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 juvenile Red-tailed Hawks in the Marin Headlands. Juveniles displayed two distinct peaks of migration, one in mid-September and a second in mid-November. In contrast, the number of migrating adults increased steadily through mid-November, and declined thereafter, with adult females migrating earlier than adult males and the mean passage date for both sexes much later than documented at other North American hawk watch sites.
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Vol. 50 • No. 1