Almost half of bald and golden eagles in the U.S. have lead poisoning.
In the largest comprehensive study of lead poisoning in eagles in the United States, biologist Vincent Slabe at Conservation Science Global and his colleagues discovered that 46% of bald eagles and 47% of golden eagles have chronic lead poisoning. Slabe and his team have been collecting tissue samples from these eagles for the past eight years and just this week released their findings in the journal Science.
Bald eagles were almost rendered extinct in the 1960s thanks to a chemical called Dichlorodiphenyltrichloroethane (DDT), a common insecticide used in agriculture. DDT was banned in 1972, and the US passed the Endangered Species Act of 1973, both of which helped the bald eagles rehabilitate to their current population of over 300,000.
Golden eagles, on the other hand, are not doing as well.
Most eagles consume lead through leftover ammunition in carcasses that hunters don’t dispose of. Because eagles, like most other animals, can’t process lead, lead can exist in the bloodstream, make its way through the liver, and even enter the bird’s bones in some circumstances.
Slabe and his team collected tissue from 1210 bald and golden eagles across 38 states. 590 samples were gathered from dead eagles, while the rest came from tagged eagles or ones that were being rehabilitated in sanctuaries. Researchers measured both acute and chronic lead poisoning in each species and were surprised to find such high levels.
Acute lead poisoning appeared in the eagles; blood, liver, and feathers, while chronic lead poisoning was measured via the bones. On top of chronic lead poisoning, 27% to 33% of bald eagles had acute lead poisoning, while 7% to 35% of golden eagles exhibited acute lead poisoning. Notably, the frequency of acute lead poisoning depended on the tissue type.
The research team then used models that compared eagle deaths by lead poisoning versus natural deaths and calculated that lead poisoning could stunt population growth in bald eagles by 3.8% and golden eagles by 0.8%.
While bald eagle population growth is more at risk from lead poisoning because they have “buffer” eagles— local, nonbreeding adults that can help reproduce if others aren’t able to. On the other hand, since few golden eagles are left, their population is more at risk. Estimates from 2016 say that around 40,000 golden eagles are left in the U.S.
However, a decrease in yearly reproduction would add up over time, much like compound interest. Other ecologists point out that eagles are not the only ones affected by lead in an ecosystem and worrying lead levels in one species likely means that there are similar issues in other birds, fish, or mammals.
The study authors don’t think that blanket hunting restrictions, such as banning lead bullets, are the most effective way to protect these animals. Instead, since many hunters are unaware of their impact on this issue, the authors believe that educating hunters about alternative ammunition and lead poisoning will make the most significant impact.