Only a small fraction of Iowa bald eagles appear to suffer from high levels of lead exposure, according to Iowa State University researchers.
“On average, we found very low levels of lead,” said Julie Blanchong, an associate professor of natural resource ecology and management who led the first-of-its-kind study.
Blanchong noted, however, that “we found measurable amounts of lead in a reasonable fraction” of the eagles sampled — a fraction she described as “less than half.”
“Bald eagles in Iowa are definitely exposed to lead but most at very low levels,” she said.
During their sampling, the researchers did not observe any behavior suggestive of a sick eagle, Blanchong said.
They also found 83 percent of the nests they visited produced at least one offspring in a given year — a nest success rate described as “very high” by researcher Stephen Dinsmore, an ISU professor of natural resource ecology and management.
Dinsmore said the study is the first to investigate the prevalence of lead exposure among the general population of bald eagles in the state rather than just sick birds admitted to wildlife rehabilitation facilities, which typically have much higher lead levels.
“Any amount of lead in an animal is unnatural and a negative thing, but it’s encouraging to know that most free-flying eagles are not experiencing the high levels of lead exposure we see in rehabilitation patients,” Blanchong said.
Encouraging, but not surprising, said Kay Neumann, director of the SOAR (Saving Our Avian Resources) raptor rehabilitation facility near Dedham in western Iowa.
“They are looking at free-flying birds in the wild. We are looking at birds that are either going to die of lead poisoning or would have without our treatment,” she said.
Unlike rehabilitation centers, which test for lead in blood samples, the ISU researchers collected excrement on the ground and tree bark below eagle nests and tested the samples for lead. They collected more than 400 samples in 2011 and 2012 from 140 different nests.
Blood samples have been the preferred method of determining lead exposure in animals, but when the subject of the testing is a powerful, tree-dwelling bird with sharp talons and a beak, logistical concessions are required, Dinsmore said.
The researchers visited three rehabilitation centers — the Macbride Raptor Project near Solon, the ISU Wildlife Care Clinic and the SOAR facility — to compare lead levels derived from blood samples with lead levels derived from excrement samples.
“We are continuing to refine our correlations. It is not a perfect correlation, but the two are comparable,” Dinsmore said.
SOAR’s Neumann agrees that the researchers’ excrement-based data is valid and welcomes excrement sampling as a non-invasive diagnostic tool that is often preferable to drawing blood from a sick and stressed bird.
Neumann said the low lead levels found by the researchers in free-flying eagles was to be expected. “It should be zero,” she said.
Jodeane Cancilla, director of the Macbride Raptor Project, said she was surprised at the low levels of lead found in the state’s general population of eagles.
A high percentage of the state’s eagles have access to gut piles and carcasses of deer shot with lead ammunition, which are believed to be the source of most eagle lead poisonings, Cancilla said.
“We see two dozen eagles a year, and all have some levels of lead in their blood,” she said.
Cancilla said she wonders if the tests derived from eagle feces are as accurate as blood tests.
Neither the researchers nor the rehabilitators think the study findings undermine efforts to replace lead ammunition with non-toxic alternatives such as steel, bismuth and copper.
“I don’t think so,” Blanchong said. “Lead is a toxin that is not natural to the environment.”
Dinsmore said lead exposure at any level is not good for the birds.
“It’s a health issue for people who eat the meat (of animals shot with lead ammunition) as well as animals that forage on it,” Cancilla said.
If anything, the fact that nearly half the free-flying eagles in Iowa have some lead in their systems “should make people more inclined to switch,” Neumann said.
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