Forensics unravels mysterious eagle deaths

Stan Bousson/For the HeraldBald eagles gather along Clinton’s north shoreline at Lock and Dam 13.

Stan Bousson

Wildlife sleuths used forensic science to determine what was killing bald eagles along the Upper Mississippi River corridor. Laboratory tests showed that nearly two-thirds of the 58 eagles examined had lead concentrations and more than one-third had clinical lead poisoning. The lab results deepened the mystery as to how these meat eating predators could have been exposed to lead.

Bald eagles live in our area year round with several nesting pairs in the Clinton area, especially within the Upper Mississippi River National Wildlife and Fish Refuge. In winter, hundreds of eagles gather along Clinton’s northern shoreline to feed on the sushi buffet floating in the tailwaters of Lock and Dam 13.

Eagles die of many causes. Refuge staff collect dead eagles and send them to the National Eagle Repository in Denver that distributes eagle feathers to Native American tribes for religious ceremonies.

In January 2012, Refuge biologists began investigation into the causes of death for 58 bald eagles. Many eagles appeared healthy externally, however, internal examination unraveled the trail of death for some.

Livers were analyzed at the U.S. Geological Survey’s National Wildlife Health Center. Liver lead levels showed that many eagles had ingested lead in amounts many times over the clinical lethal concentration. An eagle’s digestive acids dissolve the lead and it is absorbed like calcium that results in health problems including muscle paralysis, blindness, and inability to fly. Depending upon the lead dosage, eagles will die within days or will persevere until starvation.

X-rays showed that lead fragments were present in several eagle’s digestive tracts. Lead ammunition fragments were recovered and identified as shotgun pellets and slugs, .22 caliber bullets, and muzzleloader bullets.

Lead ammunition is used by many hunters for big and small game. Lead is a soft metal that fragments upon impact. This fragmentation often pierces multiple organs and results in a quick humane kill of wild game.

Deer hunting is popular in our area. Harvest reports show that counties bordering the Mississippi River often have the highest number of deer killed because of the quality deer habitat. Tens of thousands of deer are harvested annually along the river corridor.

The largest concentration of bald eagles in our area occurs during the deer hunting season. Eagles have learned that deer gut piles discarded in the field are an important source of protein and an easy meal. Refuge researchers documented that eagles quickly found deer gut piles and multiple bald eagles would feed on a single gut pile for several days.

Deer gut piles were subsequently collected from 25 hunters and X-rayed. Over one-third of the gut piles contained lead fragments with over 100 fragments in a single pile. Laboratory tests show that just over one grain of lead (a fragment smaller than a grain of rice) will kill a bald eagle, if ingested. A standard one ounce 12 gauge shotgun slug contains 437 grains of lead.

The Refuge encourages all hunters, and especially deer hunters, to use lead-free ammunition. Lead-free ammunition is commercially available with copper being the most popular choice by deer hunters. Copper is a hard metal that expands rather than fragments upon impact often resulting in deeper penetration, greater shock, and a larger blood trail.

Please help conserve our national symbol and continue the conservation legacy that hunters are known for by switching to lead-free ammunition. And please tell others about this tragic story of lead exposure in bald eagles.

Ed Britton is a Wildlife Refuge Manager on the Upper Mississippi River National Wildlife and Fish Refuge and volunteer at Bickelhaupt Arboretum.

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