The Swainson's Hawk is mainly a summer visitor to North America. It breeds locally in east-central Alaska, southwestern Yukon and Mackenzie, and locally in a narrow band through west-central British Columbia, but is more generally distributed from south-central Alberta, central Saskatchewan, southwestern Manitoba, and west and southern Minnesota south through the eastern parts of Washington and Oregon locally to the central valley of California, Arizona, New Mexico, and most of Texas. The eastern part of its range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri, northern Illinois, and southwestern Wisconsin.
Small populations winter in southeastern Florida and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico. Individuals reported, for example, on Christmas Bird Counts north of these areas are almost invariably mis-identified buteos of other species.
Immature Swainson's Hawks winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. It is not known with certainty where most of the adults spend the winter.
There are no recognized North American subspecies.
MIGRATION
The Swainson's Hawk appears to feed during the early part of fall migration but not the later stages, although this, like many details of the species' life history, requires much more study. It is presently impossible to distinguish between late summer to early fall feeding aggregations, and the early onset of migratory assemblies, if, in fact, such a distinction exists. Some obvious migrant groups have been seen feeding; for instance, 3,000 of about 50,000 birds fed on caterpillars in a field on October 12, 1984 near Falcon, Texas. It is generally reported that Central American migrants do not feed or cast pellets. The Swainson's Hawk is probably the longest long distant migrant of any North American raptor. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 14,000 miles (22,400 kilometers). Each migration can last at least two months.
Hawks leave the breeding grounds from August to October and gradually head southwards toward Central America where virtually the entire population funnels through the Isthmus of Panama. Concentrations over locations like Ancon Hill, Balboa, and Panama are spectacular.
Fall migration begins each clear day on which a wind blows in the general direction of travel. Birds gain altitude by soaring in circles on a rising thermal and then set their wings and close their tails as they glide, slowly losing altitude until they find another thermal and rise with it. Thus, waves and small groups are strung out across the sky. By early October migrant groups are passing through Kansas and Texas.
Spring migration broadens once the birds have passed through Mexico. Migrant groups are noted in the southern states in March. The earliest Swainson's Hawks arrive in southern Canada in late March with migration peaking from mid April onwards.
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