Migratory Strategies and Morphology of American Kestrels

Demanding energetic requirements associated with migration result in selective pressure on the morphology of birds to make them more aerodynamic. Birds that remain resident on the breeding grounds may not have the same selective pressure for aerodynamics. Therefore, migrant and resident individuals in the same breeding population may have differences in wing length, tail length, and overall mass. American Kestrels, the smallest falcons in North America, vary in their migratory behavior across the continent, creating excellent opportunities for comparative studies. We examined the relationships between migratory strategy and morphology of American Kestrels in Idaho’s Treasure Valley, where kestrels are partial migrants, and Camas Prairie Centennial Marsh, where kestrels are a fully migrant population. We determined the migratory status of Treasure Valley kestrels using stable hydrogen isotope analyses on claw samples taken during the breeding season. We predicted that migrant kestrels would have longer tails and wings, and lighter mass compared to resident kestrels. Overall, we found no correlation between migratory status and longer tails (p = 0.125). There is evidence that size is related to migratory status, as our findings point to birds with smaller mass and shorter wings being more likely to migrate (p = 0.001; p = 0.035). These results provide further insight into the relationships between migratory strategy and morphology within bird populations.