Then in early July, several weeks after the young birds have learned to fly, the hawks begin to roost and hunt in flocks. During the early 1900's, the Swainson's Hawk nested in lowlands throughout most of California maintaining populations as large as 17,000 pairs. Ten years ago, only 550 nesting pairs were found in California and numbers have been slowly declining. Today, most nesting is confined to the Central Valley and parts of the Great Basin. About two-thirds of the statewide population nest in the southern Sacramento Valley and northern San Joaquin Valley regions.
Moderate numbers nest in northeastern California and the Klamath Basin.
There have even been a few pairs nesting in southern Mono County and in the Owens Valley in Inyo County. One pair nests in Kern County, and another in Los Angeles County. The primary cause of the decline in Swainson's Hawk's population size is believed to be habitat destruction. Much of California's native grassland that was once their home has been converted to crop fields and pastures. This has lead to a decrease in prey, loss of nesting sites, loss of hunting habitat, and exposure to harmful chemicals used for agricultural purposes, such as pesticides.