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The availability of small, lightweight tracking devices enhances our ability to study birds during mobile phases of their lives. Tree Swallows Tachycineta bicolor, a model species of wild songbird, are well-studied during their breeding season; but our understanding of their biology at other times of the year, when they are not tied to the fixed location of a nest, is more limited. We developed a lightweight radio tag with no battery (solar nanotag) to study the movements of small animals, and we deployed it to explore the behavior of Tree Swallows after the end of their summer breeding season. We tagged 32 breeding adult swallows and 36 juveniles and monitored their presence and absence at the breeding site during the post-fledging period. Although our observations are based on very small sample sizes, the tags revealed previously unknown patterns in Tree Swallow behavior during the post-breeding season. Some Tree Swallow fledglings continued to visit the site repeatedly in the months following the nesting season, with the latest detection occurring on September 30th; by contrast, all adults had permanently departed by the end of July. These results inform future hypotheses about post-breeding movements in Tree Swallows. But, more generally, the detection of tagged swallows on their distant wintering grounds, seven months after tagging, indicates the potential of studying small passerine movements throughout their entire lifetimes, and suggests a rich array of applications for these \"Life Tags\" to study the movements of small animals world-wide.

Animals must balance various costs and benefits when deciding when to breed. The costs and benefits of breeding at different times have received much attention, but most studies have been limited to investigating short-term season-to-season fitness effects. However, breeding early, versus late, in a season may influence lifetime fitness over many years, trading off in complex ways across the breeder’s lifespan. In this study, we examined the complete life histories of 867 female tree swallows (Tachycineta bicolor) breeding in Ithaca, New York, between 2002 and 2016. Earlier breeders outperformed later breeders in short-term measures of reproductive output and offspring quality. Though there were weak indications that females paid long-term future survival costs for breeding early, lifetime fledgling output was markedly higher overall in early-breeding birds. Importantly, older females breeding later in the season did not experience compensating life history advantages that suggested an alternative equal-fitness breeding strategy. Rather, most or all of the swallows appear to be breeding as early as they can, and differences in lay dates appear to be determined primarily by differences in individual quality or condition. Lay date had a significant repeatability across breeding attempts by the same female, and the first lay date of females fledged in our population was strongly influenced by the first lay date of their mothers, indicating the potential for ongoing selection on lay date. By examining performance over the entire lifespan of a large number of individuals, we were able to clarify the relationship between timing of breeding and fitness and gain new insight into the sources of variability in this important life history trait.

Intraspecific genetic diversity across a species' geographic range is relevant to adaptive potential and long‐term population persistence, and identifying genetically distinct groups within species can direct management decisions focused on conserving species‐level genetic diversity. Comparative phylogeography using whole genome techniques allows for investigation of whether co‐distributed species exhibit shared spatial genetic differentiation at fine spatial scales, thereby facilitating a comparative approach to both landscape and conservation genetics. By sequencing over 900 low‐coverage whole genomes, we evaluated the concordance of genetic structure and diversity from 12 co‐occurring species of migratory birds whose breeding ranges span adjacent North American ecogeographic regions: the vast boreal forest belt and the temperate montane Appalachian forests. We detected concordant phylogeographic patterns in 11 of 12 species wherein populations from the southern Appalachians were genetically distinct from boreal belt populations. Our results reveal that small populations persisting in the southern Appalachian Mountains consistently harbor genetic diversity that is subtly distinct from much larger, widespread boreal populations of the same species. However, in most species, levels of standing genetic diversity were not significantly different between Appalachian and boreal populations despite the drastic difference in geographic extent of these populations. We found no evidence for shared signatures of selection across the genome, suggesting that the concordance of spatial genetic structure across species emerges from species‐specific patterns of molecular divergence across the genome rather than parallel patterns of selection. Conservation of the Appalachian ecosystem would likely support maintenance of distinct genetic diversity in several migratory avian species with widespread distributions.

Background Rapid morphological change is emerging as a consequence of climate change in many systems. It is intuitive to hypothesize that temporal morphological trends are driven by the same selective pressures that have established well-known ecogeographic patterns over spatial environmental gradients (e.g., Bergman’s and Allen’s rules). However, mechanistic understanding of contemporary morphological shifts is lacking. Results We combine morphological data and whole genome sequencing from a four-decade dataset in the migratory bird hermit thrush ( Catharus guttatus ) to test whether morphological shifts over time are accompanied by genetic change. Using genome-wide association, we identify alleles associated with body size, bill length, and wing length. Shifts in morphology and concordant shifts in morphology-associated alleles over time would support a genetic basis for the observed changes in morphology over recent decades, potentially an adaptive response to climate change. In our data, bill size decreases were paralleled by genetic shifts in bill size-associated alleles. On the other hand, alleles associated with body size showed no shift in frequency over time. Conclusions Together, our results show mixed support for evolutionary explanations of morphological response to climate change. Temporal shifts in alleles associated with bill size support the hypothesis that selection is driving temporal morphological trends. The lack of evidence for genetic shifts in body size alleles could be explained by a large role of plasticity or technical limitations associated with the likely polygenic architecture of body size, or both. Disentangling the mechanisms responsible for observed morphological response to changing environments will be vital for predicting future organismal and population responses to climate change.

Divergence in song between allopatric populations can contribute to premating reproductive isolation in territorial birds. Song divergence is typically measured by quantifying divergence in vocal traits using audio recordings, but field playback experiments provide a more direct way to behaviorally measure song divergence between allopatric populations. The White-breasted Wood-Wren (Henicorhina leucosticta; hereafter \"WBWW\") is an abundant Neotropical species with four mitochondrial clades (in Central America, the Darién, the Chocó and the Amazon) that are deeply divergent (~5-16% sequence divergence). We assessed the possibility that the WBWW as currently defined may represent multiple biological species by conducting both statistical analysis of vocal characters and field playback experiments within three clades (Central America, Chocó and Amazon). Our analysis of vocal traits revealed that Central American songs overlapped in acoustic space with Chocó songs, indicating vocal similarity between these two populations, but that Central American songs were largely divergent from Amazonian songs. Playback experiments in the Caribbean lowlands of Costa Rica revealed that Central American WBWWs typically responded aggressively to songs from the Chocó population but did not respond to playback of songs from the Amazonian population, echoing the results of the vocal trait analysis. This marked difference in behavioral response demonstrates that the songs of Central American and Amazonian WBWWs (but not Central American and Chocó WBWWs) have diverged sufficiently that Central American WBWWs no longer recognize song from Amazonian WBWWs as a signal to elicit territorial defense. This suggests that significant premating reproductive isolation has evolved between these two populations, at least from the perspective of the Central American population, and is consistent with the possibility that Central American and Amazonian populations represent distinct biological species. We conclude by advocating for the further use of field playback experiments to assess premating reproductive isolation (and species limits) between allopatric songbird populations, a situation where behavioral systematics can answer questions that phylogenetic systematics cannot.

Common names of species are important for communicating with the general public. In principle, these names should provide an accessible way to engage with and identify species. The common names of species have historically been labile without standard guidelines, even within a language. Currently, there is no systematic assessment of how often common names communicate identifiable and biologically relevant characteristics about species. This is a salient issue in ornithology, where common names are used more often than scientific names for species of birds in written and spoken English, even by professional researchers. To gain a better understanding of the types of terminology used in the English-language common names of bird species, a group of 85 professional ornithologists and non-professional contributors classified unique descriptors in the common names of all recognized species of birds. In the AvianLexiconAtlas database produced by this work, each species’ common name is assigned to one of ten categories associated with aspects of avian biology, ecology, or human culture. Across 10,906 species of birds, 89% have names describing the biology of the species, while the remaining 11% of species have names derived from human cultural references, human names, or local non-English languages. Species with common names based on features of avian biology are more likely to be related to each other or be from the same geographic region. The crowdsourced data collection also revealed that many common names contain specialized or historic terminology unknown to many of the data collectors, and we include these terms in a glossary and gazetteer alongside the dataset. The AvianLexiconAtlas can be used as a quantitative resource to assess the state of terminology in English-language common names of birds. Future research using the database can shed light on historical approaches to nomenclature and how people engage with species through their names.

Organisms are frequently exposed to challenges during development, such as poor weather and food shortage. Such challenges can initiate the hormonal stress response, which involves secretion of glucocorticoids. Although the hormonal stress response helps organisms deal with challenges, long-term exposure to high levels of glucocorticoids can have morphological, behavioral, and physiological consequences, especially during development. Glucocorticoids are also associated with telomere shortening, and they have a complex relationship with survival. To investigate whether brief, acute exposures to glucocorticoids can also produce these phenotypic effects in free-living birds, we exposed wild tree swallow (Tachycineta bicolor) nestlings to a brief exogenous dose of corticosterone once per day for 5 d and then measured their morphology, baseline and stress-induced corticosterone levels, and telomere length. We also deployed radio tags on a subset of nestlings, which allowed us to determine the age at which tagged nestlings left the nest (fledged) and their pattern of presence and absence at the natal site during the postbreeding period. Corticosterone-treated nestlings had lower mass, higher baseline and stress-induced corticosterone, and reduced telomeres; other metrics of morphology were affected weakly or not at all. Our treatment resulted in no significant effect on survival to fledging, fledge age, or age at first departure from the natal site, and we found no negative effect of corticosterone on interannual return rate. These results show that brief acute corticosterone exposure during development can have measurable effects on phenotype in free-living tree swallows. Corticosterone may therefore mediate correlations between rearing environment and phenotype in developing organisms, even in the absence of prolonged stressors.

Blood sampling is a frequendy used method of collecting genetic and physiological data in natural populations, and understanding the possible impact of blood sampling on individuals and populations is important, both for the welfare of study organisms and to avoid introducing bias into analyses using bled individuals. Most studies of birds have revealed minimal negative effects of blood sampling. However, Brown and Brown (2009. Auk 126: 853-861) found that blood sampling reduced return rates of Cliff Swallows (Petrochelidon pyrrhonotd), suggesting that these results are not always generalizable and that swallows (Hirundinidae) may be particularly sensitive to blood sampling. We examined the possible effects of blood sampling on the reproductive performance of female Tree Swallows ( Tachycineta bicolor) and the return rates of both females and offspring in a population in New York state. To reduce the chances of Type II error, we tested 15 possible effects of blood sampling on reproductive performance and return rates using generalized linear mixed models. Overall, our results suggest that blood sampling had few negative effects on within-year reproductive success or survival of adult females. The one exception was that bleeding nestlings had a negative effect on the number of young that fledged in broods of five or six nestlings. Bleeding did not negatively impact return rates of females or nestlings in our Tree Swallow population. Our results support those of other studies suggesting that blood sampling has few negative effects on within-year reproductive success or survival of adult females, and reiterate the importance of systematically testing for possible effects with datasets compiled over multiple years because such effects may be impossible to detect via direct monitoring during a single breeding season. La toma de muestras de sangre es un método frecuentemente usado para colectar datos genéticos y fisiológicos en poblaciones naturales y es importante entender el posible impacto de la toma de muestras de sangre sobre los individuos y las poblaciones, ambas razones, el bienestar de los organismos de estudio y para evitar la introducción de sesgos a los analisis usando individuos sangrados. Muchos estudios en aves han revelado que los efectos negativos de la toma de muestras de sangre son mínimos. Sin embargo, Brown y Brown (2009. Auk 126: 853–861) encontraron que la toma de muestras de sangre reducía la tasa de retorno en Petrochelidon pyrrhonota, sugiriendo que estos resultados no son siembre generalizables y que las golondrinas (Hirundinidae) pueden ser particularmente sensibles a la toma de muestras de sangre. Examinamos los posibles efectos de la toma de muestras de sangre en el desempeño reproductivo de Tachycineta bicolor y las tasas de retorno en hembras y crías en una población en el estado de Nueva York. Para reducir los chances de error Tipo II, probamos 15 efectos posibles de la toma de muestras de sangre en el desempen̄o reproductivo y las tasas de retorno usando modelos lineales mixtos generalizados. En general, nuestros resultados sugieren que la toma de muestras de sanere tuvo pocos efectos negativos sobre el éxito reproductivo intra-anual o las tasas de supervivencia de Tas hembras adultas. La única excepción fue que el sangrado de los pichones tuvo un efecto negativo en el numero de juveniles que salían del nido en las nidadas de cinco seis pichones. El sangrado no tuvo ningún impacto negativo sobre las tasas de retorno de las hembras o de los pichones en nuestra población de Tachycineta bicolor. Nuestros resultados soportan esos otros estudios que sugieren que la toma de muestras de sangre tiene pocos efectos negativos sobre el éxito reproductivo intra-anual o sobre la supervivencia de hembras adultas y reiteran la importancia de comprobar sistemáticamente los posibles efectos en las bases de datos recopiladas a través de multiples años que dichos efectos pueden ser imposibles de detectar por medio de monitoreos durante una única temporada de reproductión.

In migratory species, the temporal phases of the annual cycle are linked to seasonally shifting geographic ranges. Despite intense interest in the annual cycle ecology of migratory species, a synthetic understanding of the relationship between the biogeography of the migratory annual cycle and its phenology remains elusive. Here, we investigate the spatiotemporal structure of the annual cycle in a phylogenetic comparative framework by developing a method to demarcate the pacing of annual cycle stages using eBird, a massive avian occurrence dataset, and applying it to migratory passerine birds breeding in North America. Our analyses reveal a striking negative correlation between the durations of the breeding versus nonbreeding periods, indicating that a tradeoff between the lengths of the two stationary periods is the primary axis of variation in annual cycle pacing. Our results further show that the duration of annual occupancy in the breeding versus stationary nonbreeding ranges predicts the geographic separation of these seasonal ranges, demonstrating that the ratio of time spent on stationary breeding versus nonbreeding locations evolves in tandem with a species' migration distance. In contrast, the amount of time during which species undergo seasonal migration—that is, the duration of the seasonal periods when species' geographic ranges shift latitudinally—varies relatively little across species compared with the length of the stationary periods. Our study helps untangle the complexity of seasonal distributions and schedules to reveal the integrated evolution of the biogeography of the migratory cycle, its pacing, and life history tradeoffs among species.

Seasonally migratory animals breeding at high latitudes escape winter conditions by temporarily moving to warmer climates. Migration requires substantial time and energy, and its influence pervades migratory animals’ biology, from their morphology to their annual time budgets. Through adaptation to long-distance travel, migratory animals have few constraints on movement, even as they experience other constraints (e.g., time constraints) that nonmigratory animals do not. In light of their high mobility and constrained annual schedules, I investigated how seasonal migration influences evolutionary processes in a comparative context. My research focuses on small bird species in North America, with a particular focus in two chapters on the migratory avifauna of the boreal forest. The boreal avifauna comprises species with broadly co-distributed breeding ranges that spend the winter in disparate locations, making it a natural system for assessing consequences of variation in migratory strategy. I focus on how migration distance affects the evolutionarily consequential processes of geographic range expansion, gene flow, and life history evolution. Studies show that high mobility promotes dispersal and range size, yet some have suggested that migratory behavior restricts dispersal and range expansion because innate, spatially precise migratory behaviors do not transfer well into new spatial contexts. To test whether migration distance promotes or constrains range expansion, I conducted multivariate model comparison using songbirds breeding in North America (306 species). I measured a morphological proxy of mobility (wing shape) on over 1000 museum specimens and quantified range expansion using species distribution models based on climate information and millions of citizen science records. My results revealed that migration distance does not promote range size in North American birds. I suggest that these species are all sufficiently mobile that their geographic ranges are not meaningfully constrained by dispersal ability. Next, to analyze the relationship between migration distance and gene flow, I generated a massive multi-species population genetic dataset (~1780 genome sequences from 34 boreal-breeding species). I quantified and compared continuous spatial genetic variation across species, finding that many long-distance migrants display patterns of geographic structure that reflect reduced dispersal. These two chapters demonstrate that the relationship between mobility and spatial evolution, apparent in many taxa across the globe, breaks down in the North American avifauna. Finally, I analyzed migration from a novel perspective as a life history strategy—i.e., a strategy correlated with the life history continuum of investment in survival vs reproduction. Species that invest more in survival and less in reproduction have slower rates of molecular evolution than species at the opposite end of the continuum. In the North American boreal avifauna, my work has shown that migration is a life history axis whereby long-distance migrants invest more in survival by spending less time on breeding grounds, and short-distance migrants spend more time breeding at cost to survival. Using mitochondrial genomes from 39 species, I applied a Bayesian modeling framework to co-estimate rates of molecular evolution and their correlation with migration distance. I also used population genomic data from 27 of these species (~950 samples total) to test whether migration distance influenced the dynamics of mitochondrial selection. My results support the hypothesis that long-distance migrants have slower mitochondrial molecular evolution. Overall, my dissertation uses comparative methods to highlight the role played by time and life history constraints, rather than movement constraints, in the evolution of migratory animals.