Each autumn, nearly the entire breeding population of the Swainson's Hawk migrates from the temperate zone of North America to “wintering” areas in South America. From prairie Canada, this migration is more than 10,000 km each way, a distance second among raptors only to that of the Arctic Peregrine Falcon (Falco peregrinus tundrius).
A highly gregarious species, the Swainson's Hawk forages and migrates in flocks sometimes numbering in the thousands. Its movement through Central America has been described as among “the most impressive avian gatherings in North America, since the demise of the Passenger Pigeon” (Brown and Amadon 1968). Nearly 350,000 Swainson's Hawks have been counted passing over a single point in Panama City in October and November, and up to 845,000 have been counted in a single autumn in Veracruz, Mexico.
Although wide-ranging and common, the discovery and naming of this species did not come about until the 1820s. Dr. John Richardson, English surgeon and naturalist with the Franklin Arctic expeditions, collected the first Swainson's Hawk in 1827 at Fort Carlton near Saskatoon, Saskatchewan. The bird was illustrated by William Swainson and identified as Buteo vulgaris (now Buteo buteo, the Common Buzzard of Europe) in Richardson and Swainson's classic work Fauna Boreali-Americana . In 1838, Charles Lucien Bonaparte, realizing this was a different species, applied the name Buteo swainsoni, commemorating Swainson's earlier work, but basing his description on a plate drawn by John James Audubon for a bird collected at Fort Vancouver, Washington.
The breeding-season diet of the Swainson's Hawk is similar to that of other temperate-zone buteos; young are fed rodents, rabbits, and reptiles. When not breeding, however, this hawk is atypical because it is almost exclusively insectivorous, eating grasshoppers (Acrididae) in particular. Only by concentrating on abundant insects can large concentrations of Swainson's Hawks be sustained.
In many parts of its range, this hawk has adjusted to agricultural landscapes. Nonetheless, its numbers have declined significantly in parts of the western United States; and in the western Canadian prairie, reproduction of the Swainson's Hawk has dropped since the mid-1980s, following a decline in its main prey species, Richardson's ground squirrel (Spermophilus richardsonii). In 1995, satellite telemetry was used for the first time to investigate Swainson Hawk migration; these studies revealed that thousands of these hawks were dying in Argentina as a result of the use of pesticides.