Thousands of bald eagles migrate in winter and hundreds of eagles nest in spring along the Upper Mississippi River corridor. Their journey in America’s heartland may be a dangerous one.
Dead eagles are randomly collected by state and federal conservation agencies. The eagles are sent to the National Eagle Repository in Denver, Colorado, where the feathers are distributed to Native American tribes for religious ceremonies.
Researchers with the U.S. Fish and Wildlife Service began a study on bald eagle mortality in the Upper Midwest in 2011 and collected 168 dead eagles from conservation agencies in Minnesota, Wisconsin and Iowa. The U.S. Geological Survey’s National Wildlife Health Center in Madison, Wisconsin, was a partner in this study. A necropsy was performed on each eagle and livers were analyzed for lead concentrations to determine a cause of death.
Forty-eight percent of the bald eagles had detectable concentrations of lead and 21 percent had lethal levels. Researchers investigated potential sources of lead in the environment to determine how eagles were being exposed. Bald eagles are predators and scavengers that hunt and fish, and their diet consists entirely of animal matter. Ammunition used in hunting wild game became a focus for the lead source.
Lead is the traditional ammunition used by hunters. It is a soft metal that fragments into tiny pieces upon impact. Hunters often discard animal parts, especially entrails, in the field. Lead fragments embedded in discarded animal parts or in fatally wounded, but not retrieved game, are available to scavengers. Many scientific publications report that ammunition, especially used for deer hunting, is an exposure pathway for lead poisoning in several wildlife species.
Managed deer hunts are conducted on the Lost Mound Unit of Upper Mississippi River National Wildlife and Fish Refuge in Savanna, Illinois. These hunts provided an opportunity to investigate lead ammunition as an exposure pathway to bald eagles. During 2012 and 2013, 57 white-tailed deer were harvested that included 25 deer shot with lead. The entrails from the 25 lead-shot deer were radiographed and showed that 36 percent contained lead ranging from 1 to 107 fragments per entrail.
Spent shotgun shells were collected to quantify the amount of lead that was shot. The lead weight for each shell was determined from specifications provided at the manufacturers’ websites. The total weight of lead shot to harvest the 25 deer was 9,938 grains (644 grams).
Laboratory studies show that 1.27 grains (82.5 milligrams) of lead is a lethal dose for a bald eagle. The 9,938 grains of lead contained an equivalent of 7,825 lethal doses. The amount of lead contained in each shot showed: a 20 gauge slug averaged 344 grains, equivalent weight to 271 lethal doses per slug; a 12 gauge slug averaged 421 grains, equivalent weight to 331 lethal doses per slug; and a .50 caliber muzzleloader bullet averaged 328 grains, equivalent weight to 258 lethal doses per bullet.
Bald eagles were frequently observed circling above the hunt area. Road-killed deer and their entrails were placed in the hunt area to simulate wounded but not retrieved deer and discarded entrails. Motion sensor cameras documented a variety of scavengers dining on the deer carcasses and entrails including bald eagles, red-tailed hawks, crows, raccoons, opossums and coyotes.
Hunters are important conservationists and provide millions of dollars in license fees and excise taxes that fund wildlife programs annually. U.S. Fish and Wildlife Service researchers are promoting awareness to hunters about the potential exposure pathway from lead ammunition to bald eagles and other wildlife. Please share this story with others and encourage the voluntary use of non-lead ammunition.
Ed Britton is a wildlife refuge manager at the Upper Mississippi River National Wildlife and Fish Refuge and volunteer at Bickelhaupt Arboretum.