Research shows that Central Valley Swainson's Hawks generally migrate to Mexico, Central America, and a few to northern South America.
Most of the world’s Swainson’s Hawks nest outside of California and migrate to Argentina each year. This population is referred to as the Great Basin or Great Plains Swainson’s Hawk population. You can find lots of information on this population of 450,000 plus hawks on the web. They do not breed with California Swainson's Hawks, nor do they winter in the same locations.
To experience migration, visit Borrego Springs, California for the spring Hawk Watch - typically February 15-April 15 of each year. Check this blog for updates and download this brochure (page 1, page 2) for all you need to know to participate.
Wildlife biologists in the Sacramento area, including Mike Bradbury, Jim Estep and Dick Anderson have tracked migratory patterns by outfitting hawks with satellite transmitters. This group, and their colleagues collectively known as the Swainson's Hawk Technical Advisory Committee, have made available information, photographs and diagrams on this and linked pages. The most recent migration study --Owen's Valley-- is described in the link below.
Summary of migration study results from Mike Bradbury, 1999. Maps of migration patterns of Swainson’s Hawk courtesy of Swainson Hawk TAC Trip Reports by FOSH, Visits to Swainson's Hawk Mexican wintering grounds: 1998 Report; 2007 Report