North Coast Journal


Hawks in winter

by Lisa Ladd-Wilson

Biologist Brian Woodbridge stood staring at the hundreds of dead Swainson's hawks that littered the groves on a ranch in Argentina. The shock he felt nearly eclipsed the wonder of how he got there.

By the time he and two colleagues left the ranch, located near the town of Col. Hilaria Logos in the La Pampa Province, they had counted 714 carcasses in the two-acre plot. The farmer's account of how the birds had died suggested pesticides were the culprit.

Until the grim discovery, however, Woodbridge's study of Swainson's hawk migration had been exhilarating for both himself and raptor lovers around the nation. For the first time, a raptor had been fitted with a tiny, 28-gram radio transmitter -- a little "bird backpack" that allowed it to be tracked by satellite all the way from Northern California to Argentina.

"Satellite telemetry became available for larger animals 10 years ago," Woodbridge, a graduate of Humboldt State University, said from his office at the Klamath-Trinity National Forest in Yreka, "but only in the last two years has a transmitter been available that's small enough for a hawk."

Swainson's hawks were once the most abundant diurnal bird of prey in California, but their numbers have declined by an estimated 90 percent.

"They're considered threatened in California," Woodbridge said, but not by federal standards: Swainson's hawk populations are healthy in the Great Plains states and in Canada.

Population studies in Northern California have shown the hawk's numbers to be fairly steady, but there was a disturbing sign: Adult birds often disappeared after migrating south for the winter.

"It became fairly clear that in given years a good chunk of the adult population didn't come back," Woodbridge said, and although the loss was being made up in the breeding season, it was an ominous trend.

The problem was, nobody really knew for sure where Swainson's hawks went in the winter. Large numbers had been recorded in Mexico, and Canadian birder Stuart Houston -- arguably the biggest Swainson's hawk fan in North America -- recalled a study wherein 3,500 of the birds were spotted flying over Panama in one 24-hour period.

And although flocks had been spotted in Argentina, the low numbers had led researchers to believe that country was not the hawk's primary non-breeding home.

To find out where the hawks were going, and what was happening to them when they got there, Woodbridge captured two adult females at their nest sites in Butte Valley in the Klamath National Forest. They were fitted with the tiny radio transmitters and released.

The two birds' movements were tracked by weather satellites and relayed to a data processing center in Maryland, which sent the information over the Internet. Every morning Wood-bridge would come into work, turn on his computer and find out where the hawks were that day.

"It was very rewarding because we -- collectively, we who have been studying Swainson's hawks for years -- have been wanting to do this for such a long time," he said.

Although one of the transmitters failed early on, the second hawk's radio backpack worked like a dream: The bird left Northern California in early October 1994 and reached the Phoenix, Ariz., area Oct. 6. After about a week the hawk was on the move again, reaching the Gulf Coast of Mexico on Oct. 24, El Salvador on Oct. 30 and Nicaragua on Nov. 2.

The female hawk was in southwest Brazil by Nov. 18 and flying over Argentina by the end of that month. It settled Dec. 11 in La Pampa, where it remained until late January, when the radio transmitter shut down. (This bird was spotted again near her breeding territory in June 1995.)

"It is arguably the most amazing migration known," said Stuart Houston, a radiologist and birding hobbyist, in a telephone interview from his home in Saskatchewan.

Houston and others interested in birds of prey were at a raptor conference in Arizona last fall when word of the telemetry-tracked hawk began spreading. The resulting excitement, he said, was contagious.

Woodbridge and his colleagues -- Karen Finley of Corvallis, Ore., and S. Treant Seager of Bellingham, Wash. -- traveled to La Pampa in January after it was clear the bird had settled there for the season. It was not alone.

"The first thing we found, obviously, were live hawks. And there were flocks of thousands," Woodbridge said. "It was pretty magical to see that kind of number. And I remember thinking early on, to have this much life in one place there has to be death.

"And we found hawks hit by cars, hawks smashed up in storms -- they have severe storms down there. But we weren't expecting or prepared to see 714 dead ones."

The farmer on whose property the carcasses were found was furious about the carnage, and many of the farmers Woodbridge met expressed concern for the hawks, too. The farmers are, for the most part, "very environmentally conscious," he said, and very aware of the growing amount of pesticides being sprayed around the area.

The pest in question is the grasshopper, the major food source for the migrating hawk. It is also the bane of the many farmers in Argentina who, for economic reasons, are switching from a low-intensity agriculture (cattle) to high-yield crops such as corn, soya and sunflower.

"Future trends (do) not bode well for the species," Woodbridge and his colleagues wrote in one paper. "The current market situation created by the European Common Market, U.S. agricultural subsidies, GATT and the newly formed Latin American Common Market all put economic pressure on Argentinian farmers ... and aerial application of pesticides (is expected to) increase dramatically."

Woodbridge's study, however, is not complete. The pesticide theory is just that and requires more testing, and he's getting help from a variety of places, including the Canadian Wildlife Service, the National Fish and Wildlife Foundation, Argentina's University of Cordoba and Clemson University in South Carolina.

"What we're going to look at in the short term are some of the contaminant exposures on the hawks' wintering grounds in Argentina," Michael Goldstein, a doctoral student in Clemson's environmental toxicology department, said in a telephone interview. "What we'd like to do eventually is a collaboration with the university in Argentina to train their people ... and begin the foundations for a toxicological center in their country."

Goldstein is hoping to base his dissertation on the project. He and Woodbridge are cautiously optimistic about the hawks' future, but a big warning flare has been fired -- from a very small transmitter.

"It just makes you want to fall down and giggle," said Goldstein, when contemplating that satellites were tracking the movements of a single bird across the Earth.

"If it hadn't been for the bird with the radio on, nobody would know these birds were dying," Houston said from his home in Canada, where Swainson's hawks also are struggling with a dwindling amount of their main food, Richardson's ground squirrels.

"Brian's study makes all the difference in the world."

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