Bird of the Month: Swainson's hawk
By Andrew McCormick
Scientific Name: Buteo swainsoni
Length 19 in
Wingspan 51 in
Weight 1.9 lb
AOU Band code SWHA
The life cycle of Swainson’s Hawks provides an excellent illustration of the importance of having a hemispheric perspective on bird conservation. Using the Central and Pacific flyways this western North American species makes an annual round trip migration of about 12,000 miles to and from southern South America and the Argentine pampas. These hawks gather with Broad-winged Hawks and Turkey Vultures along high altitude “streets” of thermals in a massive migration in the North American fall. They spend the austral summer feeding on grasshoppers and dragonflies (Bechard, et al).
Although Swainson’s Hawks make their diet of insects, farmers in the past have tried to shoot as many Swainson’s Hawks as they could. Poisoning with pesticides was the method of choice in Argentina, where thousands of these hawks were killed in the mid-1990s. Once one of the most common birds in the western plains states, Swainson’s Hawks have lost much of their historical habitat which has been converted to farmland. Some population estimates indicate a loss of 90% from the bird’s historical level. However, the Swainson’s Hawks have proven to be adaptable to agriculture and they are now comfortable in alfalfa and corn fields. The pesticides that killed so many have been banned, but there is still spraying at times (Bechard, et al). Current population numbers range from 200,000 to 400,000.
Aves Argentina has worked to have the pampas declared an Important Bird Area and Lagoa de Peixe has been established as a national park in Brazil thanks to the Brazilian Institute for the Environment and Renewable Natural Resources. These South American agencies have worked with Partners in Flight and the Nature Conservancy to protect grassland habitat on both continents in the hemisphere (Wells).
The Swainson’s Hawk is in the genus Buteo Latin for hawk (Holloway). Buteos are called “buzzards” in much of the world except North America (Alderfer). They are large hawks that soar on broad, rounded wings and in general make their diet out of rodents and other small mammals, snakes, lizards and sometimes other birds. Swainson’s Hawk is named for William Swainson, a British naturalist and artist, who was for a time the Assistant Keeper of Zoology. He is an unsung natural history artist despite being an innovative and skillful one, who published more books than Thomas Bewick, Alexander Wilson and John James Audubon combined (Mearns & Mearns).
Swainson’s Hawks have lovely two-toned plumage with the flight feathers darker than the wing linings (coverts). Seen from below the light-morph form has a dark head, often with gray cheeks and white throat. The chest is dark and forms a bib (Alderfer). The belly can be white or streaked. These hawks also have an intermediate or rufous form which has plain or lightly streaked rufous color on the chest and belly. The dark morph is dark all over.
Swainson’s Hawks arrive on the breeding grounds in Washington by late April. The nest is often built on an old magpie nest. Usually 2-3 dull white eggs with brown spots are deposited and incubated by the female for a little over a month. The Swainson’s diet changes to small mammals and reptiles while feeding the young. Juveniles can fly in about six weeks but stay with the adults until fall migration.