Bald eagles are dying because they are eating the remains of deer that have been shot using lead ammunition, according to a two-year study conducted by the U.S. Fish and Wildlife Service.

The findings will be presented Tuesday, June 10, during an informational session at the Thomson, Ill., visitor center of the Upper Mississippi River National Wildlife and Fish Refuge.

Ed Britton, the manager of the refuge's Savanna, Ill., district, said the study tested the livers of 168 dead eagles collected from the refuge over a two-year period and found that 48 percent had detectable concentrations of lead while 21 percent had concentrations considered lethal.

"Additionally, most of the eagles exhibited physical signs consistent with lead exposure," he said.

The 240,000-acre refuge covers parts of Iowa, Illinois, Minnesota and Wisconsin along the Mississippi River.

To show that eagles find and eat deer, researchers deposited road-kill deer in an area with motion detectors, and "the eagles found them right away," Britton said.

That being the case, eagles also could be expected to quickly find deer gut piles — the internal parts of deer that hunters typically leave behind after they field-dress deer they have slain.

And gut piles contain lead. As part of the study, researchers collected 25 gut piles from managed hunts in the Lost Mound Unit of the refuge during 2012 and '13, and they found that 36 percent of the piles contained from one to 107 lead fragments per pile, Britton said.

"During winter months when fish are not readily accessible, eagles rely on carrion, including deer carcasses and gut piles, as a primary food source," he said.

"With thousands of bald eagles wintering and hundreds of eagles nesting on the refuge, we are concerned with the potential long-term impacts of the occurrence of lethal lead levels documented in the study," he added.

"We are also concerned that other wildlife that feed on wounded, but not retrieved, deer, discarded deer parts and deer gut piles are being exposed to lead." 

"Most hunters are conservationists at heart," Britton said, adding that he hopes that when they see the data, they will switch to non-lead ammunition.

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Although all metals are toxic, alternative metals don't fragment as much as lead, Britton explained. The chances of an eagle eating large chunks of metal are much less than with small pellets, he added.

The possibility of limiting lead ammunition by law or regulation is "very controversial," he said. 

Ed Kocal, the Quad-City area conservation officer for the Iowa Department of Natural Resources, said he does not know of any deer hunters around the Quad-City region who use non-lead ammunition. It is relatively new, difficult to find and more expensive than lead, he added.

"A lot of hunters would be against that (a restriction on lead) because of the cost," he said. Asked whether some would view it as a back-door method of gun control, he said, "Unfortunately, that's going to be the reaction of a lot of people."

The refuge study is not the first to link spent ammunition and lead poisoning in other wildlife; studies conducted throughout the United States as well as internationally have shown ammunition to be a pathway, Britton said.