Population and Morphological Changes in American Kestrels through Space and Time

Many American kestrel (Falco sparverius) populations are declining across North America. Potential causes include mortality from West Nile Virus, anticoagulant rodenticide exposure, climate change, an increase in avian predators, habitat degradation, and reduction in food availability. We analyzed American kestrel count and banding data from seven raptor migration sites throughout North America with at least 20 years of migration data. We used count data to determine the year at which the kestrel population began a significant decline and then used banding records to determine whether body mass and wing chord declined after this point. We found reductions in mass at five sites and wing chord at four sites. We also assessed wing pit fat for two sites, and this metric of energy reserves declined at one site and increased at another site. Our results indicate declines in body size at the majority of sites are consistent with the hypotheses that food availability, change in climate, predation risk, or increases in the use of anticoagulant rodenticides are causing population declines. In contrast, the sites that do not show significant trends in body size could indicate that West Nile virus is contributing to population declines.