Eagles get lead poisoning from scavenging dead animals, particularly deer and the gut piles that deer hunters leave behind. Swans are poisoned by sucking up lead pellets as they bottom-feed in ponds and marshes where waterfowl have been shot.
But how do robins and jays and squirrels and opossums find the lead that poisons them? Small birds and neighborhood mammals are lead victims, just like the eagles and swans.
Some of those victims land at the Wildlife Rehabilitation Center of Minnesota (WRC) in Roseville, where robins, squirrels and other injured or ill animals are treated. They arrive for a variety of reasons, perhaps because of peculiar behavior.
When WRC veterinarians notice coordination issues or lethargy, testing for lead is routine.
In a 26-day period in October, tests on 56 opossums produced 31 positive results, according to Dr. Leslie Reed, senior veterinarian and director of vet education at the WRC. Opossums are routinely tested (as are all waterfowl admitted).
Gray squirrels did somewhat better, with 40 positives out of 112 tested.
Opossums are opportunistic feeders, Reed told me in a phone interview. They’ll eat almost anything, dead or alive. Squirrels, she said, get lead from the soil. They also chew on houses, picking up lead from paint.
She did not have numbers for bird patients, but mentioned robins and jays along with woodpeckers, doves, pigeons and crows. Birds are getting lead from the soil. Ground feeders are at particular risk.
It doesn’t take much lead to cause sickness. “A fragment the size of the tip of a pencil will make an eagle ill,” Reed said. Smaller bird species react to smaller amounts of lead.
Because of its potent toxicity, particularly for children, lead has been well studied. It can remain relatively stable and intact in the environment for decades or centuries.
Under certain circumstances, lead can dissolve into soil. It can migrate with water, carrying contamination. It can be taken up by plants and insects, which in turn are eaten by birds and mammals.
Fuel containing lead has been banned by the federal government for general vehicle on-road use. Fuel containing lead continues to be sold for off-road uses, including aircraft, racing cars, farm equipment and marine engines.
Vehicle exhaust drift might have caused toxic levels in soils close to busy streets and highways, said an article published by the University of Minnesota Extension Service. Land near old buildings where lead-based paint has peeled or been stripped off also poses a risk.
In 1978, the federal government banned consumer use of lead-based paint, although it might be hiding under a coating of non-leaded paint.
Opossums and squirrels, said Reed, are among the mammals that ingest lead by chewing on painted wood.
Upland game birds can be a problem, both for animals and people. An article from the Environmental Contamination and Toxicology archive said upland game birds (grouse and woodcock) “may pose an unnecessary risk of lead poisoning because of the possible ingestion of lead shot, bullets, fragments or embedded shot.”
Those birds killed or wounded and not retrieved pose a problem for woodland scavengers. (The use of lead shot for hunting waterfowl was banned by the federal government in 1991, but its shelf-life means it’s still in the environment.)
Back at the rehab center, testing for lead costs from $8 to $10 per test. Treatment is the expensive part, according to Reed. Treating swans costs $50 per day, and treatment might be needed for weeks.
Half of admitted swans show lead toxicity, with 25% of those birds surviving. Lead can lodge in the folds of a swan’s stomach. Enzymes break down the metal, allowing it into the bird’s bloodstream.
Smaller birds, she said, have better survival rates; they tend to not need as many rounds of treatment.
Treatment for lead poisoning is known as chelation. Oral medication causes lead to bind with urine, and thus expelled.
The rehab center is funded by donations (www.wrcmn.org).
Read Jim Williams’ birding blog at startribune.com/wingnut.