Bald eagle groups slowly recovered from near devastation after the government banned DDT in 1972, but another ongoing problem blunted this recoil—lead poisoning from firearms ammunition.
A new study published in Wildlife Management Journal, found that despite increasing numbers of bald eagles, poisoning from eating dead carcasses or parts contaminated with lead reduced population growth by 4% to 6% annually in the Northeast.
The findings can help educate and inform policy about ammunition choices for hunters, as copper-based ammunition exists—although supplies of all ammunition have recently declined.
“We hope this report adds information that will compel hunters, as environmentalists, to consider their ammunition options,” said Kristin Schuller, associate research professor in the Department of Public Health and Ecosystem Health at Cornell University and lead author of the study.
Declining growth rates have the potential to erase the cushions that protect the population from unforeseen events.
“Although the population appears to have recovered, some disturbance may occur which may cause the vultures to retreat again,” Schuller said.
Habitat loss, climate change, West Nile virus and other infectious diseases are all threats that could affect the resilience of bald eagles and lead to a population decline, Schuller said.
While bald eagle numbers in the lower 48 states quadrupled between 2009 and 2021 to more than 316,000, according to a 2021 US Fish and Wildlife Service report, current findings on the effects of lead on eagles point to potentially negative outcomes for other species.
Human health can be affected when lead is broken down within game species and then consumed.
Many hunters wear a deer that they shot, leaving contaminated organs where the animal fell. Bald eagles have been known to excavate such carcasses, but they are not the only animals to do so. Passage cameras have shown that owls and crows, as well as species of mammals, including wolves, foxes, hunters, and bears, are also digging for the remains of hunters.
“We didn’t collect data on these other species in the same way we pay attention to vultures,” Schuller said. “We’re putting vultures out there as sort of a poster for this problem, but they’re not the only ones affected.”
Although overall eagle numbers increased across the Northeast between 1990 and 2018, the researchers’ models estimated that deaths from lead ingestion reduced the growth rate of bald eagle populations by 4.2% (for females) and 6.3% (for males).
Schuller said the study authors have released the software from their new methodology, so that others can use it to analyze similar population-level data for other species.
Reference: “Environmental lead reduces resilience of bald eagle populations.” Jan. 13, 2022, Available here. Wildlife Management Journal.
DOI: 10.1002 / jwmg.22177
The study was funded by the Morris Animal Foundation, the New York State Department of Environmental Protection, and funds from the Federal Wildlife Restoration Aid Act, which is administered by the US Fish and Wildlife Service.
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