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Species Profile: Swainson's Hawk
The Swainson's hawk is a raptor with a thin body and narrow wings that migrates impressive distances. It weighs between 1.5 and 3 pounds (693 and 1367 grams), has a wingspan of 46 to 54 inches (117 to 137 centimeters) and is 18.9 to 22 inches (48 to 56 centimeters) in length. Nearly the entire population of Swainson's hawks migrates between breeding grounds in western North America and overwintering grounds in South America. During their journey, these hawks travel roughly 12,427 miles (20,000 kilometers) round trip, making them one of the world's longest-distance migratory raptors.
The Swainson's hawk breeds in the grasslands of the Midwestern and Western United States. They often build nests in small groves of trees that border streams and agricultural fields. A pair of Swainson's hawks will lay a clutch of one to five eggs, which they incubate for more than a month before the eggs hatch.
The nestling hawks are very small when they hatch and too weak to even raise their heads. The parents must care for them for six weeks before the young birds are strong enough to leave the nest. While raising their young in the summer, the hawks have a varied diet. They feed on small animals, such as squirrels, gophers, voles, mice, birds, lizards, snakes and insects. They capture their prey by perching on tree tops, poles or posts, rocks, and higher ground. Then, they swoop down to catch their prey by surprise. Swainson's hawks have also been observed ambushing prey, for example, waiting outside a squirrel’s burrow for it to emerge.
Before leaving their breeding grounds to make the journey south, Swainson's hawks must prepare for their long trip. In late August and early September, they form flocks and begin to put on fat to fuel their upcoming journey. Having just finished raising their offspring, adults in particular need to put on more weight to survive the fall migration. Swainson's hawks put on fat by gorging on grasshoppers until winds from the north arrive, and the flocks take off to travel south.
Once they are in flight, the flocks use winds from the north and thermals to help them save energy while flying. Thermals are columns of air that rise right from the surface of warm patches of land. Birds use these thermals to gain height without flapping their wings as the column of air rises. As it cools, the birds glide down to another thermal and repeat the process. This pattern of following thermals allows the birds to use very little effort and energy.
Swainson's hawks rely heavily on thermals and typically avoid crossing water from which thermals rarely rise. This reliance on land masses to migrate means that almost the entire population of Swainson's hawks must funnel through the narrow land bridge between North and South America, often in massive flocks of hundreds of thousands of birds.
During their journey from North America to South America, Swainson's hawks spend little time feeding to refuel. Instead, they fast for most of the time. When they do eat, they feed exclusively on insects, such as grasshoppers, dragonflies, butterflies, moths and leaf beetles. They catch insects on the ground or in midair with their talons and can eat them while in flight.
Swainson's hawks spend the bulk of their nonbreeding season south of the equator in South America. To say that Swainson's hawks overwinter in South America may be a bit inaccurate, because it is summer in the Southern Hemisphere when they are there! The hawks therefore travel from summer breeding grounds in North America to summer overwintering grounds in South America. Their diet in this period is similar to their diet during migration. They almost exclusively eat insects. Their goal is to stay as healthy as possible and, especially just before returning north, to put on enough body fat to successfully migrate and raise their young.
Conserving the Swainson's Hawk
Swainson's hawks cover enormous distances each year, and they are vulnerable to threats throughout their journey, including hunting, collisions with manmade structures and poisoning from pesticides. They may also be threatened by habitat loss at both their breeding and wintering grounds.
The Mystery of the Disappearing Swainson's Hawks
Brian Woodbridge, a biologist with the U.S. Forest Service, had been studying Swainson's hawks in Butte Valley National Grasslands in northern California for 15 years when he noticed something alarming. The population of Swainson's hawks in his study area had suddenly declined. Woodbridge and his colleagues had been monitoring Swainson's hawk populations at the site since 1979 by trapping and marking birds with unique color bands during the breeding season, re-releasing marked birds and counting how many of these individuals returned each year after winter migration.
In a typical year, about 85-90 percent of banded hawks would return to the breeding grounds in the spring. "But then a year would come up when nearly half failed to return," Woodbridge said of the decline. "We were seeing an abnormally high turnover rate of adult hawks."
Woodbridge and his colleagues had to determine what was occurring on their wintering grounds that could cause such a massive decline. They knew the birds wintered somewhere in Argentina, but did not know precisely where. So from the winter of 1994 to 1995, scientists attached satellite transmitters to 50 Swainson's hawks, including two from the Butte Valley site. Signals from these transmitters were sent to National Oceanic and Atmospheric Administration Argos satellites and then relayed back via the internet where they could be accessed by Woodbridge and his team.
The data showed that the hawks were wintering in grassland and agricultural sites in the Pampas region of Argentina. These vast plains were the perfect spot for the hawks to fatten up on their preferred winter snack, grasshoppers, which make up about 95 percent of their winter diet.
But when Woodbridge made a trip down to the Pampas to investigate, he made a disturbing discovery: approximately 6,000 dead Swainson's hawks, 700 of which were on just one farm. The culprit was a pesticide that had been banned in the U.S. but was still in use in Argentina. Farmers were spraying this pesticide to control grasshoppers that were considered an agricultural pest. Those grasshoppers were in turn being eaten by the hawks, poisoning them.
This mass die-off sparked a huge public outcry and mobilized many organizations to help protect Swainson’s hawks. World governments, farmers, conservation groups and even pesticide manufacturing companies teamed up to remove the pesticide from the Pampas. The Argentine government banned the pesticide from being used on alfalfa fields where the hawks tended to congregate. The chemical companies that manufactured the pesticide agreed to stop selling the chemical in the region and recommended less harmful alternatives for farmers.
Conservation and government organizations placed TV and radio ads to get the word out about the new regulations. Researchers also shared information with Argentine scientists, so they could better report and diagnose similar problems in the future. The data from the satellite tagged birds helped determine where the regulation efforts should be concentrated. The following year, only 24 hawk deaths were reported in the area. A potential crisis for Swainson's hawk populations had been averted.