Differential Autumn Migration of Sharp-Shinned and Cooper's Hawks in Western North America

Abstract

Sharp-shinned Hawks (Accipiter stria tus) and Cooper's Hawks (A. cooperii) showed differ ential migration timing during autumn migration in New Mexico and Nevada. Age-sex classes passed through both sites in this order: juvenite females, juvenile males, adult females, and adult males. We com pared 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 be tween 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 for aging efficiency (i.e., hunting skill level) may best ex plain age-specific differential migration timing.