Lead slugs used in deer hunting could be linked to lead poisoning in bald eagles, according to ongoing studies by researchers from the University’s Raptor Center.

The research team is studying the hypothesis that bald eagles are ingesting lead by feeding on downed deer and gut piles from deer killed by hunters who use lead slugs from rifles or shotguns.

Pat Reddig, director of the Raptor Center, said many hunters leave gut piles from the dead deer in the field. These gut piles often contain lead from the slug used in the hunt and are ingested by the eagles.

Lead slugs that hit bones in the deer often break up and leave a “comet trail” of lead, Reddig said. Shots that miss bone and hit only soft tissue are often left intact, and raptors do not usually ingest them.

The eagles often experience negative side effects from increased levels of lead in their systems.

“It can make them very sick, very weak or they may die,” Reddig said.

Approximately 25 percent of the bald eagles the center treats test positive for lead poisoning.

“Up to a point, we can treat them,” Reddig said. “It depends on the levels of lead in their system.”

Researchers discovered the link between hunted deer and bald eagles after noticing an increase in lead poisoning during deer-hunting season and the presence of lead fragments and deer hair in the stomachs of sick eagles.

To study the link, the Raptor Center participated in a hunt Nov. 20 to gather 15 deer carcasses and analyze lead levels.

One hundred forty-nine hunters showed up at dawn to participate in the hunt at Elm Creek Park Reserve in Osseo, Minn.

The center is analyzing radiographs gathered from the carcasses. Radiographs are used to differentiate lead from soft tissue and to look at the amount of lead present. No conclusions have been drawn from the research yet.

The study was done in partnership with the University Veterinary Diagnostic Laboratory, Minnesota Department of Natural Resources, Three Rivers Park District and Anoka Equine Veterinary Services.

Reddig also headed a 2003 study that looked at how ingesting lead slugs affected California condors.

To protect the condors from lead exposure, the study recommended that hunters retrieve shot animals from the field, bury carcasses and gut piles, cover them with brush or rocks and use alternative, nontoxic ammunition.

“Some of the same measures may work here,” Reddig said of the Minnesota bald eagle study.

One problem, however, is gut piles are not broken down as quickly in the cooler weather of Minnesota as they are in other places.

“Eagles will find these things and feed on them,” Reddig said.

Previously, it was believed that lead poisoning in bald eagles was tied to lead slugs used in hunting waterfowl. But even after federal legislation outlawed using lead slugs to hunt waterfowl in 1991, lead poisoning levels remained the same in bald eagles.

The center has studied trends on lead poisoning in bald eagles since 1975.