Seasonal Migration, Gene Flow, and Speciation in North American Birds

Abstract

Around 20% of bird species migrate annually between distinct breeding and wintering ranges and many sedentary populations shift cyclically within a single range in response to varying climates and food availability. Migration allows populations to take advantage of seasonal resource abundance and access isolated habitat patches, but it comes with significant costs. Individuals have to build up large fat reserves to power the journey, and must navigate huge distances in their first year of life. At a population level, migratory species experience high levels of gene flow between distant areas of the breeding range, which maintains genetic diversity but makes local adaptation difficult because of a continuous inflow of alleles from off-target migrants. Conversely, when migratory behaviors are themselves heritable they can reinforce population differentiation by selecting against unfit hybrids. However, we know relatively little about migratory connectivity or genetic diversity across the range in most species, and it is unclear if the few examples of migratory variation associated with genetic differentiation are the exception or the rule. Is seasonal migration a homogenizing force during speciation, or a differentiating one? In this dissertation I study three examples of migratory birds in the early stages of speciation, and ask how migratory patterns shape genetic diversity during lineage divergence. In chapter 1, I test for gene flow and estimate species limits in the Red-Eyed Vireo (Vireo olivaceus) species complex -- a group of four to seven species that breed in disjunct habitats around the Americas and winter in sympatry in northern South America. Next I use a combination of breeding and wintering range samples from the Painted Bunting (Passerina ciris) to map migratory connectivity in the species, and identify an unexpected link between breeding populations in the lower Mississippi River Valley and wintering populations on the Yucatán Peninsula. Last, I sequence and analyze the complete genomes of the Rufous, Allen's, and Calliope Hummingbirds (Selasphorus rufus, sasin and calliope, respectively) in order to map geographic population structure and identify regions of the genome under disruptive selection across species.