Bald eagle populations have slowly recovered from near devastation after the government banned DDT in 1972, but another ongoing issue has weakened that rebound – lead poisoning from gunshot ammunition.

A new study, published January 13 in the Journal of Wildlife Management, finds that despite increasing numbers of bald eagles, poisoning from eating dead carcasses or parts contaminated by lead shot has reduced population growth by 4% to 6% annually in the Northeast.

Credit: Avian Haven/Provided

The results could help educate and inform policy on ammunition choices for hunters, as copper-based ammunition exists though supplies of all ammunitions have been low lately.

“Hopefully, this report will add information that compels hunters, as conservationists, to think about their ammunition choices,” said Krysten Schuler, assistant research professor in the ­­Department of Public and Ecosystem Health in the College of Veterinary Medicine and senior author on the study, “Environmental Lead Reduces the Resilience of Bald Eagle Populations.” Brenda Hanley, a research associate in the Department of Public and Ecosystem Health, is first author.

The diminished growth rates have the potential to erase cushions that protect populations against unforeseen events. 

“Even though the population seems like it’s recovered, some perturbation could come along that could cause eagles to decline again,” Schuler said.

Habitat loss, climate change, West Nile virus and other infectious diseases are all threats that could affect bald eagles’ resilience and lead to population declines, Schuler said.

While bald eagle numbers in the lower 48 states quadrupled between 2009 and 2021 to more than 316,000, according to a 2021 U.S. Fish and Wildlife Service report, the current findings on impacts of lead to the eagles point to potential negative outcomes for other species.

Human health can be affected when bullets fragment inside game species and are then consumed.

Many hunters ‘field dress’ a deer they shot with lead ammunition, leaving contaminated organs where the animal fell. Bald eagles are known to scavenge such carcasses, but they are not the only animals to do so. Trail cameras have shown that owls and crows, as well mammalian species including coyotes, foxes, fisher and bears also scavenge remains left by hunters.

“We haven’t collected data on these other species in the same way that we pay attention to eagles,” Schuler said. “We’re putting eagles out there as a poster species for this issue, but they’re not the only ones being impacted.”

In the study, Hanley used a mathematical representation of bald eagles’ life history that links properties of individuals to population-scale processes. The researchers also used necropsy records from 1990 to 2018 from seven northeast U.S. states. Out of 1,200 records, close to 500 had been tested for lead, and of those, the researchers looked to see if the birds had ingested toxic levels or if they had just been exposed to the metal.

Through the computer model, the researchers were able to create a hypothetical situation where the lead-exposed and poisoned birds from the necropsy records were added back into the population of living birds, to get counts of how populations would have fared if these birds had not died or been exposed to lead.

Even though total eagle numbers increased across the Northeast between 1990 and 2018, the model estimated that deaths from ingesting lead depressed the growth rate of bald eagle populations by 4.2% (for females) and 6.3% (for males).

The study’s authors have made public the software from their novel methodology, so others can use it to analyze similar population-scale data for other species, Schuler said.

The study was funded by the Morris Animal Foundation, the New York State Department of Environmental Conservation and funds from the Federal Aid in Wildlife Restoration Act, administered by the U. S. Fish and Wildlife Service.