Swainson's Hawk

Swainson's hawk is a raptor and a medium-sized member of the genus Buteo. It broadly overlaps in size with the red-tailed hawk (B. jamaicensis), a related species found as a breeding resident almost throughout North America. Swainson's hawk is on average a little shorter in length, 43–56 cm (17–22 in) long, and weighs a bit less, 0.5–1.7 kg (1.1–3.7 lb). However, Swainson's hawk has a slightly longer wingspan at 117–137 cm (46–54 in), with more slender, elongated wings, than the red-tailed hawk. Female Swainson's hawks, at an average weight of 1.15 kg (2.5 lb), are somewhat larger and heavier than males, at an average of 0.81 kg (1.8 lb). Among standard measurements, the wing chord is 36.2–42.7 cm (14.3–16.8 in), the tail is 18.5–23.4 cm (7.3–9.2 in), the tarsus is 6.2–8 cm (2.4–3.1 in) and the bill (from the gape) is 3–3.5 cm (1.2–1.4 in). 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% of individuals are light-morph; the dark morph is most common in the far west of the range: 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. Juveniles 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. 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 hawk inhabits North America mainly in the spring and summer, and winters 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 its range includes Minnesota, northwestern Iowa, most of Nebraska, Kansas, and Oklahoma, and all but eastern Texas. It periodically occurs in Iowa and rarely in northwestern Missouri, northern Illinois, and southwestern Wisconsin. Small populations winter in southeastern Florida and along the Texas coast, probably having failed to find the way south around the Gulf of Mexico. Individuals reported north of these areas in winter (for example, on Christmas Bird Counts) are almost invariably misidentified buteos of other species. Swainson's hawks mostly winter on the pampas of South America in Argentina, Uruguay, and southern Brazil. The populations of Swainson's hawks breeding in California's Central Valley also winter in western Mexico and central America. The Swainson's hawk is the second longest migrant of any North American raptor, after arctic nesting Peregrine falcons. The flight from breeding ground to South American pampas in southern Brazil or Argentina can be as long as 7,100 mi (11,400 km). Each migration can last at least two months. 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. Thus, waves and small groups are strung out across the sky. The birds gradually head southwards toward Central America where virtually the entire population funnels through the Isthmus of Panama. Concentrations over locations like Ancon Hill, Balboa, and Panama City are spectacular. In the Andes, it migrates along a narrow corridor and rarely strays off course; for example, it was only recorded in the Serranía de las Quinchas of Colombia – just 100 km (62 mi) or so off its usual migration route – in 2000/2001. In Brazil, migrating birds pass through the western states of Acre and Mato Grosso, while wintering birds may stray to the southern states of Paraná, Rio Grande do Sul and São Paulo. But surprisingly, the occasional Swainson's hawk — including birds one or two years of age — has also been recorded in the eastern states of Maranhão, Pará, Pernambuco, Piauí and Tocantins, thousands of kilometers away from their usual migration route and wintering grounds and sometimes in mid-summer. This suggests that individuals occasionally become lost during migration, and/or that they may spend a whole year in the tropical regions and range about, rather than just overwintering at one site. In Uruguay, the first dedicated studies show it to be not uncommon but patchily distributed across the country in winter. Notably, it had been underreported in Flores and Paysandú Departments, where it seems in fact to be a regular visitor. In recent years, the first birds were seen in early November, and some stayed until late February. Numbers increase throughout November and peak in December, when flocks of many dozen roam the open lands. But many stay only for a scant few weeks before leaving again. Spring migration broadens once the birds have passed through Mexico as they disperse through the breeding range. Migrant groups are noted in the southern U.S. states in March. The earliest Swainson's hawks arrive in southern Canada in late March, with migration peaking from mid April onwards.