For much of the past 2 million years, ancestors of a keen-eyed raptor known as the Swainson's hawk have circled above the vast grasslands of the Western Hemisphere.
For the past 15 years, Brian Woodbridge has been studying the hawks and their annual migration to and from South America from his vantage point in the Butte Valley National Grasslands in northern California.
What he saw, he says, was troubling.
Typically, 85 to 90 percent of the valley's hawks return each year from their winter stay in Argentina. "But then a year would come up when nearly half failed to return," says the wildlife biologist with the US Forest Service. "We were seeing an abnormally high turnover rate of adult hawks."
The culprit? Pesticides
During the winter of 1994-95, he set out to discover the reason. Research over the next two years uncovered the culprit: widespread misuse of pesticides on the Argentine Pampas that would lead to the largest hawk kill ever recorded. The discovery sparked an unprecedented international rescue effort that showed its first signs of success this past winter.
In addition, it may become a model for attempts to arrest the decline of hundreds of other species of migratory birds. "We know so little about these birds - we don't know anything about the role they play in the ecosystems they inhabit," says Michael Hooper, a wildlife toxicologist at Clemson University's Institute of Wildlife and Environmental Toxicology in Clemson, S.C.
Researchers are coming to appreciate that long-range migratory birds like the Swainson's hawk "reflect the link between ecosystems in North America and South America," Dr. Hooper says. "The plains of Argentina are not separated from the plains of North America, because the Swainson's hawk holds them together."
Indeed, until Mr. Woodbridge's discovery, no one was certain where in Argentina the Swainson's hawks spent their winters. Based on conversations with colleagues in Canada, no one could spot problems with northern habitats that would lead to the heavy losses the birds were experiencing. "This got my curiosity going," Woodbridge says, "and it suggested a need to focus on Latin America."
Up until the winter of 1994-95, he says, researchers banded birds before they headed south, then tallied the number of banded birds that returned to their breeding grounds in the United States and Canada. So few birds could be banded compared with the species' overall population of from 450,000 to 800,000 that the odds of someone in Argentina finding one, let alone reporting its location, were slim.
So Woodbridge trapped two hawks, equipped them with battery-operated radio transmitters, and released them for their 20,000 kilometer (12,427 mile) round trip. Special receivers aboard National Oceanic and Atmospheric Administration weather satellites picked up the bird-borne beacons, and relayed the information to ground stations. The data pinpointing the birds' locations went via Internet to Woodbridge.
The wintering spot for virtually all the Swainson's hawks in the world turned out to be in Argentina's La Pampa province. The flat expanse of plains is rich in the hawks' favorite winter treat, grasshoppers, which make up 95 percent of their diet while there.
Once he discovered the birds' location, Woodbridge toured the area to size up conditions. "It was an amazing sight to see 7,000 Swainson's hawks sitting on the ground eating grasshoppers," he says of one stop.
But the trip also yielded a grim discovery: 700 dead hawks on one farm. With the cooperation of the Argentine government, university researchers, conservation groups, and local farmers, researchers found that the birds had eaten grasshoppers killed with the pesticide monocrotophos, which has been banned in the US for more than 10 years and to which raptors are said to be vulnerable. The following year, the largest hawk kill on record occurred - some 20,000 birds died after eating pesticide-laced grasshoppers, what some researchers call a conservative estimate. One casualty was the oldest Swainson's hawk on record: At 19, the bird, which summered in Colorado, had logged nearly 248,000 miles.
Such staggering losses galvanized an "alphabet soup" of people - from government agencies in Canada, the US, and Argentina to conservation groups, university researchers, and five or six chemical companies, Clemson's Hooper says. Working with the Argentine government, conservation groups, and farmers, chemical companies agreed to pull monocrotophos from the area and replace it with another pesticide deemed less harmful to the hawks. Meanwhile, research groups such as Hooper's shared information with their Argentine counterparts on how to deal with bird fatalities - from basic reporting approaches to the forensic science needed to determine what killed the birds.
A cautious victory
This past winter, Woodbridge says, only 24 dead birds were found. Earlier this month, the US Fish and Wildlife Service gave its special commendation award to three Argentine agencies for their roles in the hawk salvage effort.
Yet Woodbridge and others are cautious about declaring victory. For one thing, the pesticide in question is still available in other parts of Argentina, and little is known about its effects on other non-raptor species. Moreover, though banned in the US, the pesticide is widely used in Africa and Asia at the ends of other migratory-bird flyways. Woodbridge also notes that this past winter's grasshopper infestation on the Pampas was not particularly severe, so farmers may have felt less impelled to apply pesticides.
"This is still a work in progress," he says. "We'll need a couple of years of bad grasshoppers before we can say the worst is over."
Still, he says, the effort yields lessons. "This really shows the rewards of a cooperative approach even when in the short term that seems counterproductive. There was no barking. We went to these groups and gave it some time."