Lead exposure in bald eagles in the Upper Midwest
During winter, bald eagles congregate in high numbers along the Upper Mississippi River and other large waterways in the states of Illinois, Iowa, Wisconsin and Minnesota. Biologists with the U.S. Fish and Wildlife Service, including myself, Contaminants Biologists Sarah Warner and Mike Coffey, and Wildlife Biologist Drew Becker, began a study in 2011 on lead exposure in bald eagles that inhabit the Upper Midwest Region including the Upper Mississippi River National Wildlife and Fish Refuge.
Thousands of bald eagles winter and hundreds of eagles nest on the Refuge annually. On December 11, 2013 Refuge staff counted 1,100 bald eagles at one location near Fulton, Ill.
Each year, dead bald eagles are found throughout the Region and are subsequently turned in to federal and state conservation offices. These offices ship the eagles to the National Eagle Repository in Denver, Colorado, where the feathers are distributed to Native American tribes for religious ceremonies. We sent a note to neighboring conservation offices inquiring if they would send us dead eagles for a research project on bald eagle mortality and we would send the carcasses to the Repository.
Fifty-eight bald eagles were collected as a result of our inquiry. Most of these eagles came from Iowa and Wisconsin and one from Minnesota. We necropsied each eagle and the livers were harvested and then were analyzed for lead concentration by U.S. Geological Survey’s National Wildlife Health Center in Madison, Wis. The liver analysis showed that 60 percent of the eagles had detectable concentrations of lead. More significantly, 38 percent had liver lead concentrations within the lethal range for lead poisoning and most had corresponding clinical signs consistently found in lead exposure cases.
Our external examination of the carcasses showed only one eagle with green bile staining around the vent, a classic symptom of lead poisoning. The others had fair to good body condition with no other visual external signs of lead poisoning. However, the internal examination of organs showed clinical signs and gross lesions of lead poisoning that included distended and bile engorged gallbladders.
The high concentrations of lead (as much as 10 times the lethal dose) indicated that many eagles had acute exposure. In these acute cases, liver levels had become elevated prior to exhibiting some of the typical clinical signs of lead poisoning and most of the eagles probably died within a few days of ingestion. This is important to note because field biologists typically examine eagle carcasses for external characteristics to hypothesize cause of death. In our study, only one eagle exhibited external signs of lead poisoning.
The high percentage of eagles having lethal liver lead concentrations prompted an investigation into the potential sources of lead in the environment. The bald eagle’s diet is primarily fish, however, during winter months when the waterways are frozen, eagles rely on carrion as a primary food source, especially deer carcasses and offal (gut piles) left in the field after hunting events. This fact led us to focus on lead ammunition used in deer hunting as a source of lead available to eagles.
The Refuge extends 261 miles through the states of Illinois, Iowa, Wisconsin and Minnesota, with more than 200,000 acres open to public hunting. Non-toxic shot ammunition is required on the Refuge but does not include rifle bullets and shotgun slugs used for deer, squirrel and non-game such as coyote or crow. Thousands of hunters participate annually in these activities with many using lead ammunition. During the 2012-13 hunting season, there were 645,317 deer reported harvested with firearms in the states of IL, IA, WI and MN. Lead bullets were used by many hunters and often fragment inside a deer, especially when bone is hit.
The Refuge’s Lost Mound Unit in northwest Illinois conducts managed deer hunts for youth and disabled adults that provided an opportunity to investigate the prevalence of lead in offal piles. Fifty-seven white-tailed deer were harvested during the managed hunts held in 2012 and 2013.
Offal piles from 25 deer shot with lead ammunition were collected and radiographed. The radiographs showed that 36 percent of the offal piles, which would have been discarded on the Refuge, contained lead ranging from 1-107 fragments per offal. These results indicate that bullet fragments embedded in deer offal are very likely a pathway for lead exposure in bald eagles.
This study is continuing with an additional 115 bald eagles currently being necropsied at the National Wildlife Health Center. Most of these eagles were collected in Wisconsin with the remainder from Illinois and Iowa. Liver lead analysis is being conducted and full body scans are being taken on each eagle to collect any lead fragments that may be present in the intestines.
-- Ed Britton, Project Leader,
Upper Mississippi River NW&FR – Savanna District