The banning of the pesticide DDT appears to have helped the endangered peregrine falcon make a remarkable recovery across the West, according to the environmentalist group HawkWatch International.
The falcons were devastated by the spread of the pesticide DDT beginning in the 1940s, but now they appear to be rebounding, according to data gathered by the conservation group.And populations of two other migratory birds of prey, or raptors - merlins and osprey - appear to be rising, said Steve Hoffman of HawkWatch.
The banning of DDT in the United States in 1972 helped the birds, said Hoffman, who founded HawkWatch in Albuquerque 10 years ago and moved the organization's headquarters to Salt Lake City in 1992.
"You would not expect a species of raptor that lays a few eggs and lives a long time to be increasing at this fast a rate," he said.
"It indicates the populations were very suppressed as a result of pesticides and now they're just exploding," Hoffman said. "The big question is, when are they going to level off? They can't grow forever."
It has taken several years for bird populations to recover because DDT does not break down readily in the environment.
DDT disrupts calcium metabolism in female birds, causing them to lay eggs with unusually thin shells that are easily crushed when adult birds sit on them during nesting.
Merlins and peregrines have absorbed particularly high concentrations of DDT because they only eat other birds and so are near the top of the food chain.
"The banning of DDT was, of course, very important," said Steve Spangle, an endangered species biologist with the U.S. Fish and Wildlife Service.
"I think there's a better awareness now about the importance of raptors, whereas they were more likely to be shot in the old days," he said.
HawkWatch researchers have gathered 18 years worth of data from single sites in Nevada and Utah and two sites in New Mexico.
The average annual population increase for up to 18 years was 12.1 percent for peregrines, 14.6 percent for merlins and 8.9 percent for ospreys, Hoffman said.
Spangle said peregrines have been helped by captive breeding programs and reintroduction efforts.
Hoffman said ospreys might have benefited from the West's dams and the fish in the reservoirs behind them.
Ospreys also have been helped by the efforts of conservation groups to restore the birds' habitat, he said.
Red-tailed hawks increased 6.7 percent annually at the Nevada and Utah sites, but no significant change was detected in New Mexico, according to the data.
Red-tailed hawks, which prefer open areas, appear to be benefiting from an increase in logging on federal land in the West, Hoffman said.
But he said the numbers of forest-dependent northern goshawks have declined at the Utah and Nevada sites.
Golden eagles also declined at the Utah site, he said.
The causes of the declines detected in the study might be confined to habitat changes in the Great Basin region, Hoffman said.
HawkWatch also has kept track of turkey vultures, and there appear to be more of them now than in the 1970s and 1980s, he said.
Turkey vultures, which feed on carrion, are not raptors, but they were included in the survey anyway.
An increase in road kills due to road-building has improved conditions for turkey vultures, Hoffman said.
HawkWatch, with an annual budget of $425,000, relies on federal and private money.