By Mary Jo DiLonardo │TreeHugger│ 3 min

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When eagles go scavenging, they can pick up all sorts of things in the entrails of the animals they eat. One dangerous substance is lead, often from bullets found in the prey they are eating.

A long-term study has discovered widespread, frequent lead poisoning in bald eagles and golden eagles in North America. The levels are high enough to negatively impact populations of both species.

“This research was started because there had never been any nationwide studies of the effects of lead on eagle populations,” study author Todd Katzner, U.S. Geological Survey wildlife biologist, tells Treehugger.

“There are many local studies that have shown that eagles are lead exposed but there was no understanding if this lead exposure was affecting growth rates of eagle populations. This study clearly shows that lead is having measurable and relevant consequences for growth rates of both eagle populations.”

For their study, scientists from the U.S. Geological Survey, Conservation Science Global, Inc., and U.S. Fish and Wildlife Service, evaluated lead exposure in bald and golden eagles from 2010 to 2018. They looked for lead exposure in samples from 1,210 bald and golden eagles from 38 states across North America. Their study group included 620 live eagles.

“Prior to this study, we had good evidence of effects to individual eagles from lead poisoning and we even had some local studies that looked at effect to eagle populations,” Katzner says. “This is the first study of any eagle species to show continent-wide effects from lead poisoning on population growth rates.”

Lead Exposure Sources

Wildlife can be exposed to lead from many different sources, but they often encounter it while scavenging the bodies of animals that have been shot with lead ammunition.

“When a lead bullet enters an animal, it is designed to spread out, or break apart into a lot of pieces,” study author and research wildlife biologist Vincent Slabe of Conservation Science Global tells Treehugger. “Those pieces can be small, but when ingested, can kill an eagle that accidentally consumes even one of them.”

Almost 50% of the eagles in the study showed repeated exposure to lead, which was measured in bone samples. About one-third demonstrated short-term exposure, which was calculated in feathers, blood, and liver samples.

“It was really 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,” Slabe says. “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.”

Researchers also found that the frequency of lead poisoning was influenced by the birds’ age. For bald eagles, it was also affected by region and by season. Levels were higher in winter when eagles rely more heavily on using dead animals as a food source because live prey is more difficult to find.

Modeling suggests that poisoning at this rate is causing population growth to slow by 3.8% for bald eagles annually and by 0.8% for golden eagles each year.

The results were published in the journal Science.

Conservation Outlook

Researchers say the study findings are key to helping with conservation tactics for eagles.

“As apex predators, eagles are important to ecosystems and they are also important to people, for example as our national symbol. It is therefore highly relevant that they are exposed to lead so frequently and that, at a continental scale, lead is suppressing their populations,” Slabe says. “It is also important to think about how these results can be used.”

At Conservation Science Global, he says the group has started programs to familiarize hunters with non-lead ammunition. Hunters are offered free or discounted ammo to try out options that are safer for scavenging eagles.

Slabe says, “As a result, many hunters voluntarily switch to non-lead ammunition so the offal they leave behind do not negatively impact eagles by poisoning their food source.”