How do Raptors Get Lead Poisoning?

Ingestion of lead bullet fragments is deadly for wildlife.

Lead is a soft, pliable heavy metal that fragments easily. Historically, lead ammunition has been frequently used in big game hunting, including deer, elk, and moose hunting. Even when a lead core bullet passes all the way through its intended target, as much as a third or more of the bullet’s total weight will be broken away and remain inside the animal. A normal practice for hunters is to remove the internal organs of the shot deer or other game animal, and simply leave the “gut pile” in the fields after removing the body of the animal. Nearly all of the gut piles contain tiny shards of the lead ammunition. Radiographs of animals shot by hunters show that these small lead fragments can lodge as much as 18 inches from the main wound channel —the pathway of the bullet.

If consumed, a fragment of lead the size of a grain of rice is enough to kill an eagle. 

While eagles and other raptors regularly expel indigestible material (fur, feathers, some bones) about 12 to 24 hours after eating, lead can be quickly eroded by stomach acid—particularly very small fragments.  Even small amounts of ingested lead can also cause detrimental cumulative effects for avian scavengers.

Radiograph of lead fragments in the stomach of a Bald Eagle patient.
Radiograph of lead fragments in the stomach of a Bald Eagle patient.

Multiple studies have noted a correlation between big game hunting season and an increase in lead-poisoned birds. This is a problem that is found throughout North America in a variety of species – Bald Eagles in Minnesota and Wyomingvultures in Californiaravens in Canada and the southern Yellowstone ecosystem. Lead exposure has also been a significant problem for the reintroduction of the California Condor in the western United States.

This problem is often even more serious when hunters or farmers shoot nuisance animals, such as groundhogs, because the entire animal is left in the field, and is typically out in the open where eagles and other avian scavengers can find it. 

For eagles and other raptors, lead is most toxic when consumed, as opposed to lead bullets or shot simply lodging in muscle tissue. Exposure to digestive fluids and stomach acids breaks down the lead, allowing it to be absorbed into the bloodstream and distributed to internal organs, the nervous system, the respiratory system, and the renal system. Lead may also leach from lead fragments lodged in joints and in bone marrow.  Even if the actual projectile has moved through the digestive tract and out of the body, dangerous amounts of dissolved lead can still be circulating in the blood or stored in the bones or internal organs of the bird’s body. 

Unlike organic toxins, lead is a heavy metal; an eagle’s internal organs are not able to easily purge the lead in the bird’s bloodstream, and the toxin is cumulative. Lead accumulates in the bones of the affected bird; if the bird is exposed to additional lead in its diet, the amount of the toxin will increase over time, eventually affecting the bird’s ability to survive.  The cumulative impacts can last for years, and can only get worse over time. 

Treatment

Lead-positive animals are treated with chelation therapy since no level of lead in the body is considered “safe”. The course of treatment depends on the amount of lead measured on the lead test as well as whether the animal has clinical symptoms. For clinical cases, treatment consists of a course of CaEDTA injections; this “chelator” binds to the lead to take it out of the animal’s blood – essentially “scrubbing” the blood clean. Treated patients also receive an oral chelation medication. After the first round of chelation therapy, affected animals typically have a rest period and then are tested for lead again. Sometimes an animal must receive several rounds of chelation therapy.

Sadly, even if animals survive lead poisoning and chelation therapy is effective, patients can have permanent long-term deficits that prevent their release. Studies show that lead poisoning can cause permanent cardiac damage, which affects stamina and may prevent release.  The vision of affected animals can also be permanently impaired.

More Information

Infographic showing pathways to ingestion of lead hunting fragments