Speciation Via Loss of Migration and Correlated Morphological Evolution in Fork-tailed Flycatchers

A mechanism by which animals cope with seasonal changes is by moving up to thousands of kilometers every year. These seasonal migrations require navigational, morphological and physiological adaptations for journeys to be successfully accomplished at the correct time and direction, in an energetically efficient manner. Migratory behavior contrasts with a year-round resident strategy in which animals adapt to climate swings in temperate environments, or to the challenges of breeding in the tropics (such as nest parasites and high nest predation rates). Among birds, migratory species are often closely related to year-round residents, implying migration is frequently gained and/or lost. In theory, these switches in migratory strategies may result in the formation of new species. I empirically tested this hypothesis, showing that loss of migration promoted speciation in flycatchers that have both migratory and year-round resident populations. I found evidence that loss of migration in Fork-tailed Flycatchers (Tyrannus savana) induced shifts in breeding schedules, which coupled with geographic isolation of breeding populations has led to the evolution of premating isolation and correlated evolution of morphology (i.e., in wing shape and tail length). Moreover, loss of migration has influenced conspecific communication of non-vocal acoustic signals produced by males with wing feathers; morphological evolution likely associated with flight efficiency has significantly changed the frequency at which wing feathers flutter. Finally, morphological evolution associated with migratory behavior has also led to differences in early stages of the life cycle, particularly in egg shape. These trends follow patterns observed at higher taxonomic levels, in which eggs from birds with longer wings tend to have more elliptical eggs, supporting the hypothesis that flight may impose constraints to egg shape. In conclusion, correlated evolution associated with losing migration in Fork-tailed Flycatchers has driven differentiation across multiple traits, enhancing the speciation process.