Birds
Swainson's Hawk
Buteo swainsoni · also called Swainson's hawks
A kettle of thousands of Swainson’s hawks spirals upward on a single rising thermal over the Great Plains, the visible beginning of a migration that will carry them all the way to the grasslands of Argentina before the year is out.
Overview
Swainson’s hawk, Buteo swainsoni, is a long-distance migrant native to open grassland and agricultural habitat across western North America. Its extraordinarily long annual migration to South America, dramatic seasonal dietary shift toward grasshoppers, and history of large-scale pesticide poisoning on its wintering grounds make it one of the more ecologically significant and historically consequential raptors on the continent.
A hawk built for the long haul
Few North American raptors commit to a journey as demanding as Swainson’s hawk’s annual round trip to Argentina, a migration so long that the species has had to adapt its very diet along the way to make the trip sustainable.
Status
Least Concern — recovered from significant losses following pesticide poisoning on its Argentine wintering grounds in the 1990s, now widespread across suitable grassland habitat.
Swainson's Hawk: common questions
How far does Swainson's hawk actually migrate?
This species undertakes a round-trip migration of roughly 12,000 miles or more between its breeding grounds in western North America and its wintering areas on the pampas of Argentina, among the longest migratory journeys of any raptor found on the continent.
Why does Swainson's hawk eat mostly grasshoppers for part of the year?
Unlike most Buteo hawks, which rely heavily on rodents year-round, Swainson's hawk shifts largely to grasshoppers and other large insects once it is no longer feeding nestlings, a dietary flexibility that suits both its long migration and its time spent on insect-rich South American grassland.
What happened to Swainson's hawks in Argentina in the 1990s?
Large numbers of wintering Swainson's hawks died after exposure to the pesticide monocrotophos, which was sprayed on agricultural fields where the hawks fed heavily on grasshoppers and other insects, a poisoning event significant enough to prompt international conservation action.
Why do Swainson's hawks gather in such large flocks during migration?
This species forms enormous migratory flocks called kettles, sometimes numbering in the thousands, allowing individuals to ride the same rising thermals together and conserve energy across a journey that can stretch thousands of miles each way.