Half a century ago, when DDT was widely used in much of the United States, conventional wisdom held that peregrine falcons nesting in the cliffs of Alaska's upper Yukon River were too far away from where the pesticide was used to be harmed by it.
That turned out to be wrong. But as recently published long-term research found, those falcons have managed a remarkable recovery that began soon after the pesticide was banned.
By the 1970s, the birds occupied only about a dozen nesting sites along a 165-mile stretch of the Yukon River. Now, decades after DDT was banned in the United States (and its use dramatically curtailed elsewhere), the number of nest sites has increased fivefold to about 60, and stabilized there.
That seems to be the natural level of abundance in a non-DDT world, said the scientist who likely knows the upper Yukon peregrines best.
"Really, we're just learning now, probably, what it was prior to DDT," said Skip Ambrose, a former U.S. Fish and Wildlife Service biologist who has traveled to that stretch of river every year since the 1970s to keep tabs on the birds.
The results of those decades of work are detailed in a paper published online in the Journal of Wildlife Management.
The paper tallies observations made from 1977 to 2015 by Ambrose and his wife and research partner, Chris Florian, another former Fish and Wildlife Service biologist. In all, the authors observed 1,602 "occupied territories" and 2,349 nestlings over the years. The study also includes data from earlier observations at that stretch of river that date back to 1960.
DDT became notorious in the 20th century for its harm to birds, specifically its damage to eggs and reproduction. Because it accumulated in animals and grew more concentrated as it traveled up the food chain, the pesticide had particularly pernicious effects on birds of prey like peregrine falcons. The species was hard hit by DDT around North America and was classified as endangered in 1971. By then, peregrine falcons had largely disappeared from the East Coast and farther west, some populations had dropped by 90 percent or more, according to the Fish and Wildlife Service.
In its heyday, DDT was used heavily in the Lower 48 states -- though only very sparingly in Alaska, at a few sites including military bases, Ambrose said.
Although some signs of contamination were seen among the Yukon River peregrines -- the pesticide found in the 1960s in eggs, as well as some of the eggshell thinning that plagued birds elsewhere -- the belief was that the Alaska birds had largely escaped DDT damage. They hadn't.
The Alaska-nesting peregrine were most likely exposed to the pesticide when they flew south, Ambrose said. The Yukon River peregrines migrate as far south as Argentina. And even when the birds were in Alaska, their main prey was smaller birds, many of which also migrate to and from areas far to the south, including territory where DDT was heavily used.
That's a more probable explanation for how the Alaska falcons were exposed to the pesticide than the ongoing long-range atmospheric and oceanic transport that still brings contaminants from the south to the Arctic and subarctic, Ambrose said. Traces of DDT and its metabolites are still in the global environment, and still being transported to Alaska and elsewhere in the North. Also, they are among the persistent organic pollutants that have been banned or curtailed by international treaty and are being monitored by the eight-nation Arctic Council, among other organizations. But the current levels of DDT and DDT-related metabolites in the environment are far lower than those that caused great harm decades ago, Ambrose said.
Elsewhere in North America, peregrine falcons have recovered robustly in the post-DDT era. The species, which has three subspecies in North America, was delisted in 1999, and populations are so abundant now that peregrines have settled in even some urban sites, making nests on skyscrapers and other tall structures.
What it remarkable about the upper Yukon River peregrine recovery, however, is that it did not require any of the direct, on-site human intervention needed elsewhere, the study's authors said. Such human assistance came in the form of captive-breeding programs or nest-site manipulations, none of which was used in Alaska, said David Payer, a former Fish and Wildlife service biologist, now with the National Park Service, who is a co-author of the study.
The upper Yukon peregrines apparently were able to recover on their own, with a gradual -- but noticeable -- increase beginning soon after DDT was banned, according to the study.
"It just shows how rapidly the population recovers once we remove the contaminant that was causing the eggshell thinning," Payer said.
That section of the Yukon River flows through the Yukon-Charley Rivers National Preserve, and peregrines and other migratory birds are protected by federal law and international treaty, so those factors probably also helped bring about recovery, Payer said.
Another key lesson from the study, Ambrose said, is the importance of very long-term monitoring "year after year after year," which is rare for any species or wildlife population.
It is something he was happy to do with the peregrines during his long tenure at the Fish and Wildlife Service, and during a subsequent period with the National Park Service.
"I'm lucky. I was in the right place at the right time, and I like peregrines," he said.
Ambrose and Florian are so dedicated to keeping up with the peregrines that, even in their retirement in Utah, they continue to travel on their own dime to the upper Yukon each year to continue their research. They will be back in Alaska next month, and plan to launch their boat as soon as the ice in the river melts out, Ambrose said.