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

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.