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The Bald Eagle’s comeback is one of America’s most famous conservation success stories. From an all-time low of 417 pairs throughout the United States in 1963, the species numbers 71,467 pairs as of 2021. The primary reason for the bird’s turnaround was the crucial 1972 ban of the pesticide DDT. The birds have bounced back that the government is even upping the amount of Bald Eagles that industry can accidentally kill without penalty. 

But despite the boom in their numbers, Bald Eagles still face many threats, including poisoning from ingesting lead bullets. And the risk posed by this toxic ammunition appears to be significant at the population level, according to a new published in the Journal of Wildlife Management. Researchers found that eating fragments of lead bullets used for hunting reduced Bald Eagle population growth by 4 to 6 percent in the northeastern United States due to lead bullets used for hunting. The study analyzed Bald Eagle count data and lead-poisoning veterinary reports from 1990 to 2018 across seven states. 

While Bald Eagles are often portrayed as regal hunters, they are opportunists and are just as likely to scavenge for their food, feeding on carcasses of dead animals, including those shot by hunters. That’s when they and other scavengers like California Condors  get into trouble with lead. When a lead bullet hits an animal, it breaks up into tiny fragments, which the eagles accidentally eat along with their food. 

“Once the lead is in the very acidic stomach of the bird, it is dissolved and absorbed into the bloodstream, causing chaos and damage,” said Victoria Hall, a veterinary practitioner at The Raptor Center at the University of Minnesota, in an email. According to Hall, as little as .5 to 1 centimers of lead—roughly the size of a rice grain—can poison a Bald Eagle. 

Lead ravages a bird’s nervous system, causing seizures and irreversible damage to the brain. It can also cause depression and weakness, organ failure, and anemia, leading to death. Even if the bird survives, the injuries are often so debilitating that it can’t survive long in the wild. As a result, eagles showing signs of severe lead poisoning frequently have to be euthanized when brought into clinics. Hall says the center sees between 160 to 180 Bald Eagles a year, of which 85 to 90 percent test positive for some level of lead, and about 30 percent have levels high enough to be fatal. 

Although lead is a potent toxic substance and known to affect a wide range of animals and bird species, lead ammunition is currently only banned for hunting waterfowl. (A late-term Obama administration ban of all lead ammunition and fishing tackle on federal lands was swiftly rolled back by the Trump administration.) Currently, California is the sole state that has banned lead bullets for hunting big game. 

So, when wildlife managers at the New York State Department of Environmental Conservation (NYSDEC) started noticing reintroduced Bald Eagles dying of lead poisoning in New York, they knew that lead bullets were to blame. “As the population [of Bald Eagles] started to grow, we were starting to see more cases where the actual lead particles themselves were present in the digestive tract,” says Kevin Hynes, a pathologist and wildlife manager at NYSDEC, and an author of the study. “Upwards of 14 percent of the [Bald Eagles] we were looking at every year were succumbing to lead poisoning.”

These observations led Hynes and other researchers to wonder if the number of birds dying from lead poisoning was significant enough to have an impact on the larger population. “It’s been a big question for me for a long time,” says Krysten Schuler, a disease ecologist at Cornell Wildlife Health Lab and an author of the study. 

Bald Eagle numbers, like those of any other bird, depend on the survival and fertility rates of chicks, breeding adults, and non-breeding juveniles. These factors determine how the population grows with time. To estimate how lead might affect these rates, the researchers used a complex mathematical model harnessing 30 years of population data. Then, they constructed two hypothetical “what if” scenarios to estimate the growth rates: one scenario if lead were reduced to a point where birds could still be negatively affected but none would die from ingestion, and another scenario where lead was completely removed from the environment and the eagles would thrive to their fullest, experiencing no fatal or other debilitating effects caused by poisoning. 

“This math problem was the hardest one I’ve had to solve in my life,” says Brenda Hanley, the quantitative mathematician from Cornell University who built the model. “It was a real brain buster.” 

By comparing the results of the two hypothetical scenarios with the real population growth numbers, the scientists could precisely say that lead hampered the overall growth rates of Bald Eagles by 4.2 percent in males and 6.3 percent in females. “To lay eggs is hard work for the females and I think that could be a role there,” says André Dhondt, an ornithologist at Cornell University who was involved in the study, addressing the discrepancy between the sexes. 

While the model showed non-breeding juveniles to fare well in both scenarios, chicks and breeding adults were negatively affected, hurting the “resiliency” of the population to bounce back if the numbers fluctuate in the future due to a variety of stressors, including climate change, collisions with wind turbines and power lines, and disease outbreaks. “Think of resiliency as holding a rubber band,” Hanley says. “If you pull the rubber band, it will stretch for a while, but at some point that rubber band will break.” 

Knowing that lead could cause an underlying population weakness is a valuable insight for conservation. “It’s clear that [lead poisoning] has been an issue for wildlife for a very long time,” says Kyle Elliott, an ornithologist at McGill University, who studies the ecology of avian predators and was not part of this study. “But being able to propagate that up and understand what’s happening at the level of the population is really the strength of this study.” 

Though the findings are worrisome, Bald Eagle and other wildlife deaths due to lead poisoning are preventable. “Eliminating lead shot from key areas is something that’s tangible,” Elliott says. Instead of lead bullets, hunters can switch to copper bullets, for example. In California, where lead bullets are illegal, wildlife organizations even hunters with free, non-lead ammunition to encourage the behavior. In other states where lead bullets are still legal, hunters  should—remove all remnants of carcasess contaminated with lead bullets to prevent scavengers from eating them. 

The researchers believe making hunters more aware of how harmful lead is to the environment and to raptors like Bald Eagles is vital to tackling this problem. “A large part of the problem, as I see it, is a general lack of knowledge about this cause of death,” Hynes says. “If [hunters] realized that their ammunition choice was potentially going to poison the eagles, they would more than likely make the switch.” 

Andrea Jones, director of bird conservation for , agrees, noting that the use of lead for hunting can also have a negative impact on humans who “I think education is really important,” she says. “So if people care about what they're hunting and feeding their families, they should care about this.”

Bans on lead ammunition and tackle are a controversial issue that can get in political crosswinds, but as this study further proves, the potential upside for wildlife is clear. “There are so many environmental problems that have such complex or difficult solutions, and this is one where the solution couldn’t be simpler,” Hynes says. “It’s kind of a no-brainer.”