Peter Sharpe dangled 100 feet beneath a helicopter, secured by a harness around his chest. In his hands, the research ecologist clutched a small box containing smooth oval objects sculpted from resin to resemble a bald eagle’s chalky eggs. The chopper pilot swung the craft above a wide bald eagle nest on a rock ledge high on a cliff—and Sharpe climbed in. Among the sticks and nesting material, he spotted two eggs with shells that would likely be crushed by the weight of brooding parents. In a flash, Sharpe swapped the eggs for the resin dummies, then signaled to the pilot that he was ready to go.

Scenes like this played out up to four times a year between 1989 and 2009 on Santa Catalina and Santa Cruz, two of the eight Channel Islands off the coast of Santa Barbara, Calif. Sharpe would transport the eggs to the San Francisco Zoo for incubation and return the hatchlings weeks later for the nesting parents to raise. Biologists also hiked to nests to check on eggs and collect them for off-site incubation as necessary.

These egg-swapping missions might sound extreme—but at the time, so were the circumstances for the national bird of the U.S. By 1963 the number of nesting pairs of bald eagles had dropped to a low of 417 in the contiguous 48 states. When the project in California began in 1989, that national number had climbed to 2,680, yet the U.S. population remained quite vulnerable. On the Channel Islands, the effects of the potent insecticide DDT (dichlorodiphenyltrichloroethane), which was widely used in U.S. agriculture in the mid-20th century, had already helped wipe out a population of 35 bald eagle pairs by the mid-1950s. DDT thinned some eggshells so much that they were easily squashed, and the insecticide dehydrated others: it enlarged shell pores during maturation, allowing the fluid inside to evaporate.

Today 60 bald eagles live on the Channel Islands. And more than 316,000 of them thrive across the country, according to a 2020 survey recently released by the U.S. Fish and Wildlife Service (FWS). Captive-breeding efforts such as those Sharpe helped with, along with natural population growth—thanks to federal protections, including a DDT ban—have yet to restore bald eagle numbers to what they were before the widespread use of the pesticide. But populations of bald eagles are rebounding, as are those of other raptor species affected by DDT, such as peregrine falcons and osprey.

The progress for bald eagles has resulted from many public and private efforts to protect the species regionally and nationally. Captive breeding programs and other conservation efforts have also given today’s biologists more accurate insights into fluctuations in bald-eagle numbers, the critical role these birds play in ecosystems and how to best protect them.

The successes for bald eagles were decades in the making. In 1972 the outlook for them and other raptors took a first significant turn for the better when the federal government banned DDT. A year later Congress passed the Endangered Species Act, which protected bald eagles.

“If you would have surveyed ornithologists in 1972 and asked them, ‘Which groups of birds are you most concerned about?’ you know raptors would have been right at the top of the list,” says Brian Millsap, national raptor coordinator at FWS. “In the decades in between the1970s and now, because of the banning of DDT, raptors as a group have massively improved their status.”

But these historic measures are only part of the bald eagle success story. A crucial but often overlooked moment for the birds in California, where they were thought to perhaps be genetically unique, came in 1986. As part of the effort to save the iconic raptors in the state, conservation groups and government agencies formed a coalition to initiate what would become one of the largest bald eagle captive-breeding-and-reintroduction programs in the country.

To start, wildlife biologists and ornithologists took eaglets from nests in Washington State and Alaska, as well as British Columbia, to repopulate the Channel Islands. Biologists took others from northern California to habitats at the San Francisco Zoo, which already operated a captive-breeding program for barn owls and peregrine falcons. By the mid-1990s the zoo’s Avian Conservation Center housed eight breeding pairs of bald eagles. Their offspring were reintroduced in the wild on the Channel Islands and in the Big Sur region to help California’s wild bald eagle population recover.

Once couples started to pair up again on the Channel Islands, Sharpe began the egg-swapping missions. DDT and a chemical it breaks down to—dichlorodiphenyldichloroethylene (DDE)—persisted in the marine environment, concentrating in animals as it moved up the food chain, after manufacturers released industrial waste off the California coast. (Researchers also just announced new evidence suggesting barrel upon barrel of DDT was dumped into the ocean off the coast of Santa Catalina.) Eggs often failed to hatch back at the San Francisco Zoo, perhaps because they were unhealthy or were moved too soon after being laid. In such cases, Sharpe and his colleagues would transfer hatchlings from the zoo’s captive eagles to the wild nests.

Waiting for the population to recover naturally after the DDT ban—a possibility after numbers increased enough for eagles to explore unclaimed territories—could have taken a century or longer. So the captive-breeding and reintroduction effort successfully jump-started the restoration in California, says Sharpe, who works at the Institute for Wildlife Studies (IWS), a biodiversity-research nonprofit organization that coordinated recovery efforts with other groups.

In 2005 IWS began incubating bald eagle eggs on its own so it could cut back on transporting the eggs and eaglets, and the collaboration with the San Francisco Zoo came to an end. Just one year later came the most significant turning point for the bald eagles on the Channel Islands: the first egg hatched in the wild, something Sharpe had not been sure he would see in his lifetime.

To determine whether bald eagle populations were recovering nationwide, FWS tallied biologists’ nest counts from U.S. states. By 2007 the numbers had risen to the point that the agency removed the birds from the Endangered Species List. Two years later—as part of its post-delisting monitoring requirement under the Endangered Species Act—FWS committed to updating the nation’s bald eagle count using aerial surveys, a task now completed every six years.

For its most recent bald eagle survey, conducted between 2018 and 2019, FWS wildlife biologists partnered with the Cornell University Lab of Ornithology, adding another set of data to the aerial counts. Researchers Orin Robinson and Erica Stuber. both then at Cornell, analyzed hundreds of thousands of bird checklists submitted to eBird, an online database of species observed in the wild by birders. FWS scientists used both sets of data in a sophisticated population model. The result was a higher and more accurate estimate of bald eagle numbers, compared with previous years: quadruple the number found in the prior aerial survey, conducted in 2009.

“We were looking for ways to see how we can leverage [eBird] as a resource to help us make conservation and management decisions for these species,” Stuber says. The collaboration between Cornell and FWS could soon paint a more detailed picture of raptor populations nationally. The two are currently discussing a project to evaluate golden eagle numbers.

Researchers have now banded 90 percent of the Channel Islands bald eagles to continue assessing populations there. Whether the ecosystem will fully recover to its pre-DDT state is still an open question. Since that first chick in 2006, more than 150 eaglets have hatched from wild nests on the islands, Sharpe says. And the news keeps getting better: eagle pairs raised 22 fledglings there in 2019, more than ever before.

If the surging bald eagle numbers on the Channel Islands and in the rest of the U.S. tell us anything, Millsap says, it is that we can slow or even reverse population declines and help species recover. “Once we’ve identified a threat, if we can address that threat directly and head-on,” he adds, “we do have the potential to save a species that otherwise is clearly going to go extinct.”