Rapid growth of the Swainson's Hawk population in California since 2005

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 population trend since then, we analyzed data from 1,038 locations surveyed throughout California in either 2005, 2006, 2016, or 2018. We estimated a total statewide population of 18,810 breeding pairs (95% CI: 11,353-37,228) in 2018, and found that alfalfa (Medicago sativa, lucerne) cultivation, agricultural crop diversity, and the occurrence of non-agricultural trees for nesting were positively associated with hawk density. We also concluded that California's Swainson's Hawk summering population grew rapidly between 2005 and 2018 at a rate of 13.9% per year (95% CI: 7.8-19.2%). 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. Our results are consistent with increasing raptor populations in North America and Europe that contrast with overall global declines especially in the tropics. Lay Summary center dot Swainson's Hawk (Buteo swainsoni) is listed as a threatened species in California where its population dropped to as low as 375 breeding pairs in 1979. center dot We surveyed for the species throughout its breeding range in California in 2005, 2006, 2016, and 2018. center dot We found that the Swainson's Hawk population in California has increased rapidly at an average annual rate of 14% during this time period to over 18,000 pairs in 2018, but the causes for the increase remain unclear. center dot Although raptor populations are declining globally, especially in the tropics, our findings are consistent with recent recoveries of raptor populations in North America and Europe.

Automated summary

The population of Swainson's Hawks in California has experienced rapid growth in recent decades, although they remain nearly extinct in Southern California. The expansion of orchards and vineyards may have contributed to the increase in hawk numbers.