Wing and bill measurements of Tyrannus round specimens identified to subspecies

Morphology is closely linked to locomotion and diet in animals. In animals
that undertake long-distance migrations, limb-morphology is under
selection to maximize mobility and minimize energy expenditure. Migratory
behaviors also interact with diet, such that migratory animals tend to be
dietary generalists, while sedentary taxa tend to be dietary specialists.
Despite a hypothesized link between migration status and morphology,
phylogenetic comparative studies have yielded conflicting findings. We
tested for evolutionary associations between migratory status and limb and
bill morphology across kingbirds, a pan-American genus of birds with
migratory, partially migratory, and sedentary taxa. Migratory kingbirds
had longer wings, in agreement with expectations if selection favors
improved aerodynamics for long-distance migration. We also found an
association between migratory status and bill shape, such that more
migratory taxa had wider, deeper, and shorter bills compared to sedentary
taxa. However, there was no difference in intraspecific morphological
variation among migrants, partial migrants, and residents, suggesting that
dietary specialization has evolved independently of migration strategy.
The evolutionary links between migration, diet, and morphology in
kingbirds uncovered here further strengthen ecomorphological associations
that underlie long-distance seasonal movements in animals.