Species Profile: Swainson’s Hawk

Photo: Ashok Khosla

The Swainson’s Hawk is built to travel. Flying on long wings that are relatively tapered for a “buteo,” this large hawk is highly migratory, making a round trip of up to 17,000 miles between its breeding grounds in North America and wintering grounds on the pampas of Argentina.

The Swainson’s is a hawk of the west, breeding throughout much of the Rocky Mountains and western Great Plains, from southern Alberta and Saskatchewan to northern Mexico. It is most often found in grass, shrublands, and agricultural areas, where both open land for foraging and trees for roosting and nesting are available. Here ground squirrels, gophers, voles, mice, small birds, lizards, and snakes form the bulk of the hawks’ prey—a diet high in protein for raising young. Sometimes they hunt on the ground, lurking near ground squirrel holes until their prey emerges. On its wintering grounds, however, the Swainson’s Hawk is largely insectivorous, subsisting on grasshoppers, dragonflies, butterflies, moths, and beetles. In Argentina, Swainson’s Hawks have been observed feeding amongst swarms of migratory dragonflies.

Outside of the breeding season, this hawk is gregarious, foraging for food and roosting overnight in groups. Numbers of Swainson’s Hawks are often seen following prairie fires or farm equipment, waiting to snatch up any exposed insect prey.

Swainson’s also migrate in large flocks of up to ten thousand birds. South-bound they travel only over land, where they can exploit rising thermals to carry them over long distances with little effort. Mexico and Central America act like a funnel, concentrating Swainson’s and other hawks in huge numbers. Hawk watch sites in Panama and Veracruz, Mexico have recorded staggering counts of Swainson’s Hawks—up to 845,000 during one fall migration season at one site.

Declines in Swainson’s Hawk populations have been reported across much of the species’ range over the past 50 years, resulting in its inclusion on the WatchList. Loss or degradation of nesting, foraging, wintering, and migration stop-over habitat are among the primary reasons, but illegal shooting and electrocutions on power lines have also played a role. The hawk’s insect diet also makes it especially vulnerable to pesticide poisoning in agricultural fields. In one infamous example a decade ago, tens of thousands of wintering Swainson’s Hawks died in Argentina over a period of two years after monocrotophos was used to control grasshoppers in alfalfa and sunflower fields. Although the pesticide had been banned in the United States in 1991, it was still being used in Argentina with dire consequences.

In 1996, ABC spearheaded an international campaign to remove the threat of monocrotophos from Swainson’s Hawk wintering habitat, and to educate Argentinean farmers on safer pest control techniques. ABC successfully urged Ciba-Geigy (now Novartis) to halt distribution of monocrotophos to Argentina, and persuaded the Argentinean government to stop all uses, a major international conservation victory.

Numbers of Swainson’s Hawks have recently begun to stabilize. Data gathered by HawkWatch International from western migration sites demonstrate mostly stable, if not increasing, trends for Swainson’s Hawks for the last 15 years. This positive trend may correlate with a decrease in mortality due to the removal of monocrotophos in Argentina, and is an encouraging sign for this neotropical migrant raptor.