Evolutionary Consequences of Seasonal Migration in North American Birds

Abstract

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.