Differential autumn migration of Sharp-shinned and Cooper's Hawks in western North America

Abstract

Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii) showed differential migration timing during autumn migration in New Mexico and Nevada. Age-sex classes passed through both sites in this order: juvenile females, juvenile males, adult females, and adult males. We compared the magnitude of differential migration timing in these two species. The number of days separating mean passage dates of age classes (juvenile and adult) was greater for Sharp-shinned Hawks than for Cooper's Hawks. The number of days separating the mean passage dates of sex classes was mostly similar between the two species. In the Manzano Mountains of New Mexico, however, adult male and female Sharp-shinned Hawks were more separated than adult male and female Cooper's Hawks. In Sharp-shinned and Cooper's Hawks, it seems likely that differences in rate of travel between males and females may best explain sex-specific differential migration timing and that foraging efficiency (i.e., hunting skill level) may best explain age-specific differential migration timing.