Migration Patterns in Birds of the New World: Seasonal, Morphomefric and Physiological Considerations

Tyrannus is a highly diverse genus with respect to migration strategies within the largest adaptive radiation of birds in the world (Tyrannidae). Within the genus, there are Nearctic- Neotropical migrant, Neotropical austral migrant, intratropical migrant, and resident species and subspecies. The overall goal of this dissertation was to use a variety of methods to explain the diversity of migration patterns found in the New World (focusing on the Tyrannus genus) to gain a better understanding of the ecology and evolution of migration in birds.;In my first chapter I developed a phylogeny to account for phylogenetic relatedness in analyses demonstrating that natural selection has acted on two morphologies relevant to the strategy-specific niches of migratory versus resident Tyrannus (wing pointedness), which may be part of a suite of co-adaptations that are adaptive for migratory populations. Then I showed that New World migratory passerines are tracking with different aspects of seasonality in the Northern versus the Southern Hemisphere, better equipping further research exploring differential impacts of climate change on different types of migrants. In my final chapter I developed a novel gas-chromatography mass-spectrometry (GC/MS) protocol to discover evidence that austral migrant Fork-tailed flycatchers had higher concentrations of oleic and linoleic acid in their diets compared to residents, demonstrating that a diet switch from insects to fruit is important to fuel spring migration, via the accumulation of polyunsaturated fatty acids (PUFAs).;This study has improved our understanding of the ecology and evolution of migration in birds by extending hypotheses developed for Nearctic migrants into a genus with both Nearctic and austral migrants. Taken together, my findings demonstrate important new information for proximate versus ultimate mechanisms driving the evolution of migration in birds, as well as elucidating important ecological patterns of Nearctic-Neotropical versus Neotropical austral migration that may be important for developing species distribution models and conservation plans under climate change scenarios where both future temperature and rainfall regimes are expected to change.