The Swainson's Hawk soars over open ground with wings held in a medium to strong dihedral. It occasionally courses low over the ground like a harrier and also hovers like a Red-tailed or Rough-legged hawk. The Swainson's Hawk commonly perches on the ground both during migration and on the breeding grounds. During migration, it typically roosts for the night on bare ground with scattered trees, a habit that distinguishes them from fellow long distance migrants such as the Broad-winged Hawk, which roosts in closed canopy woodlands.
The Swainson's Hawk uses several hunting 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 in Oklahoma. 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 closely follows both tractors and wild fires for injured or fleeing food. 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 defends its territory from other buteos. Breeding densities apparently vary from one area to the next with one pair per 2.59 square miles (6.7 square kilometers) in eastern Wyoming and one pair per 2.39 square miles (6.2 square kilometres) in Wyoming. Home range estimates have varied from 1.0 square miles (2.6 square kilometers) in Wyoming to 1.58 square miles (4.1 square kilometers) per pair in Utah. 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 defend territories against each other. In many parts of the plains these three species nest in the same general area and exploit much the same prey base. 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. The bird is attracted to haying, mowing, and ploughing operations. House Sparrows, European Starlings, and other small birds may nest in, or near a Swainson's Hawk's nest.
The courtship displays of the Swainson's Hawk are poorly 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. During treading one of the birds calls.
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. Wind storms and hail caused 30 percent nest failure in one study. 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.
ADAPTATIONS
Swainson's Hawk nestlings are exposed to hours of heat and parching sun each day and yet are unable to drink. They must obtain the moisture they need from the prey that their parents bring to the nest. This may be why adult Swainson's Hawks switch from insect to vertebrate prey for their young. Small mammals, such as young ground squirrels and young cottontails, form the bulk of the nestlings' diet.
The Swainson's Hawk has long, narrow wings for a buteo. Long wings may be an adaptation for long distance migration since a rough correlation exists between wing length and length of migration in several bird groups.
