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A once common raptor, the American kestrel (Falco sparverius) has experienced population declines in the last two decades throughout North America. Many hypotheses exist about the decline, including mortality from West Nile virus, rodenticide poisoning, climate change, an increase in predators, and core habitat loss or degradation, which could influence food availability. Food availability is key to raptor survival and reproduction, and changes in food availability throughout the year can have lifelong effects on size and body condition. Here we examine how morphology, specifically mass and wing chord, has changed at seven migration sites throughout North America as kestrel populations have declined. We hypothesized that if kestrel populations were declining due to lower food availability, there would also be declines in body size. Our results show a decrease in kestrel populations at all sites and a decline in mass and wing chord at five and four sites, respectively. We examined fat scores at two intermountain region migration sites and found that fat scores increased at one site and decreased at another. These results implicate a role for food availability in driving declines in kestrel populations, most likely during the breeding season. We also found differences in body mass and wing chord among migration sites. Despite being correlated within sites, variation in body mass and wing chord across sites differed, giving rise to variation in sexual size dimorphism and wing loading across sites. This variation may be due to selective forces acting on traits, though random divergence due to low gene flow may be driving variation in wing chord. For body mass, regional variation in males and females could be a response to ecological processes. Overall, we can conclude that lower food availability is affecting some sites and not others, and that kestrels show regional morphological variation.
Advisor: John P. DeLong