- Eagles eat the entrails and remains of game animals shot and left by hunters, in the process often ingesting pieces of lead ammunition, which end up poisoning the birds.
- Scientists sampled the blood from living and dead bald eagles and golden eagles across 38 U.S. states and found that nearly 50% showed evidence of repeated lead exposure.
- Lead is a neurotoxin, and long-term exposure can lead to impaired movement, lower sperm quality, and a weakened immune system; higher doses can lead to death.
- Lead poisoning, the researchers found, is causing population growth rates to slow by 3.8% for bald eagles and 0.8% for golden eagles, annually.
Eagles, regarded by some as a symbol for American freedom, are being poisoned by lead from ammunition.
Scientists sampled the blood from 1,210 living and dead bald eagles (Haliaeetus leucocephalus) and golden eagles (Aquila chrysaetos) across 38 U.S. states over eight years. Nearly half of all the sampled birds showed evidence of repeated lead exposure, according to a newly published study in the journal Science.
How does lead getting into the eagles? Mostly from bullets — but eagles aren’t directly in the crosshairs. Rather, they ingest the lead when they eat the remains of animals killed by hunters. Hunters often do what’s known as field dressing of their game, taking much of the animal but leaving the entrails (organs and viscera) behind. The explosion from a shotgun shell can lodge dozens of small lead balls inside these internal animal parts, giving scavengers a dose of lead with their meal.
Lead is soft and easily eroded and oxidized inside the guts of birds. Once inside, it can easily make its way into the bloodstream. Long-term exposure to lead, a neurotoxin, can lead to impaired movement, lower sperm quality, and a weakened immune system. Higher doses can lead to death.
Lead poisoning, the researchers found, is causing population growth rates to slow by 3.8% for bald eagles and 0.8% for golden eagles, annually.
“This new study is the first to show population-level consequences from lead poisoning to these majestic species at such a wide scale,” Anne Kinsinger, associate director for ecosystems at the U.S. Geological Survey, said in a statement.
Of the live birds sampled, 9% of golden eagles and 28% of bald eagles had concentrations of lead in their blood high enough to cause death. Lead accumulates over time in bones, so more frequent chronic lead poisoning was found in older eagles. Because eagles aren’t the only ones eating these remnants, scientists suspect lead poisoning is more widespread among animals than we may know.
The dangers of lead poisoning from discarded animal remains have been known for more than a decade, leading to measures to reduce the use of lead ammunition for hunting. In 2006, a state-funded program in Arizona offered free non-lead ammunition to hunters. Of those that received the ammunition, two-thirds both used the ammo and said they would recommend it to others. However, without these subsidized programs, non-lead ammo can be more difficult to acquire and more expensive than lead-laden shots, so these conservation measures haven’t really taken flight.
Although lead reduces the rate of population growth for both species of eagles, the impact isn’t as bad for bald eagles, said Brian Millsap, national raptor coordinator at the U.S. Fish and Wildlife Service and a co-author of the recent study.
The U.S. bald eagle population is on the rise, growing at around 10% per year. After dropping to just 417 known nesting pairs in 1963, the bald eagle was one of the first species listed under what’s now known as the Endangered Species Act. This protected status, along with conservation measures, have been a soaring success.
“Bald eagles have returned to areas of the United States where they hadn’t been seen in a century or more because we prioritized their protection,” Noah Greenwald, endangered species director at the Arizona-based Center for Biological Diversity, said in a statement.
“In contrast,” Millsap said, “the golden eagle’s population is not as stable, and any additional mortality could tip it towards a decline.”
Starvation is the leading cause of death among young golden eagles, meaning that food availability may be the limiting factor throughout their annual cycles. Golden eagle populations are decreasing by around 1% per year. At that rate, they could vanish within the century.
USGS wildlife biologist and study lead author Todd Katzner said this first nationwide study into lead poisoning in eagles “demonstrates the unseen challenges facing these birds of prey.”
Citations:
Slabe, V. A., Anderson, J. T., Millsap, B. A., Cooper, J. L., Harmata, A. R., Restani, M., … Katzner, T. E. (2022). Demographic implications of lead poisoning for eagles across North America. Science, 375(6582), 779-782. doi:10.1126/science.abj3068
Bedrosian, B., Craighead, D., & Crandall, R. (2012). Lead exposure in bald eagles from big game hunting, the continental implications and successful mitigation efforts. PLOS ONE, 7(12), e51978. doi:10.1371/journal.pone.0051978
Chase, L., & Rabe, M. J. (2015). Reducing lead on the landscape: Anticipating hunter behavior in absence of a free nonlead ammunition program. PLOS ONE, 10(6), e0128355. doi:10.1371/journal.pone.0128355
Banner image of a bald eagle in Alaska by Andy Morffew via Wikipedia (CC BY 2.o).
Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough_
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