This article first appeared in the May 2003 issue of the San Antonio Audubon News.
I don’t remember exactly where I was on October 1, 1997, but I do know
exactly where a certain Swainson’s Hawk was on that day. She was just
northeast of town, having come down across the Panhandle from Colorado. A week
later she was in Oaxaca. By early November she had traveled south along the
Andes clear down to Argentina. The following spring she returned, passing some
distance east of here on her way back to her breeding territory in Minnesota.
All of this we know due to the miracle of satellite tracking (see www.raptor.cvm.umn.edu/content.asp?page
Swainson’s Hawks are unique among hawks in their genus in that, like many kite species, they spend most of their life flocking with others of their kind. Broad-winged Hawks also migrate in flocks, but these little hawks flock only on migration, and separate again upon reaching the forests of Central and South America. Swainson’s Hawks, on the other hand, commonly migrate clear to the pampas of Argentina, and roost communally all winter in flocks numbering up to thousands of hawks. A ready explanation for this unusual behavior lies in the fact that, like those aforementioned kite species, these large but lightly-built hawks subsist upon insects for most of the year, feeding primarily on crickets, grasshoppers and dragonflies.
Swainson’s Hawks arrive on their North American breeding grounds later than most raptors. Once on their breeding range they behave much like other hawks, pairing off and defending territories. They also undergo an abrupt dietary shift, forsaking insect prey in favor of mice, small rabbits and ground squirrels. They will also take small birds, reptiles and amphibians.
On their breeding grounds, Swainson’s Hawks must contend with Red-tailed and/or Ferruginous Hawks, both of which take many of the same prey items, and are already on territory when the Swainson’s arrive. Red-tails have successfully adapted to a wider variety of habitats, relative to Swainson’s Hawks, though they are heavier on the wing, and require tall perches from which to hunt. The light build and long wings of the Swainson’s allow it to hunt efficiently from the air, and it requires only enough trees for nesting and roosting. The Ferruginous Hawk is also a bird of open country, but prefers drier habitats, commonly takes larger prey items and is less tolerant of human activity and habitat alteration.
Swainson’s Hawk populations declined significantly with the passing of the prairie. Modern agriculture still provides open country and to some extent the large insect and small vertebrate prey that this species requires. Indeed, this hawk may be regarded as a staunch ally of the farmer; in addition to feeding upon rodents in season, by late summer these hawks are gathering in flocks again and seeking insect prey. A single hawk may consume more than 200 grasshoppers per day, perhaps laying on fat for migration, during which time it is believed they eat very little. Likely, it was the search for late-summer grasshoppers that drew hawk #29855 southeast across the plains from Minnesota to Colorado, in company with others of her kind.
Swainson’s hawk populations have been declining again in recent years. Poisoning by pesticides may be a major cause, the same hordes of insects which attract the hawks are also likely to be subject to control efforts by affected farmers. On the wintering grounds in Argentina, applications of insecticides on occasion caused the death of hundreds of these hawks at a time. Thankfully, education programs aimed at farmers have been at least partially successful.
I’m not sure where #28955 is just now… Back in Minnesota catching mice, I hope.
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