Swainson’s Hawks (Buteo swainsoni) breed in California’s Central Valley, and most migrate from there south to spend the winter from central Mexico to central South America. Prior to 1978, there were no confirmed winter records for Swainson’s Hawks in central or northern California. Beginning in the early 1980s, however, one to several were found to be wintering annually in the Central Valley. From 1987 to 1994, winter numbers increased, especially in the Sacramento-San Joaquin Delta. Almost 30 hawks were observed in the winters 1991–1992 and 1993–1994 at an evening roost on Andrus Island. The large, flat agricultural islands in the west-central part of the Delta near the confluence of central California’s two largest rivers appear to be the main area where these hawks gathered to forage and to roost. On 4 January 2025, birders participating in the Sherman Island Christmas Bird Count on the west side of the Delta counted up to 23 Swainson’s Hawks. This was the highest count since 1994, and it motivated us to document recent numbers, examine and summarize previous winter records of Swainson’s Hawks in the Delta, and to consider factors that might explain why the Delta is a key wintering location in the Central Valley.
Swarth et al. (Thu,) studied this question.