Swainson's hawks are large soaring raptors of the open country. These birds exist in two main color variations: a light-morph and the dark morph. Light-morph adults are white on the underparts with a dark, reddish "bib" on the chest and a noticeable white throat and face patch. The underwings, seen as the bird soars, have light linings (leading edge) and dark flight feathers (trailing edge), a pattern unique among North American raptors. The tail is gray-brown with about six narrow dark bands and one wider subterminal band. The upperparts are brown. Dark-morph birds are dark brown except for a light patch under the tail. There is a rufous variant that is lighter on the underparts with reddish bars. The tails of both these forms resemble those of the light morph.
Swainson's hawks are found in North America mainly in the spring and summer, and winter in South America. Breeding areas include south-central Alberta, central Saskatchewan, southwestern Manitoba, and west and southern Minnesota. They will breed as far north as east-central Alaska, and southwestern Yukon. Breeding continues south through the eastern parts of Washington and Oregon, locally to the central valley of California, Arizona, New Mexico, Colorado, and most of Texas. The eastern part of their range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. Small populations winter in southeastern Florida and along the Texas coast. Immature Swainson's hawks winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. These raptors inhabit grasslands, savannahs, shrublands, and prairies. They favor wild prairie, hayfields, and pastures over wheat fields and alfalfa fields, which may offer their prey too much cover.
Swainson's hawks are long-distance migrants. They leave the breeding grounds from August to October. 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. During migration, they typically roost for the night on bare ground with scattered trees. Swainson's hawks are territorial but will gather in groups for feeding and migrating. They hunt using various methods. Many still-hunt, watching for prey activity from a perch such as a tree, bush, pylon, telephone pole, hummock or other high objects. Others hunt by soaring over open ground, using their stellar vision to watch for prey activity below. They also occasionally course low over the ground or hover while hunting. While hunting on the ground, almost entirely for large insects, their gait can appear awkward but these raptors are often successful in pinning down several insects per day. Swainson's hawks often follow tractors and other agricultural equipment during haying or ploughing, where rodents are exposed for the birds to capture, or insects are uncovered after crop cutting. In South American grass fires, Swainson's hawks frequently wait around the edges of the fire, picking off not only insects but also vertebrates including nothuras, lizards and snakes.
Swainson's hawks are carnivores and eat mainly insects and mammals. Insect prey includes grasshoppers, crickets, and locusts. Breeding birds rely heavily upon small mammals such as young ground squirrels, young cottontails, pocket gophers, mice, young jackrabbits, and, at least locally, small birds and other vertebrates including reptiles and amphibians. Birds taken include Mallards, Sage grouse and also young lark buntings.
Swainson's hawks are monogamous and form long-lasting pair bonds. The birds arrive at their nesting grounds in March or April and 7 to 15 days after, the males begin constructing nests in isolated trees or bushes, shelterbelts, riparian groves, or around abandoned homesteads. Nests are located from 9 to 15 ft (2.7 to 4.6 m) above the ground, often in the shaded canopy but near the top of the tree. The nest consists of twigs and grasses and can take up to two weeks to complete. Occasionally, a pair may nest on the ground or on a bank or ledge. The courtship displays of Swainson's hawk are not well known. One activity involves circling and diving above a potential nest site with flashed underwings and rump. The display may end with one bird diving to land on the edge of the nest. During the breeding season, Swainson's hawks become very aggressive and chase away any intruders. The female lays 1 to 4 eggs which she incubates for 34 to 35 days, while the male brings food. The chicks are altricial; they are hatched naked and helpless. Flight feathers begin to emerge at 9 to 11 days. The young begin to leave the nest for surrounding branches at 33 to 37 days, and fledging occurs at about 38 to 46 days. The chicks are usually dependent upon their parents for 4 to 5 weeks.
The main threats to Swainson's hawks include collisions with traffic, illegal shooting, electrocution, and even severe prairie weather such as hailstorms. When sharing a grove (a small group of threes) with nesting Great horned owls, the hawks suffer much egg loss due to owl predation. These raptors also suffer from frequent, unexplained egg infertility. Western populations, like that of Oregon, and southern California, have drastically declined, often due to habitat loss or incompatible agricultural practices. Although often nesting close to human activity, some Swainson's hawks are very easily disturbed at the nest and often a desert, especially early in the season. These birds are often quite tame and easy target for shooters who travel isolated prairie roads.
According to the All About Birds resource, the global breeding population of the Swain’s hawk is 580,000 individuals. According to the California Department of Fish and Wildlife resource, the total population size of the species in the state of California is over 17,000 birds. Overall, currently, Swain’s hawks are classified as Least Concern (LC) on the IUCN Red List and their numbers today remain stable.