Migration and winter ranges of ferruginous hawks from Washington: Final Report

Between 1999 and 2003, 13 adult and 15 juvenile ferruginous hawks (Buteo regalis) from Washington were monitored via satellite telemetry to assess their migration, range use, and survival. These factors were studied to better understand ferruginous hawk ecology and population dynamics, and to promote recovery of this species from state Threatened status. Adult hawks migrated twice. Post-nesting migration of 10 hawks was longitudinal to the eastern front of the Rocky Mountains in Alberta and Montana, and was a functional response to populations of Richardson’s ground squirrels (Spermophilus richardsoni). Post-nesting migration of 3 adult hawks was to the Great Basin. In the fall, hawks migrated latitudinally to winter ranges. Eight hawks wintered in central California and fed on California ground squirrels (S. Beecheyi), 2 hawks wintered in the Central Plains, and 1 bird returned to his territory in Washington. The Continental Divide was not a physical barrier to hawk migration, and the population exhibited broad-fronted migration without using specific flight corridors. Adult males migrated through or remained on territories in route to winter ranges, whereas most females migrated through the Central Plains. Sexual differences in migratory patterns may have resulted from differences in prey preference and territory affinity. Six adults monitored for 2 seasons exhibited markedly similar migration patterns, and high winter range philopatry. There was a 100% return rate of adults to breeding territories in spite of drought conditions and declining productivity in this localized area of the nesting range. Cumulative adult survival over 4 years was 0.46 (±0.15), and no consistent cause of mortality was identified for adult hawks.

Juvenile hawk migration was characterized by high initial mortality and exceptional movements. At least 3 of 8 juveniles died from starvation, and cumulative juvenile survival was 0.43 (±0.10) at 1 year. Juveniles wandered extensively for an average of 6,139 km throughout western North America during 90 days prior to their settling on winter ranges in California, the Central Plains, and Mexico. Young and adults from the same nests migrated independently and followed dissimilar migration patterns. Two young migrated over 2000 kilometers less to winter ranges in their second year, compared to the first year. These factors suggested there was a strong learning component to the establishment of individual adult migration routes.

A lack of nomadism, high adult fidelity to breeding sites, and movement of immature birds back to the Pacific Northwest during the nesting season indicate the population of ferruginous hawks in Washington is self-sustained. The population is maintained by hawks that range widely during 49% of the year when they are exposed to multiple mortality sources. Direct mortality of adults in winter may be less of an issue to survival rates than indirect effects of habitat loss. Maintenance of prey and habitat resources, principally in the Northern Plains and central valley of California, are important to sustain hawks through the winter and replenish adult fat reserves for reproduction the following spring. Widespread agricultural conversion and urbanization are real and significant threats to these habitats. Juvenile survival, although less important than adult survival to population maintenance, is most impacted by poor foraging conditions in Washington, likely a result of depressed prey populations and drought. Protection of this population, and other migratory ferruginous hawk populations, will require regional and international cooperation.