An alarming number of North American bald and golden eagles are suffering from lead poisoning as the result of scavenging on animals shot with lead bullets. The research is a call to action for conservationists — and also hunters — to mitigate this disturbing trend.
New research in Science shows that lead, a toxic metal, is surprisingly prevalent in eagles across North America. The degree of lead poisoning among these protected birds was found to vary depending on location, season, and age, and it’s having a measurable effect on their ability to reproduce. As this lead primarily comes from ammunition, the new research highlights it as a previously underappreciated environmental toxin.
“It was surprising to me that nearly 50% of the eagles in our study showed evidence of repeated exposure to lead over the course of their lives,” Vincent Slabe, author of the study and a research scientist at West Virginia University, explained to me in an email. “Previously, I knew that eagles encountered lead, but now that we understand how prevalent the issue is, we can start thinking about solutions to the problem.”
While these eagles are not currently listed as endangered, many protective laws still exist at the federal level. Other toxins, such as rat poison, continue to make life difficult for these majestic birds.
That predatory birds are susceptible to lead poisoning was already known, but precise data has been lacking. The new study is the first to demonstrate the extent of the problem at the continental scale, as opposed to studies done regionally or locally.
The birds primarily consume the lead by ingesting ammunition fragments embedded in dead animals. When a lead bullet pierces an animal, “it is designed to spread out, or break apart into a lot of pieces,” said Slabe. “Those pieces can be small, but when ingested, can kill an eagle that accidentally consumes even one of them.” Other sources of lead include lead paint, fishing tackle, and runoff from mining activities, he added.
Lead can wreak havoc on virtually every system in an eagle’s body. Slabe said poisoned birds are less likely to survive and reproduce, as they become less able to source and digest prey. Lead poisoning makes eagles sluggish and can paralyse their digestive system, prevent them from eating, trigger vomiting, and induce brain damage. It can result in blindness, seizures, weakness, and death, and even at low doses it can “impair the functioning of the brain, organs, and immune and reproductive systems,” he explained.
Slabe and his colleagues analysed both bald and golden eagles across 38 U.S. states, including Alaska, gathering data from 2010 to 2018, in what is an impressively thorough study. The researchers looked for lead by testing the blood of live eagles and by analysing bones, livers, and feathers of dead eagles. They did not track lead exposure over time, but lead poisoning was more prevalent in older birds, which is likely due to the steady accumulation of lead over time.
Of the 1,210 birds analysed, 47% of bald eagles and 46% of golden eagles exhibited concentrations consistent with chronic lead poisoning, the result of repeated exposure. Short-term exposures were more frequent during the winter, which tracks, as dead animals are an important food source for eagles during these challenging months.
This environmental toxin appears to be reducing the rate of population growth for both species of eagles. According to the study, the growth rate of bald eagles is declining 3.8% each year, while the growth rate for golden eagles is dropping 0.8% annually.
Eagles living in the central, interior portions of North America — specifically a region known as the central flyway zone — exhibited higher levels of lead than their counterparts living near the Atlantic and Pacific. A possible explanation is that eagles have “unexplained differential scavenging rates” in the various zones, as the scientists write. In an email, Todd Katzner, a wildlife conservation ecologist with the U.S. Geological Survey and a co-author of the paper, said “eagles in the central flyway were more likely to be lead poisoned than other eagles,” but the new data “provide no insight into why that is the case.” The team will now seek “to try to understand these patterns,” he added.
The scientists also documented the rate of acute lead poisoning, that is, high exposures to lead across short timescales, among North American eagles. Nearly 30% of bald eagles and 9% of golden eagles exhibited this severe type of lead poisoning. Adults appeared to be most affected, it tended to happen during the autumn and winter months, and it was most prominent in the central flyway zone.
Slabe is currently organising non-lead ammunition programs to mitigate the problem of lead poisoning among North American eagles. In fact, the extent of the problem compelled him to take his “career in a new direction” upon completing his PhD. Slabe and his colleagues are currently running these programs in southeast Wyoming and southwest Montana.
“There is really good scientific evidence that, when presented with information on this problem, hunters lead the way in switching to non-lead ammunition,” Slabe told me. “Hunters are the solution to the problem of lead poisoning of eagles and other scavengers,” and the “hunting community is extremely valuable to conservation.”
To which he added: “As a hunter myself, this problem and the solution resonate with me on multiple levels.”