The Swainson’s Hawk (Buteo swainsoni) is a large hawk named after William Swainson, a British naturalist. It is found on prairies and in dry grasslands throughout the western plains of North America.
This hawk requires trees for nesting and is therefore found around aspen groves, rivers, streams and farmlands. It prefers wild prairie, hayfields, and pastures instead of wheat fields and alfalfa fields which may offer its prey too much cover. It requires elevated perches for hunting and a supply of small mammals such as young ground squirrels as prey for its nestlings. The breeding distribution of the Swainson’s hawk is tied closely to the distribution of various small mammals for this reason. In Saskatchewan, for example, the distribution of Richardson’s ground squirrel and the Swainson’s hawk are precisely the same.
This species spends its winters in the pampas of South America and is considered to be one of the most highly migratory of all North American hawks. In a life span of seven or eight years, a Swainson’s hawk might cover as many as 15,000 miles (24,140 km) during its migrations north and south.
This slender raptor is slightly smaller in size than a red-tailed hawk with the exception of its slightly longer wingspan and slimmer wings. A common color pattern for the adult Swainson’s hawk is dark brown plumage with a brown chest and a pale belly.
In flight, the Swainson’s hawk holds its wings in a shallow “V” dihedral and teeters in flight similar to a turkey vulture. The long, slim, pointed wings are indicative when they show the two-toned effect of pale wing linings and dark flight feathers. No other buteo shows such consistently dark flight feathers.
Adult birds have a pale body with a dark “bib” on the chest and a noticeable white throat patch. There are two main color variations:
- Light morph birds are white on the underparts and underwings; they have a grey tail with dark bars and a reddish breast. Their head is brown with a white patch on the chin.
- 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.
Perched Swainson’s will have wings that extend to the tip of the tail or beyond. A light colored belly sometimes barred, especially on flanks; brown back and upper wing surfaces; a gray-brown tail with about six narrow dark bands and one wider subterminal band.
The Swainson’s hawk is largely insectivorous, except when nesting. Insect prey includes grasshoppers, crickets, and locusts. Pairs bring vertebrate prey to their nestlings, relying heavily upon small mammals such as young ground squirrels, young cottontails, pocket gophers, mice, young jack rabbits, small birds and occasionally reptiles and amphibians. Birds taken include large birds such as mallards, and sage grouse which may have been injured initially. Other bird prey can include American kestrel, young short-eared owls and lark buntings taken at their fledging time. Reptiles prey includes snakes such as racers, gopher snakes and striped whipsnakes, and lizards. Amphibians may include tiger salamanders and toads. The Swainson’s hawk is an opportunistic feeder which responds quickly to local concentrations of food. Its habit of gorging on outbreaks of crickets and grasshoppers has earned it the popular name “Grasshopper Hawk”.
In Argentina, flocks of immature Swainson’s hawks feed on flocks of the migratory dragonfly Aeshna bonariensis, following the hordes of insects and feeding mostly on the wing. Local outbreaks of locusts may also be exploited for food by one or more age-classes of birds. Juveniles wintering in southern Florida will feed upon either insects, mice, or both, when turned up from field plowing. They move from one freshly plowed field to the next.
While hunting, Swainson’s hawk will use several strategies. It hunts insects, such as dragonflies or dobson flies while in flight, flapping little as it rides a wind current and stoops upon a fly, grabbing it with its foot and immediately transferring the prey to its bill. It uses a similar strategy to grab individual free-tailed bats from flying streams of bats. Also, when dragonfly hordes are grounded by weather, the Swainson’s hawk will stand near groups sheltering from the wind and pluck at individual insects. The Swainson’s hawk follows tractors and wild fires closely for injured or fleeing prey. It will also run down insect prey on the ground. Occasionally a hawk will stand still on a dirt bank or elevated mound waiting for prey to appear. It commonly hunts from elevated perches such as telephone poles, stooping on prey when it’s sighted.
The Swainson’s hawk will defend its territory from other buteos. Breeding densities vary slightly from one area to another but studies have shown that there is an average of one pair per 2.5 square miles. The average home range estimate for this hawk is 1 to 2 square miles. The Swainson’s hawk gathers in groups for feeding and migrating. However, in each case, such gathering is not social, but motivated by good feeding or migrating conditions.
The Swainson’s hawk, red-tailed hawk, and Ferruginous hawk compete for territory, and in turn defend territories against each other. In many parts of the plains these three species nest in the same general area and hunt for similar prey. Although diets overlap greatly, habitats may not overlap as much. In Oregon, the Swainson’s hawk selects nesting trees having a different configuration than those used by red-tailed or ferruginous hawks. In southern Alberta, different nesting habitats help reduce food competition, with the Swainson’s hawk favoring areas with scattered trees or riparian borders, while red-tailed hawks nest in stands of tall trees, and the ferruginous hawk nests on the open plains.
Reduced reproductive success may result from the Swainson’s hawk’s nesting proximity to these two other buteos. The Swainson’s hawk is generally tolerant of people and is attracted to hay harvesting, mowing, and plowing operations. House sparrows, European starlings, and other small birds may nest in, or near a Swainson’s hawk’s nest.
When Swainson’s Hawks arrive at their nesting sites in March or April, they may return to their original nests as these hawks are known to be monogamous. Research indicates that they have a high degree of mate and territorial fidelity, an unusual occurrence in a long distance migrant. Seven to 15 days after the birds arrive, the males begin constructing nests on the ground, ledges or in a trees. The nest consists of twigs and grasses and can take up to two weeks to complete. New nests may be constructed, old nests refurbished, or the nests of other species, including those of common raven, black-billed magpie, and American crow, refurbished.
Swainson’s hawks typically nests in isolated trees or bushes, shelterbelts, riparian groves, or around abandoned homesteads. Occasionally, a pair will nest on the ground or on a bank or ledge. Nest trees and bushes include ponderosa pine, Douglas-fir, spruce, cottonwood, domestic poplar, aspen, elm, mesquite, willow, saguaro cactus, and soaptree yuccas. Nests are located from nine to 15 feet above the ground, often in the shaded canopy but near the top of the tree. Nests are flimsy structures, usually smaller than the nests of the red-tailed hawk, and often blow down after nesting season.
Outside the breeding season, the Swainson’s hawk is a usually silent species. During summer the most common vocalization is a shrill, aggravated-sounding “kearrrrrrr”, weaker than the similar call of a red-tailed hawk. The voice of the females is lower pitched than the males. This call is given when people approach the nest and in other aggressive encounters.
The courtship displays of the Swainson’s Hawk are not well known. One activity involves circling and diving above a potential nest site. The underwings and rump are flashed and the birds call. The display may end with one bird diving to land on the edge of the nest. Copulation occurs mainly in the morning and evening on the dead limbs of trees. The female may assume the receptive position without a prior display.
Clutch size ranges from 1-4 eggs, but averages 2-3 eggs. Each egg is approximately 2.25 inches long and 1.8 inches wide. The egg is smooth and white, often tinted bluish or greenish. During incubation the shell color quickly turns to dull white. Some eggs are plain; others are lightly marked with spots and blotches of light brown. The incubation period is 34 to 35 days, with the female incubating while the male brings food.
Juvenile flight feathers begin to emerge at 9-11 days. High nestling mortality often occurs when the young are 15-30 days old and may be a function of fratricide. The young begin to leave the nest at 33-37 days and fledging occurs at about 38-46 days. The fledglings are dependent upon their parents for at least 4-5 weeks. This species has one brood a year and apparently does not lay replacement clutches.
The oldest wild Swainson’s Hawk on record is 15 years 11 months. Swainson’s Hawks die because of collisions with traffic, illegal shooting, electrocution, and even during severe prairie weather such as hailstorms. In one particular study, a 30% nest failure was seen due to wind storms and hail. When sharing a grove with nesting great horned owls, hawks suffer much egg loss due to owl predation. The species also suffers from frequent, unexplained egg infertility.
Due to loss of habitat this species is seeing a slight decline. Additionally, the Swainson’s Hawk still encounters pesticide use when it migrates in the winter to Argentina. Known as the locust hawk, the hawks will eat numerous amounts of these insects and in turn ingest a high amount of toxin, which causes a thinning of the egg shells.