Sex, body size, and winter weather explain migration strategies in a partial migrant population of American Kestrels

Authors: Sadie C. Ranck, Christina M. Garsvo, Darin M. Schwartz, Linda M. Reynard, Matthew J. Kohn, and Julie A. Heath

Year: 2023

Publication: Ornithology

Publication Link:

Keywords: arrival time, climate change, hydrogen, isotopes, movement ecology, phenology

Abstract: Given increasing evidence that climate change affects the annual cycles of birds, it is important to understand the mechanisms underlying individual
migration strategies and population-level patterns in partial migrants. In this study, we found that thermoregulation (body size and winter
temperatures) was a key driver of American Kestrel (Falco sparverius) migration decisions. The annual proportion of migrants in the population,
however, was not explained by winter weather and may be the result of differential survival. We measured stable hydrogen isotope values
(δD) of talon tissues collected from 501 breeding and overwintering birds to distinguish migrant from resident kestrels in a partially migratory
population of American Kestrels in southwestern Idaho in 2013–2021. We then evaluated drivers of migration decisions by assessing potential
correlates of migration strategies, whether individuals switched migration strategies between years, and whether the proportion of migrants in
the population changed over time or was correlated with winter weather. Male kestrels were 1.6 times more likely to migrate than females, and
in colder than average winters, smaller birds of both sexes were more likely to migrate than larger birds. Only 27% of 26 recaptured individuals
showed evidence of switching their migration strategies on an annual basis. There was no temporal trend in the proportion of migrants in the
population, but proportions varied between years. Interestingly, there was no association between winter minimum temperature anomalies
and annual migrant proportions in the population, suggesting that differential over-winter survival, or other stochastic processes, may play an
important role in population composition. As winters continue to warm, fewer kestrels may migrate and more may remain resident on breeding
grounds. However, it is unclear how changes in migration strategies might affect population-level patterns and resilience to climate change.

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