Swainson's Hawk
Swainson's Hawks feed largely on mammals, birds, and reptiles during the breeding season, but are almost exclusively insectivorous for the rest of the year.

It has the outward appearance of a typical hawk of open country, but in its life history, Swainson's Hawk is far from ordinary. The species spends only about one-third of the year in North America, where it breeds. During this time, Swainson's Hawks subsist on mammals, birds, and reptiles. After breeding, however, their diet shifts almost completely to insects. In late summer, they form great flocks and migrate south, passing over Central America in concentrated waves, in one of the hemisphere's great avian spectacles. Most Swainson's Hawks "winter" in Argentina, during the austral summer. Often reaching more than 10,000 miles round-trip, their migration is the longest of any hawk of North America.

Early written accounts of Swainson's Hawk widely note its abundant numbers, but the species is now thought to be much less numerous and widespread than in the 19th and early 20th centuries. The reasons for apparent declines are not fully understood. In the mid-1990s, new research using satellite telemetry revealed wintering grounds in Argentina, where almost 6,000 Swainson's Hawks died as a result of acute exposure to organophosphate insecticides used to control grasshoppers in agricultural fields.

Swainson's Hawks breed in western North America, from the Great Plains west to the Great Basin and the Central Valley of California, and from central Saskatchewan and Alberta south to northern Mexico. They inhabit various open-country habitats, including grasslands, shrublands, and agricultural fields. Swainson's Hawks do not frequent areas where crops grow higher than native vegetation, but do take readily to many types of cropland, including wheat and alfalfa fields.
Swainson's Hawk Range Map

Swainson's Hawks feed mostly on vertebrates during the breeding season. Specific composition of diet varies by region. In Arizona, they feed mostly on lizards and snakes; in Utah, on rabbits; in Canada, on Richardson's ground squirrel; and in California, on other rodents. Swainson's Hawks forage communally for rodents in cultivated areas during and after harvest, when prey are abundant and easily located, and during flood irrigation and burning of fields, when prey tend to concentrate on field margins.

They nest in isolated trees, small groves, or riparian areas adjoining their open foraging habitat. As with many other hawks, the nest of Swainson's Hawk is a large, rough structure made of sticks. Females may begin laying eggs from late April to late May, with intervals of up to 25 days between first laying dates even for neighboring pairs. Clutches generally consist of two or three eggs, which are usually plain and colorless, unlike the speckled or blotchy eggs of many other buteos. Nestlings generally fledge by mid-August.

In late summer, Swainson's Hawks begin to assemble in flocks. Flocking Swainson's Hawks forage communally, often consuming copious amounts of grasshoppers. After fattening up, and upon the incidence of northerly winds to aid their flight, Swainson's Hawks begin their migration south. After traveling through North America in large flocks of up to 10,000 individuals, they converge over the narrow land masses of Mexico and Central America, sometimes in staggering numbers. In the 1996 migration season alone, one team of observers counted 845,000 Swainson's Hawks passing above Veracruz, Mexico.

For most Swainson's Hawks, the long journey ends in Argentina, as much as 6,000 miles from their breeding grounds. There, they spend the Southern Hemisphere summer in native grasslands and agricultural fields, feeding almost exclusively on insects. They begin their return journey to North America in late February or early March, arriving at their breeding grounds in April or May.

Description: Swainson's Hawks are medium-sized birds of prey. Length ranges from 19 to 22 inches, about the same as the much more common and widespread Red-tailed Hawk. Swainson's Hawk has the general proportions shared by all buteos, but with a relatively slimmer body and long wings. Females are slightly larger than males, but otherwise sexes are similar in appearance.

Like many other hawks, Swainson's Hawks show a wide variety of color morphs. Light adults make up the majority of the species. They have dark brown heads and upperparts, with a white face and chin and mostly white underparts, with a broad brown band across the upper chest. Narrow white band at base of tail may be visible on the upper side of a Swainson's Hawk in flight. Flight feathers, when viewed from below, are dark, in contrast to the white body and wing linings. The underparts of darker adults have various amounts and shades of brown. The darkest adults show almost no white at all, except on the undertail coverts, with little if any contrast between flight feathers and wing linings.

Juveniles also vary widely in coloration, with wing patterns similar to adults of their particular morph. Juveniles are generally mottled above and marked with heavy spots below.

Soaring Swainson's Hawks hold their wings slightly above horizontal. They glide with wings bowed downward. When perched, they may closely resemble Prairie Falcon; relative length of wingtips (extending past tail in Swainson's Hawk, stopping short of tail tip in Prairie Falcon) may aid attempts to distinguish the two.

Voice: Call is a shrill keeeah, variable in duration and often fading at the end.


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