RAPID GROWTH OF THE SWAINSON'S HAWK (BUTEO SWAINSONI) POPULATION IN CALIFORNIA
|Brett J Furnas; California Department of Fish and Wildlife; email@example.com; David H. Wright, Erin N. Tennant, Reagen M. O’Leary, Michael J. Kuehn, Peter H. Bloom, Carie L. Battistone|
By 1979 Swainson’s Hawks (Buteo swainsoni) had declined to as low as 375 breeding pairs throughout their summer range in California. Shortly thereafter the species was listed as threatened in the state. To evaluate the hawk’s recent population trend, we analyzed data from 1,038 locations surveyed throughout California in either 2005, 2006, 2016, or 2018. We estimated a total statewide population of 15,907 breeding pairs (95CI: 10,414–28,505) in 2018, and found that alfalfa (Medicago sativa, lucerne) cultivation, agricultural crop diversity, and the occurrence of non-agricultural trees for nesting best explained spatial variation in hawk density. We also concluded that California’s Swainson’s Hawk summering population has grown rapidly since 2005 at a rate of 13.5% per year (95CI: 9.3–17.8%). Despite strong evidence that the species has rebounded overall in California, Swainson’s Hawks remain largely extirpated from Southern California where they were historically common. Further, we note that the increase in Swainson’s Hawks has been coincident with expanded orchard and vineyard cultivation which is not considered suitable for nesting. Therefore, we recommend more frequent, improved surveys to monitor the stability of the species’ potential recovery and to better understand the causes.