Natal Dispersal Distance and Population Origins of Migrant Red-Tailed Hawks and Cooper's Hawks

Abstract

We used a two-step process to determine region of geographic origin for migratory Red-tailed Hawks (Buteo jamaicensis) and Cooper's Hawks (Accipiter cooperii). First, we used encounter data from the USGS Bird Banding Laboratory to estimate natal dispersal distances of Red-tailed Hawks and Cooper's Hawks. Then we used this information to assess whether hawks banded at migration stations and later encountered as breeding birds could be assigned to an approximate region of origin. Across the USA, the average apparent natal dispersal distance of Red-tailed Hawks was 137 ± 287 km (n = 127, median = 23 km) and of Cooper's Hawks was 40 ± 90 km (n = 70, median = 13.5 km). There were no differences in apparent natal dispersal as a function of latitude or longitude across the continent. From data on birds banded at migration stations and then encountered later as likely breeders, we inferred that these encounter locations will, on average, reflect these birds' region of origin. Because individuals of these two species tend to have short natal dispersal distances, we can infer regions of origin from breeding-season encounters of banded individuals originally trapped at migration sites across North America. As an example, we used data from migrating Red-tailed and Cooper's Hawks trapped in the Marin Headlands, California, that were later encountered as likely breeders to assign the region of origin of these individuals. Using this information, we inferred that early-migration-season Red-tailed Hawks originated in central California and that later-migration-season Red-tailed Hawks were a mix of individuals originating from central California and eastern Oregon. Cooper's Hawks captured during migration at Marin originated west of the Cascade and Sierra Nevada Mountains, including areas in northern California, Oregon, and British Columbia. Encounter data for hawks banded during migration then captured later during the breeding season may help migration researchers better understand origins of birds captured at migration sites and better link population trends at migration stations with population trends of breeding birds.