Sex, Body Size, and Winter Weather Explain Migration Strategies in a Partial Migrant Population of American Kestrels (<i>Falco sparverius</i>)

Partially migratory systems describe populations that consist of both individuals that migrate away from the breeding grounds for the winter, and others that remain resident near their nesting sites year-round. Partial migration is the most common type of migration across all animal taxa, but the evolution, maintenance, and consequences associated with different movement strategies are still poorly understood. Studying the factors that drive migratory strategies and the associated consequences of those decisions is important to understand how migratory animals may adapt to climate change. Partial migrant populations offer a great opportunity for which to study these questions because individuals with different movement strategies live in the same environment for a portion of their annual cycle. This allows researchers to directly compare individuals exhibiting both migration strategies in the same study area. In our study, we examined the correlates, patterns, and consequences associated with migratory decisions in the American Kestrel ( Falco sparverius ), a small falcon species that exhibits partial migration tendencies across its range in North America. To investigate the factors associated with an individual’s migratory strategy, and to assess the migratory patterns in a population of kestrels in southwestern Idaho, we used stable hydrogen isotope values (δD) extracted from talon tissues to distinguish migrant from resident kestrels during the 2013 – 2021 breeding seasons. We determined the migratory status for 349 birds with a previously unknown strategy and saw that males were more likely to migrate than females, and that in colder than average winters, smaller birds of both sexes were more likely to migrate than larger birds. Additionally, kestrels can switch migration patterns on an annual basis, and the proportion of individuals with a migrant or resident strategy in the local southwestern Idaho population is not consistent over time. Broadly, these results offer support for the body size hypothesis in that larger individuals may be better equipped to withstand harsher winters compared to smaller-bodied individuals. We also examined the consequences associated with an individual’s movement strategy by comparing a bird’s stable hydrogen isotope value to the date that they initiated breeding. For this work, we collaborated with project partners to collect talon samples from breeding kestrels across seven sites in Indiana, Kansas, Wisconsin, Wyoming, New York, Washington, and Saskatchewan, Canada; some of which were on Department of Defense lands. There was no significant association between an individual’s isotope composition and lay date suggesting that individuals that wintered further south did not initiate breeding later than individuals that migrated shorter distances. Although these results contrast with what we predicted, they are consistent with results found in similar taxa. Overall, this study elucidates the factors influencing migration decisions in American Kestrels and uncovers previously unknown phenomena in this species such as migration strategy switching. Lastly, we determined that extracting stable hydrogen isotope values from talon tissue is a viable option for researchers seeking to categorize individuals into discrete migratory strategies in studies of animal migration.

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