the flanks and throat. In contrast, mean chromatic plumage divergence was not significantly correlated with genetic distance, and chromatic plumage divergence was only significantly positively correlated with genetic distance for flanks, with most other regions showing non-significant correlations in the opposite direction. A positive correlation between genetic distance and plumage distance is not surprising; as genotypic differences accumulate between species, differences in phenotype are also likely to increase (Coyne and Orr 2004). The fact that chromatic divergence did not increase with genetic distance is perhaps more interesting, and may relate to the apparently high lability exhibited by sexual traits

(Kusmierski et al. 1997; Omland and Lanyon 2000; Cardoso and Mota 2008). Indeed, many of the taxonomic groups wherein sexual selection has been implicated in speciation show exceptional phenotypic divergence among closely related species (Lukhtanov et al.

2005; Losos et al. 2006; Seehausen et al. 2008; Tobias and Seddon 2009). The

81 implication is that this phenotypic divergence in ornamental traits is in part what allows closely related species to coexist.

We also discovered that pairs of closely related warbler species tended to exhibit high degrees of sympatry; such overlap between closely related species is intriguing.

Most studies report a positive relationship between genetic and geographic distances

(Barraclough and Vogler 2000; Coyne and Orr 2004). This pattern is expected due to isolation by distance; as geographic distance between populations increases, the likelihood of interbreeding decreases (Wright 1943). Thus, populations in close contact continue to exchange genes more often, whereas geographically separated populations do so less frequently, ultimately promoting a clinal decrease in genetic relatedness with geographic distance (Endler 1977). Some studies have also used the amount of geographic overlap between species to infer modes of speciation (Lynch 1989; Chesser and Zink 1994; Barraclough and Vogler 2000). For example, an increase in genetic distance correlated with a decrease in the amount of overlap would imply sympatric speciation, wherein species diverged in sympatry, and slowly decreased in amount of overlap as their ranges expanded into allopatry (Coyne and Orr 2004). On the other hand, allopatric speciation would show a pattern of increased sympatry as genetic divergence increases; geographically isolated and genetically distinct populations would slowly expand into each other's ranges over evolutionary time (Barraclough and Vogler

2000; Coyne and Orr 2004).

While our findings of closely related sympatric assemblages follow a pattern suggestive of sympatric speciation, previous research on warbler speciation does not support such a claim (Lovette and Hochachka 2006; Rabosky and Lovette 2008), and we

82 do not wish to imply this here. Instead, our findings, in conjunction with previous research, may explain how a pattern of low genetic divergence and high sympatry is possible in the absence of sympatric speciation. Warbler diversification is thought to have begun in the tropics, with range expansion during the interglacial periods allowing populations to expand northward (Morse 1989). The unique opportunities for diversification across a broad range of habitats in the temperate zone presumably enabled several populations to become established and diverge in allopatry. Our results may indicate the expansion of the most closely related species into a higher degree of sympatry. Recent research has suggested that speciation has occurred more rapidly in the temperate zone than in the tropics, even though species diversity is much higher in the tropics (Weir and Schluter 2007). Rapid sympatry has been proposed to increase colour pattern divergence in temperate birds, with frequent secondary contact driving character displacement in species recognition traits (Martin et al. 2009). Although pamlids have been reported to hybridize in both natural and captive populations (Cox 1973; Gill 1980;

Graves 1988, 1993, 1996; Rohwer and Wood 1998; Rohwer et al. 2000; Gill 2004;

McCarthy 2006; Irwin et al. 2009), natural hybridization between species is relatively low considering the extent of sympatric overlap. A reasonable explanation for such low hybridization rates in the face of high sympatry is the existence of very strong premating isolation mechanisms between these species, such as plumage coloration or song (Price

1998; Uy et al. 2009). Low hybridization rates again support a model of allopatric speciation; such strong premating isolation indicates species evolved separately and secondary contact simply reinforced assortative mating (Coyne and Orr 2004).

83 In summary, our study represents one of the first comprehensive examinations of the influence of sympatry on plumage colour divergence in birds. We found that in pamlid warblers, plumage divergence was significantly positively correlated with increased sympatry, which may function to enhance reproductive isolation between overlapping species (Pfennig and Pfennig 2009). This pattern was more pronounced with chromatic coloration, suggesting that chromatic components of coloration likely function more effectively as indicators of species identity. We also found that more closely related species tended to share high degrees of sympatry. This high diversification in plumage ornaments among closely related sympatric species presents a unique situation where highly labile plumage ornaments appear to be diverging rapidly via selection for species recognition (Rundell and Price 2009). Species often overlap in distribution with several other closely related species, and selection for species recognition likely favors divergent appearance among all of them. Our approach therefore offers an effective method for investigating the broad influence of sympatry on plumage divergence.

Overall, our findings suggest that diversification in the plumage ornaments of warblers has played an important role in the evolution of such a speciose and widespread avian family, and our approach should be applicable to similar evolutionary questions in a variety of taxa.

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89 Table 3.1: List of species used in our study including the number of museum specimens from which we obtained plumage reflectance and GenBank Accession numbers for cytochrome b gene sequences used to calculate genetic distance. Specimens Accession Latin Name Common Name measured Number Basileuterus culicivorus Golden-crowned warbler 5 AF383106 Basileuterus flaveolus Flavescent warbler 2 AF382994 Basileuterus fulvicauda Buff-mmped warbler 5 AY340216 Basileuterus rivularis Riverbank warbler 2 AY340209 Basileuterus rufifrons Rufous-capped warbler 5 AY216847 Basileuterus tristriatus Three-striped warbler 5 AF383013 Catharopeza bishopi Whistling warbler 1 AF383024 Cardellina rubrifrons Red-faced warbler 5 AF383026 Dendroica adelaidae Adelaide's warbler 2 AF256504 Dendroica caerulescens Black-throated blue warbler 5 EU815674 Dendroica castanea Bay-breasted warbler 5 AY216835 Dendroica cerulea Cemlean warbler 5 EU815676 Dendroica coronata Yellow-rumped warbler 5 EU815677 Dendroica discolor Prairie warbler 5 EU815678 Dendroica dominica Yellow-throated warbler 5 EU815679 Dendroica fusca Blackbumian warbler 5 AY340208 Dendroica graciae Grace's warbler 5 EU815680 Dendroica magnolia Magnolia warbler 4 EU815682 Dendroica nigrescens Black-throated gray warbler 5 EU815683 Dendroica occidentalis Hermit warbler 5 EU815684 Dendroica palmarum Palm warbler 5 AY216834 Dendroica pensylvanica Chestnut-sided warbler 5 EU815686 Dendroica petechia Yellow warbler 5 EU815686 Dendroica pharetra Arrow-headed warbler 1 AY216844 Dendroica pinus Pine warbler 5 AF383027 Dendroica striata Blackpoll warbler 5 EU815688 Dendroica tigrina Cape May warbler 5 EU815689 Dendroica townsendi Townsend's warbler 5 EU815690 Dendroica virens Black-throated green warbler 5 EU815691 Dendroica vitellina Vitelline warbler 5 AY216832 Euthlypis lachrymosa Fan-tailed warbler 5 AF383009 Ergaticus ruber Red warbler 5 AF383010 Geothlypis aequinoctialis Masked yellowthroat 5 AY030121 Granatellus pelzelni Rose-breasted chat 1 AF382995 Granatellus sallaei Gray-throated chat 5 EF530030 Geothlypis speciosa Black-polled yellowthroat 2 AY216849 Geothlypis trichas Common yellowthroat 5 AF383003 Granatellus venustus Red-breasted chat 4 EF530029

90 Helmitheros vermivorus Worm-eating warbler 5 AF383004 Icteria virens Yellow-breasted chat 5 EF529955 Limnothlypis swainsonii Swainson's warbler 5 AF383005 Myioborus albifrons White-fronted redstart 1 AY968861 Myioborus brunniceps Brown-capped redstart 3 AF383031 Myioborus flavivertex Yellow-crowned redstart 1 AY968873 Myioborus melanocephalus Spectacled redstart 5 AY968851 Myioborus miniatus Slate-throated redstart 5 AY968854 Myioborus ornatus Golden-fronted redstart 3 AY968882 Microligea palustris Green-tailed warbler 2 AF383021 Myioborus pictus Painted redstart 5 AF383011 Myioborus torquatus Collared redstart 1 AY968886 Mniotilta varia Black-and-white warbler 5 AF383006 Oporornis agilis Connecticut warbler 5 AY216854 Oporornis formosus Kentucky warbler 5 AF383017 Oporornis tolmiei MacGillivray's warbler 5 AF383029 Parula americana Northern pamla 5 EU815692 Parula gutturalis Flame-throated warbler 5 AF256508 Parula pitiayumi Tropical pamla 5 EU815693 Parula superciliosa Crescent-chested warbler 5 AF256506 Protonotaria citrea Prothonotary warbler 5 EF529954 Seiurus aurocapillus Ovenbird 5 AF3 83007 Seiurus motacilla Louisiana waterthrush 5 AY216856 Seiurus noveboracensis Northern waterthrush 5 AF383001 Setophaga ruticilla American redstart 5 EU815694 Teretistris fernandinae Yellow-headed warbler 1 AF382999 Vermivora celata Orange-crowned warbler 5 AY030123 Vermivora chrysoptera Golden-winged warbler 5 AY216819 Vermivora crissalis Colima warbler 5 AY216815 Vermivora luciae Lucy's warbler 5 AY216802 Vermivora peregrina Tennessee warbler 5 AY216809 Vermivora pinus Blue-winged warbler 5 AY216821 Vermivora ruflcapilla Nashville warbler 5 AY216806 Vermivora virginiae Virginia's warbler 5 AY216808 Wilsonia canadensis Canada warbler 5 AF383016 Wilsonia citrina Hooded warbler 5 EU815695 Wilsonia pusilla Wilson's warbler 5 AY216865 Xenoligea montana White-winged warbler 1 AF383022 Zeledonia coronata Wrenthmsh 5 AF382998

91 I/) 00 c n o

u

u0) c

u

5?

300 400 500 600 700

Wavelength (nm)

Figure 3.1: A) Factor loadings for principal components analysis (PCA) conducted on plumage reflectance in the 77 species used in this study. PCI, PC2 and PC3 are represented by blue, red and yellow lines, respectively. B) Plumage reflectance spectra from four different species of warbler in the genus Dendroica. Each spectmm is the mean reflectance for the nape region across five males for the given species as follows: yellow - Townsend's warbler, D. townsendi; green - black-throated green warbler, D.virens; orange - blackbumian warbler, D. fusca; blue - black-throated blue warbler, D. caerulescens. Photos of each of these species are provided in Fig 3.2.

92 93 Figure 3.2: Two pairs of species in the genus Dendroica which have very different degrees of sympatric overlap. The A) Townsend's warbler D. townsendi breeding range is depicted in blue in panel C, and the B) black-throated green warbler D. virens breeding range is shown in yellow. There appears to be zero overlap between these two species. In contrast, the D) blackbumian warbler D. fusca (blue range in panel F) and E) black- throated blue warbler D. caerulescens (yellow range) are almost completely sympatric, with nearly all of the black-throated blue warbler's breeding range encompassed by the blackbumian warbler. All photographs copyrighted by respective photographers and used with permission; see Acknowledgements for photo credits. Figures depicting breeding ranges redrawn from the Birds of North America Online (Wright et al. 1998; Morse 2004; Holmes et al 2005; Morse and Poole 2005).

94 CHAPTER 4: GENERAL DISCUSSION THESIS SUMMARY AND DISCUSSION

Wood warblers have long intrigued amateur ornithologists and established researchers alike; their bold plumages, melodic voices, and widespread diversity are only a few characteristics that have stimulated substantial research on their behaviour and evolution (MacArthur 1958; Shutler and Weatherhead 1990; Bermingham et al. 1992;

Fotheringham et al. 1997; Lovette and Bermingham 1999; Kelly and Hutto 2005). Aside from taxonomic and phylogentic studies, however, most research on wood warblers has been completed at the level of species, with very few studies investigating broad, evolutionary questions about the family as a whole (but see: Lovette and Bermingham

2002; Lovette and Hochachka 2006; Rabosky and Lovette 2008). The goal of my thesis was to investigate the potential mechanisms driving interspecific divergence in plumage among the Pamlidae, and how these selective factors could have influenced the evolution of such a diverse group of birds.

In Chapter 2, we investigated the evolution of plumage colour using an avian visual modelling approach, which allowed us to investigate plumage contrast and divergence in the appropriate signalling context (Vorobyev et al. 1998; Cuthill et al.

2000). First, we found that male plumage coloration contrasted more against the visual background than female plumage coloration. Additionally, male contrast against the background was significantly positively correlated with sexual dichromatism, as has been shown in neotropical manakins (Doucet et al. 2007). Together with other studies

(Yezerinac and Weatherhead 1997; Reudink et al. 2009), our findings suggest that sexual selection may have played had an important role in the evolution of warbler plumage coloration. We also found that sympatric species subdivided the signalling environment

96 where they chose to broadcast their displays, and that this translated into significant interspecific differences in their visual display environments. By utilizing different signalling environments, sympatric warblers presumably avoid overlapping with other species when attracting mates, which could potentially reduce the potential for heterospecific mating errors (Pfennig and Pfennig 2009). Plumage coloration of warblers was also found to be highly correlated with coloration of the visual background, suggesting that sensory drive has influenced the evolution of plumage ornaments in warblers (Endler and Basolo 1998) as has been documented in a few other species

(Endler and Thery 1996; Heindl and Winkler 2003; Uy and Endler 2004). A correlation between plumage ornaments and the environment, in combination with divergent signalling locations, would set the stage for divergent evolution of plumage ornaments based on species-specific environments (Endler 1992; Boughman 2002). However, we did not find that species' plumage ornaments were most conspicuous in their specific display environments. Our study represents the first measurement and evaluation of visual environment in the temperate zone across several locally sympatric species.

Because temperate environments are so much more variable than most tropical environments over the course of the breeding season, it may not be possible for sympatric temperate species to exploit subsets of the signalling environment as has been shown in tropical species (Endler 1993; Thery 2001). In addition, high local sympatry could select for divergent coloration among closely related species (Lemmon 2009). Because particular habitats present only a limited range of optimally conspicuous contrasting colours, selection for conspicuousness and distinctiveness could be at odds in highly sympatric wood warblers.

97 In Chapter 3, we explored the influence of sympatry more directly, and with a broader scope, by investigating how range overlap could potentially drive divergence in plumage coloration. We found that plumage coloration in warblers was more divergent when species exhibited greater sympatric overlap, which is expected if such divergence enhances reproductive isolation between closely related species in sympatry. Our findings agree with the few previous studies investigating plumage divergence in sympatry (Alatalo et al. 1994; Saetre et al. 1997; Rolshausen et al. 2009). Future studies should consider expanding the number of species included in investigations of sympatric divergence, since multiple sympatric interactions may promote broad scale patterns of divergence. Additionally, reflectance spectrometry represents the most objective method available to researchers wishing to quantify coloration (Montgomerie 2006), and we have demonstrated that measuring distinct regions allows accurate comparison between species. We also found that degree of sympatry was negatively correlated with genetic distance, indicating that species in sympatric overlap are more closely related than those in allopatry (Barraclough and Vogler 2000). Such a pattern of diversification is typically suggestive of sympatric speciation, but this pattern is also possible through original allopatric speciation followed by rapid diversification in the temperate zone (Weir and

Schluter 2007). As a result, closely related species which have diverged relatively recently may exist in sympatry through effective premating isolation mechanisms, such as divergent plumage ornaments between species (Price 1998; Price 2008). Collectively, these chapters describe potential mechanisms which may have favored the evolution of such dramatic plumage diversification in wood warblers.

98 FUTURE DIRECTIONS

While numerous studies have been conducted on warblers, very few have investigated broad questions about the group's evolution as a whole. Further studies investigating the evolution of this diverse group, including studies focused on other potential isolating mechanisms, such as song, are clearly warranted. Recent analysis of more comprehensive molecular genetics data in the Pamlidae, including currently under- represented tropical species, will enable more efficient comparisons between species while controlling for phylogeny (Lovette, unpub. data). In general, my thesis presents valid methodology for exploring the morphological diversity of several taxa; utilizing the vast specimen collections of natural history museums is a non-invasive and relatively quick method for answering wide-ranging questions concerning broad scale selective pressures.

Visual Modelling

Using visual modelling when investigating ornamental colour traits presents a recent advancement in colour research (Bennett and Thery 2007), and it will likely become more prevalent as data on perceptual abilities become available for a variety of taxa. In our study, it would have been favourable to use species-specific cone sensitivities in our visual model. However, collecting cone sensitivity information is technologically demanding and very invasive, as it requires sacrificing birds; perhaps alternative methods, such as opsin gene sequencing, will come to provide species-specific visual properties in a less invasive way (Yokoyama and Yokoyama 1996). If cone- sensitivity information were to be published in the future, it might be interesting to determine whether using species-specific sensitivities would alter our results. In

99 addition, it would be interesting to evaluate plumage coloration within a single species across its entire range to determine whether plumage ornaments diverge according to geographic distance, varying degrees of sympatry across its range, and variation in the available signalling environment. Geographic variation in plumage coloration has been investigated a couple of different wood warblers (Pitocchelli 1992; Dunn et al. 2008), and it is likely that several species experience geographic variation in plumage traits across their widespread ranges.

One possible concern in our study was using the coloration of museum specimens to compare with present day environmental coloration. It may have been be more appropriate to use live birds in their current environments to measure plumage conspicuousness. However, recent studies have suggested there are minimal differences between coloration of live birds versus museum specimens (Armenta et al. 2008; Chui and Doucet 2009; Doucet and Hill 2009). Our study would probably have been enhanced by a greater sample size for each warbler species we measured. In Chapter 2, a few species had only two or three male display locations representing the species as a whole.

To ensure that these few males are not anomalies in the population, it would be beneficial to obtain larger sample sizes. Also, female plumage contrast measures were calculated using locations where males were found displaying; a more comprehensive examination of the locations of female warblers within the forest, such as while foraging or at the nest, would provide a more accurate representation of the conspicuousness or crypsis of female plumage.

100 Influence of Sympatry

The most obvious future direction following from my investigation of plumage divergence and sympatry, in Chapter 3, would be a more comprehensive sampling of genetic information for tropical species of warblers. We measured the plumage coloration of 105 species, but due to lack of genetic data, we were only able to complete the analysis for 77 species. With a greater sample of tropical species, we might be able to compare the divergent evolutionary pressures between tropical and temperate warblers.

A general observation of warbler distributions suggests that there is less sympatry between closely related species in the tropics (Mistakidis and Doucet, pers. obs), highlighting the different selective pressures that may be acting on temperate and tropical species. Additionally, genetic distance could be calculated using a variety of mitochondrial and nuclear genes to obtain a more accurate representation of divergence between species (Lovette and Bermingham 2002).

SUMMARY AND SIGNIFICANCE

My thesis presents evidence that various selective pressures have influenced plumage divergence in warblers, and that this divergence may have facilitated speciation in this group. Plumage coloration has been extensively studied in various species, but few studies have investigated its evolution at the family level or beyond (Doucet et al.

2007; Cardoso and Mota 2008). Through the influence of sexual selection and its interplay with the signalling environment, plumage ornaments have been driven to increase in conspicuousness among males. Rapid sympatric overlap in the temperate zone also appears to have promoted divergence in sexual signals through increased secondary contact between previously isolated species (Martin et al. 2009). Divergence

101 in plumage ornaments, which appear to be an important premating isolation mechanism in warblers, may have enabled several species to maintain extensive sympatric overlap with minimal hybridization (McCarthy 2006). In conclusion, environmental pressures and selection for accurate species recognition can have a significant impact on the evolution of sexual ornaments, which in turn may influence patterns of speciation.

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105 VITA AUCTORIS

NAME: Allison Flora Mistakidis

PLACE OF BIRTH: Windsor, Ontario

DATE OF BIRTH: 1984

EDUCATION: Honourable W. C. Kennedy Collegiate Institute Windsor, Ontario 1998-2003

University of Windsor Windsor, Ontario 2003-2007 B.Sc. Honours Biological Sciences with Thesis Minor: Chemistry

University of Windsor Windsor, Ontario 2007-2010 M.Sc. Biological Sciences