Differential Migration and Phenology of Adult Red-Tailed Hawks in California

Abstract

Abstract Differential migration by age and sex has been observed within several raptor species. Here, we report differential autumn migration timing between adult and juvenile Red-tailed Hawks (Buteo jamaicensis), and between adult males and females at the Golden Gate Raptor Observatory in the Marin Headlands of northern California. Because Red-tailed Hawks cannot be easily sexed in hand, we used discriminant function analysis (DFA) to identify morphometric measures useful for sexing adult Red-tailed Hawks captured during migration in the Marin Headlands and created a flowchart for in-hand field sexing of adult Red-tailed Hawks. The DFA correctly assigned sex 95% of the time, and provides an improved method for sexing adult Red-tailed Hawks in the Marin Headlands when compared to existing DFAs developed for other populations of this species in the western United States. Our ability to sex adult Red-tailed Hawks permitted examination of fall migration phenology, which differed markedly for adult and juveni...