Swainson's Hawk
Buteo swainsoni

Taxonomy and Distinguishing Characteristics

The Swainson's Hawk, like other diurnal raptors belongs to the Order Falconiformes.   It is in the Accipitridae family, which includes hawks and eagles.  It is a member of the Buteo genus, which includes soaring hawks with long, broad wings and wide, fanned tails.  Compared to other Buteos, the Swainson's Hawk is slimmer, with a relatively long tail and long, pointed wings.  It soars with wings held in a dihedral position.  The plumage varies among juveniles and adults, and among light and dark morphs (see images).

Males and females look similar in appearance, but females (weighing 937-1,367 grams and 51-56 cm long) are larger than males (weighing 693-936 grams and 48-51 cm long).

Distribution

The breeding range of Swainson's Hawks can be seen in blue in the range map below.  From north to south, the breeding range extends from Alberta, Canada, down to the northern part of Mexico.  From west to east, the breeding range extends from northeastern California to western Minnesota.  The winter range is primarily in the Pampas of Argentina and can be seen in red in the range map below.  Notice how much smaller their winter range is compared to their breeding range.

Migration

Nearly the entire population of Swainson's Hawks migrates twice each year between the wintering range in the Pampas of Argentina and the breeding grounds in North America.  For those individuals that breed in Canada, the trip can exceed 10,000 km each way!  They take advantage of rising thermals of hot air, as well as updrafts along mountain ranges to help power their flights.  They migrate in flocks that can be as large as 5,000-10,000 individuals.  When these huge numbers of birds pass through the narrow migration route through Central America, and they are joined by numerous other species of hawks, this "river" of hawks creates one of the most impressive migration spectacles in the world!

The spring migration begins in late February, when the birds begin to move northward from Argentina.  They arrive on their breeding grounds between early April and mid-May, depending on how far north they breed (the farther north they breed, the longer it takes to get there).  By late August and early September, just prior to the fall migration, huge flocks of adults and juveniles begin to gather, fattening up on grasshoppers to provide them with the energy to make the huge trip south.  They take advantage of northerly winds (that blow from north to south) to power their flights, and arrive back in Argentina some time in November.

Habitat

On their breeding range, Swainson's Hawks forage in open, grassy habitat, sparse shrublands, and small, open woodlands.  They have adapted well to foraging in agricultural areas, but only in crops that don't grow too tall, since it is difficult for them to find their prey in tall vegetation.  They typically nest in scattered trees within these grassland, shrubland, or agricultural landscapes.  During migration, the birds rest and feed in grasslands and harvested fields, especially where grasshoppers are numerous, and often perch on fence posts, telephone poles, and power poles.  At night, large flocks roost together in trees.  On their "winter" range in the Pampas of Argentina (it's summertime in Argentina, but winter in North America) they forage in agricultural fields and natural grasslands.  At night, they often roost in Eucalyptus groves.

Food

During the breeding season, the Swainson's Hawks eat mostly vertebrates, including mammals (such as rodents and rabbits), birds and reptiles, and some invertebrates.  On their wintering grounds, they eat almost exclusively insects, which is quite unusual for a Buteo.  This habit of feeding in areas with an abundance of insects is what sustains such large wintering flocks of Swainson's Hawks. 

Courtship Behavior and Breeding

Swainson's Hawks are generally monogamous.  A mated pair engages in acrobatic courtship displays, beginning with the pair soaring in circles high above the nest site, with their wings and tails spread.  The male then breaks into a rapid dive toward the nest and then flies back up again.  He repeats this several times until he perches near the nest, followed by the female, and then the pair copulates.  Though the male appears to select the location of the nest, both members of the pair help in building the nest, a task that takes 1-2 weeks.  The female typically lays 1-4 eggs. The female incubates the eggs, except when she leaves the nest for brief periods to forage; then the male takes over. The eggs hatch after 35 days, and the hatchlings are altricial.  The male provides most of the food for the female and the brood, but the female begins to hunt more frequently as the young get older.  Fledging occurs at about 38-46 days after hatching.  The fledglings remain with their parents for another 30 days, but the parents and juveniles separate before migration.

Conservation Status

The Swainson's Hawk was once very abundant throughout the western U.S., but is now greatly reduced in numbers and distribution.  Though not listed as endangered, it is considered a Species of Special Concern in some states and is listed as Threatened in California.  Up until 1955, a major cause for their decline was shooting by hunters and farmers.  Since 1955, shooting has become much less of a problem, but habitat destruction on both their breeding and wintering grounds is eliminating them from many areas.  The destruction of suitable trees in which they build their nests may also be limiting their populations.  The use of organophosphate insecticides by farmers in Argentina to control grasshopper outbreaks in agricultural fields resulted in the deaths of thousands of Swainson's Hawks (see Swainson's Hawk Research for more details).  Though scientists and conservationists managed to convince farmers to stop using this particular toxic insecticide, many other potentially harmful pesticides are still being used, although their effects on this species are not well understood yet.      

References and Further Reading

England, A.S., M.J. Bechard, and C.S. Houstan.  1997.  Swainson's Hawk (Buteo swainsoni).  In The Birds of North America, No. 265. (A. Poole and F. Gill, editors).  The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.

 

Sibley, D.A.  2000.  Pages 114-125 in National Audubon Society The Sibley Guide to Birds,.  Alfred A. Knopf, Inc., New York.