Wintering Areas and Migration Characteristics of Swainson’s Hawks That Breed in the Central Valley of California

Abstract

We used satellite telemetry during 2011–2015 to identify and characterize wintering areas and migration patterns of Swainson's Hawks (Buteo swainsoni) that bred in California's Central Valley. Twenty tracked hawks wintered across 7500 km from western Mexico to central South America. Wintering areas in Mexico, Central America, and central South America were dominated by agriculture and in northern South America by shrub-scrub. All hawks followed similar migration routes, through the interior of California, Sonoran Desert, and western Mexico, with some continuing through Central America to South America. Compared to northerly wintering birds, birds wintering farther south spent more time in migration, flew greater distance per day during southward (but not northward) migration, spent less time in wintering areas, and arrived later at the breeding area. Central Valley birds substantially used stopover areas (x = 53 d/ individual) during southward migration but returned more directly from wintering to breeding areas (x=0.3 stopover d/individual). Most stopovers occurred in western Mexico and the San Joaquin Valley, California. Land cover in stopover areas was dominated by agriculture and shrub-scrub. Use of varied wintering areas that have been altered from native habitat may indicate that migratory and wintering patterns have changed for this population. Advantages of using northern wintering areas may include less time spent in migration and earlier arrival in breeding areas. The diversity of wintering areas may provide resilience from effects of human activities compared to collective wintering in one area. Central Valley Swainson's Hawks’ migration routes and wintering areas differ markedly from those of the rest of the species’ North American population.