Swainson’s Hawk

Swainson's HawkSwainson’s hawk (Buteo swainsoni), is a large Buteo hawk of the Falconiformes, which is on the decline in Southern California, though it appears to be holding its own elsewhere.
As a medium sized raptor, Swainson’s hawk is on average 17 to 22 inches long and weighs 1.1 pounds to 3.7 pounds. It has a 46 to 54 inch wingspan. It has wings that are more slender and elongated than the red-tailed hawk, which it resembles. Female Swainson’s hawks, at an average weight of 2.5 pounds are somewhat larger and heavier than males, at an average 1.8 pounds. Among standard measurements, the wing chord is 14.3 to 16.8 inches, the tail is 7.3 to 9.2 inches, the tarsus is 2.4 is 3.1 inches and the bill from the gape is 1.2 to 1.4 inches. In flight, Swainson’s hawk holds its wings in a slight dihedral; it tips back and forth slightly while soaring.
There are two main color variations. Over 90 percent are light-morph, which 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 on the leading edge and dark flight feathers on the 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. Juveniles are are similar but dark areas have pale mottling and light areas, especially the flanks, have dark mottling. The chest is pale with some darker marks. The subterminal band of the tail is less obvious. Birds in their first spring may have pale heads because of feather wear.
Roughly ten percent of Swainson’s hawk are dark morph, which are most common in the far west of the range, and 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 hawk inhabits North America mainly in the spring and summer and winters in South America.
Swainson’s hawk is probably the longest migrant of any North American raptor. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 14,000 miles. Each migration can last at least two months.
They leave the breeding grounds from August to October. Fall migration begins when a wind blows in the general direction of travel. Birds gain altitude by soaring in circles on a rising thermal draft and then set their wings and close their tails as they glide, slowly losing altitude until they find another thermal and rise with it.
The birds head southwards toward Central America and funnel through the Isthmus of Panama.
It occasionally courses low over the ground or hovers while hunting, using their stellar vision to watch for prey activity below. It commonly perches on the ground both during migration and on the breeding grounds. It hunts on the ground, pinning down several insects per day. During migration, it typically roosts for the night on bare ground with scattered trees.
They also catch insects in flight. They take advantage of insects turned up by farm equipment or driven out by fire. It hunts insects such as dragonflies or dobsonflies 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 will prey upon free-tailed bats from flying streams of bats
Swainson’s hawk, the red-tailed hawk (B. jamaicensis) and the ferruginous hawk (B. regalis) compete for territory, and defend territories against each other.
Swainson’s hawks may be largely insectivorous except when nesting. Insect prey commonly taken includes grasshoppers, crickets, and locusts. However, breeding birds switch mainly to capturing vertebrate prey, which pairs then bring to their nestlings. Breeding Swainson’s hawks 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 large birds such as Mallards, and Sage Grouse which may have been injured.
Their breeding habitat is prairie and dry grasslands and shrubs and cliff edges, upon which they will build a stick nest in a tree or shrub or on a cliff edge.
When Swainson’s hawks arrive at their nesting sites in March or April, they may return to their original nests as these hawks are noted to be monogamous. Research indicates that they have a high degree of mate and territorial fidelity. This is unusual in a long-distance migrant.
Clutch size ranges from one to four eggs, but averages two to three. Each egg is elliptical in shape, about 2.25 inches long and 1.8 inches wide. The egg is smooth with fine granulations and the ground color is white, often tinted bluish or greenish. The incubation period is 34 to 35 days, with the female incubating while the male brings food.
Young Swainson’s hawks are fed small, young mammals. Flight feathers begin to emerge on the young at 9 to 11 days. High nestling mortality often occurs when the young are 15 to 30 days old and may be a result of fratricide. The young begin to leave the nest for surrounding branches at 33 to 37 days, fledging occurs at about 38 to 46 days. The fledglings are dependent upon their parents for 4 to 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 24 years.

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