A study published yesterday in the journal Science reports that nearly half of Bald and Golden Eagles in the United States exhibit chronic lead poisoning. And the team of researchers say that lead poisoning is an important barrier to the growth of eagle populations across North America. Their paper is the first to show continent-wide consequences of lead poisoning on any species of wildlife.

Lead is a highly toxic heavy metal that is ingested by eagles and other scavenging wildlife when feeding on the remains of animals that have been shot with lead ammunition. “This is the first study of lead poisoning of wildlife at a nationwide scale, and it demonstrates the unseen challenges facing these two iconic eagle species,” says lead author Vince Slabe, from Conservation Science Global. “Medical science tells us that, for humans, there is no safe amount of lead. Today we also know that redistributed lead in our environment is harming eagle populations across North America.”

The study evaluated lead exposure in more than 1,200 Bald and Golden Eagles sampled between 2010 and 2018 in 38 states, including Alaska, and found age-related and seasonal variation in lead poisoning.

Poisoning at the levels found in the study is causing population growth rates to slow for Bald Eagles by 3.8 percent and Golden Eagles by 0.8 percent annually. Previously, evaluations of lead exposure and its impact on eagle populations were only performed in local and regional studies. This study documents how lead poisoning inhibits both species’ population growth across North America. 

“This is the first study of lead poisoning of wildlife at a nationwide scale, and it demonstrates the unseen challenges facing these birds of prey. We now know more about how lead in our environment is negatively impacting North America’s eagles,” said Todd Katzner, USGS wildlife biologist and lead USGS author.  

In August 2018, this adult female Golden Eagle was captured and outfitted with a GPS transmitter. You can see the transmitter on the eagle’s back, below the head. This was the first Golden Eagle ever captured as part of a research project in Yellowstone National Park. In early December 2018, this eagle was recovered dead approximately 1.5 miles from where it nested in the park after movements ceased. The eagle was necropsied at the USGS National Wildlife Health Center where the cause of death was determined to be lead poisoning. Liver lead concentrations measured at 48 parts per million dry weight, which is in the range for lethal toxicity. Photo by Connor J. Meyer

Exposure in eagles more frequent in winter

In the study, almost 50 percent of the birds sampled showed evidence of repeated exposure to lead. Short-term exposure was more frequent in winter months. Both eagle species are scavengers and use dead animals as a food source year-round, but they particularly rely on them during the winter months when live prey is harder to find. Lead poisoning typically occurs when an eagle eats lead ammunition fragments lodged inside an animal carcass or in gut piles left behind when game is dressed in the field. The frequency of chronic lead poisoning found in both species increased with age because lead accumulates in bone as eagles are repeatedly exposed to the heavy metal throughout their lives. 

“The study’s modeling shows that lead reduces the rate of population growth for both of these protected species,” says Brian Millsap, U.S. Fish and Wildlife Service National Raptor Coordinator and co-author. “That is not as impactful for Bald Eagles since this endemic species population is growing at 10 percent per year across the U.S. In contrast, the Golden Eagle’s population is not as stable, and any additional mortality could tip it towards a decline.”

“With an ever-growing body of scientific findings, we hunters have an ever-improving understanding of the details of preventable exposure,” says Chris Parish, president and CEO of The Peregrine Fund and co-founder of the North American Non-lead Partnership. “The hunting community has a long-standing tradition of conservation of wildlife in the United States, and we are the key to solving the problem. Through the efforts of the North American Non-lead Partnership, we are finding that with appropriately delivered information, hunters and anglers who are asked to help are eager to do their part in improving ecosystem and wildlife health. Wildlife agencies, sporting groups, and tribal communities, are coming together to increase awareness and solve this problem on behalf of wildlife across North America.”

“Now that we have evidence that lead is affecting eagles nationwide, we are poised to address this problem,” says Slabe. “Already, Native American tribes, groups like the North American Non-lead Partnership, Conservation Science Global, and others, are encouraging voluntary use of non-lead ammunition. As a scientist and hunter, I value our conservation heritage and I see this study as another opportunity to grow awareness of this issue.”

The study included authors from academia, nonprofits, consulting services, industry, state, federal and international agencies. Funding was provided by nonprofit foundations and state and federal agencies.

Thanks to Conservation Science Global, Inc. and the U.S. Geological Survey for providing this news.

Previously: Lead ammo stunts Bald Eagle recovery

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