Kodiak National Wildlife Refuge. Credit: Dave Menke/USFWS

Why do our bald eagles have high levels of lead, mercury?

A recently released report shows that toxic levels of mercury and lead were found in the livers of bald eagle carcasses recovered in New England. The researchers, Steve Mierzykowski of our Maine Field Office, Charles Todd of the Maine Department of Inland Fisheries and Wildlife, and Mark Pokras of Tufts University Cummings School of Veterinary Medicine, share their insights with us today.

Steve Mierzykowski, a senior fish and wildlife biologist in our Maine Field Office, holds an immature bald eagle. Credit: USFWS

Steve Mierzykowski, a senior fish and wildlife biologist in our Maine Field Office, holds an immature bald eagle. Credit: USFWS

What did they find?

Between 2001 and 2012, the researchers collected liver samples from 127 bald eagle carcasses from Connecticut, Maine (mostly), Massachusetts and New Hampshire. They analyzed the liver tissue for lead and mercury.

  • 14 percent had lead concentrations indicative of poisoning. Other North America eagle studies report similar or higher percentages.
  • The average mercury level (13.49 parts per million dry weight) was higher than average levels recorded in eagles from British Columbia, several Great Lakes states and Alaska.

Check out the rest of the report.

Q. Why are we concerned about lead and mercury levels in eagles? A. Lead and mercury are both potentially long-lasting in the environment. At low levels of exposure, their direct influence may go unnoticed.

However, long-lived animals like bald eagles may suffer from chronic accumulations that eventually impair their reproduction or survival. Lead is rapidly mobilized in an eagle’s gastrointestinal tract. Certain bacteria can methylate mercury: the new organic by-product, methylmercury, passes through the food web in increasing concentrations to top-level predators like eagles.


An X-ray of this bald eagle shows lead in its gizzards — see the bottom right of the eagle’s body. Credit: Mark Pokras, Tufts University

Although we have a good understanding of the acutely toxic effects of lead on bald eagles (and other species), we feel that it is particularly important to enhance our understanding of chronic, low-level effects. Lead has significant effects on the brain, heart, kidneys and many other organ systems. There are indications that such levels may predispose eagles to traumatic injury as well as impairing reproduction and survival.

Documenting the effects of toxic materials like lead and mercury in charismatic species like bald eagles can also help to alert policy makers and the public to current and emerging wildlife health threats.

Q. What has interested or surprised you during your research on this topic? A. High exposure rates to mercury to bald eagles in Maine were a surprise to many.

Point sources for mercury contamination are more often associated with industrialized regions, but this contaminant does circulate widely in the atmosphere. Maine is downwind of many industrialized regions of North America.

The high rate of exposure to lead among Maine eagles during fall, winter and spring is also troubling. Whether this is an artifact of increasing eagle numbers in the state, or possibly related to milder winters so that more eagles remain in the state year round, it is clear that scavenging carrion that may contain lead is a risk for eagles during periods of the year when ice cover restricts normal fishing habits of Maine eagles.


Q. Will you continue to study these levels in eagles? A. As budgets allow, we will continue to investigate contaminant levels in bald eagles. Lead, mercury and newly emerging contaminants like flame retardants will be measured in tissues of bald eagle when funds are available. The monitoring plan for bald eagles (after their recovery under the Endangered Species Act) has a contaminant component, and everyone recognizes that keeping track of contaminant levels over time and periodically assessing these levels are important to the species.

Because eagles are a highly charismatic species that sparks great public interest, and because they sit atop the food chain in a variety of habitats, bald eagles are an important sentinel species for representing a variety of environmental threats. Lead, mercury and other contaminants are known to affect their health, and the health of a wide variety of other species.

Q. How might people be able to help reduce this problem in the long term? A. Clearly, more research and public education are needed regarding non-toxic bullets, shot and other sporting gear, such as fishing weights. Unrecovered lead bullets and shotgun pellets in animal carcasses present a continuing health threat to scavenging species like the bald eagle.

See other contaminants research from our Maine office, including contaminants in crossed bill bald eagles, in eagles at Acadia park and our Maine coastal refuges, in eagle eggs and brook trout.

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