Almost half of bald and golden eagles in the United States have lead poisoning, according to the largest study of its kind. The findings could spell bad news for the recovery of both species.
“It’s a strong paper that pulls together quite a bit of information,” says Bryan Watts, an ecologist at the College of William & Mary who wasn’t involved with the work. The study, he notes, surveyed eagles in 38 states, whereas previous work focused on a single region or just a handful of states. “This really solid analysis allows us to look at the eagles’ entire range.”
Bald eagles (Haliaeetus leucocephalus) were close to extinction by the 1960s thanks in part to dichlorodiphenyltrichloroethane (DDT). The pesticide washed into waterways, contaminated the fish the eagles ate, and poisoned the birds, weakening their eggshells and killing hatchlings. After DDT was banned in 1972, and the Endangered Species Act of 1973 protected bald eagle habitat, population numbers started to tick up. There are more than 300,000 bald eagles alive in the wild today.
“Bald eagle populations are doing brilliantly in the U.S.,” explains Todd Katzner, a conservation ecologist with the U.S. Geological Servey and an author of the new paper. But other birds of prey, such as golden eagles (Aquila chrysaetos) are on shakier ground, and contaminants other than DDT, including lead ammunition, still litter the landscape.
When an eagle eats lead—usually in the form of ammunition left behind in deer and other carcasses—it shows up in the bloodstream, filters through the liver, and can build up in the bones if the bird eats enough lead throughout its lifetime.
Wildlife rehabilitation clinics have long reported incidents of eagles with bullet fragments in their stomachs, and studies sampling lead in local eagle populations hinted that poisoning could be a widespread problem.
So Vincent Slabe, a wildlife biologist at the nonprofit Conservation Science Global, and his colleagues began to collect samples from birds. Over a period of 8 years, they gathered tissue from 1210 bald and golden eagles. Wildlife monitors banding birds with tracking transmitters and veterinarians rehabilitating sick or injured individuals sent feathers and blood to the team, and the researchers gathered liver and bone samples from dead eagles.
The team then measured lead levels, looking for acute exposure in blood, liver, and feathers and indications of chronic poisoning in bones. Nearly half of the birds showed signs of chronic lead poisoning—46% of bald and 47% of golden eagles. The scientists also found signs of more immediate lead exposure in 27% to 33% of bald eagles and 7% to 35% of golden eagles (the proportion depended on the tissue type). Models comparing natural and lead-caused deaths revealed lead levels would stunt annual population growth by 3.8% in bald eagles and 0.8% in golden eagles each year, the team reports today at the annual meeting of AAAS (which publishes Science) and online today in Science.
Watts isn’t so sure a 3.8% dip in population growth will put a meaningful dent in bald eagle recovery, because many local populations include a “buffer” group of nonbreeding adults that could swoop in and reproduce if others are lost. He says the losses are of more concern for golden eagles. Katzner agrees. “Their populations are much more on the edge,” he says. Counts from 2016 estimate about 40,000 golden eagles in the United States.
But even for bald eagles, Katzner and Slabe say a yearly reduction of that size will add up over the years. “It’s like compound interest,” Katzner explains.
Krysten Schuler, an ecologist at Cornell University who wasn’t involved with the paper but has studied lead’s effects on eagles in the northeastern United States, agrees. She also notes that eagles aren’t the only animals hurt by lead. “They’re a poster species for this issue,” she says, but fish, mammals, and other birds are likely also affected.
Conservation groups have pushed for bans on lead ammunition, and California barred its use in 2019, in part to protect the imperiled California condor (Gymnogyps californianus). But the authors of the study say educating hunters about lead poisoning and suggesting alternative ammunition such as copper bullets can be more fruitful than blanket restrictions. “The hunting community remains mostly unaware” of lead’s impact on eagles, Slabe explains. “But in the hunter education programs I’ve done, they’re really receptive to this issue.”