A 2022 study in the Northeast United States showed the resurgent bald eagle population is being held back by lead poisoning.
The toxin found in ammunition makes its way into the bodies of the birds and other animals when they eat the discarded tissue of creatures killed by hunters.
As a result, the long-term growth rates of male and female bald eagles were depressed by 6.3% and 4.2%, respectively, according to the study published in The Journal of Wildlife Management.
Bald eagles have been the country's national bird since 1782. Just 417 known breeding pairs remained in the contiguous U.S. in 1963. The population rebounded to 10,000 in 2007 and more than 300,000, including 71,400 nesting pairs, in 2020.
"Banned in the United States from paint in 1977, plumbing used for drinking water in 1986, and gasoline in 1996, [lead] is still widely used in ammunition to hunt big game and predators in terrestrial habitats and in shot for small game and upland gamebirds," the study authors wrote.
Environmental Health News reported that lead shot for waterfowl hunting was banned in 1991 and that California banned lead ammunition in 2019 to aid the endangered California condor. "Lead poisoning is the biggest threat" to the largest bird in North America, according to the National Park Service.
"Most hunters have no idea this is happening," Kevin Hynes, a wildlife biologist in the New York State Department of Environmental Conservation, told EHN. "It's not on their radar, and they just don't know about it."
Another issue with lead ammunition is its risks to human health. A 2009 study showed 32% of ground meat from hunter-killed deer contained at least one lead fragment.
Lead exposure is particularly dangerous for young children. It can cause damage to the brain and nervous system, learning and behavioral difficulties, slowed growth, hearing problems, and headaches. Adults can suffer reproductive issues, high blood pressure and hypertension, nerve disorders, memory and concentration problems, and muscle and joint pain.
The bald eagle study featured 1,232 dead or dying birds that were collected from 1990 to 2018 in Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, New York, and Vermont. The authors noted 30.6% had non-zero levels of lead in their tissues at the time of death and 11.3% had measurements that surpassed the criteria for toxicity.
Another 2022 study, in which researchers gathered feather, blood, liver, and bone samples from 1,210 eagles in 38 states over eight years, found 46% of bald eagles and 47% of golden eagles showed signs of chronic lead poisoning.
EHN reported one or two tiny pieces of lead can kill an adult bird.
"It really doesn't take much," Hynes told EHN.
"We shouldn't be killing these animals," Krysten Schuler, a wildlife disease ecologist at the Cornell Wildlife Health Lab, told EHN. "This is a totally preventable source of mortality, based on how we choose to exist in our environment."
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