Around the world, lead poisoning affects nearly all predator birds and scavenging birds of prey, as well as crows and ravens, and some insect-eating species and water birds.
But data on lead poisoning in predatory birds historically only reflected regional exposures and population impacts.
A new 10-year study of bald and golden eagles expands that scale — and exposes some alarming patterns.
The research appears in the journal Science.
Lead concentrations in the sampled eagle’s bones indicated repeated exposure in nearly half of the 1,200 birds studied across 38 states. Chronic poisoning occurred more often in adults than in juveniles, and more commonly in the central U.S. than the coastal zones.
What's more, blood, liver and feather tissues showed signs of acute poisoning in one-third of bald and one-tenth of golden eagles. Those toxic events occurred mainly during winter, when prey grow scarcer and the eagles turn to scavenging.
"In the case of bald and golden eagles, lead is shown to have negative effects on every system in their bodies. It can affect their ability to survive, to acquire and digest prey, to fly or to successfully reproduce," said lead author Vincent Slabe of Conservation Science Global in Bozeman, Montana.
The authors' model-based analyses suggest that, without lead poisoning, bald eagle populations would grow 4% faster and golden eagles would increase 1% faster.
"That 1% seems small, but when the population is barely stable, it can tip the scales towards a decline. And, multiplied over several generations, that results in a considerably visible decline in population size," said co-author Brian Millsap, national raptor coordinator for the U.S. Fish and Wildlife Service.
Co-author Todd Katzner, a research wildlife biologist at the U.S. Geological Survey in Boise, Idaho, said the group did not source the lead, but plans to do so in an upcoming study.
That said, the authors agreed the results match known patterns of eagles consuming lead ammunition as they scavenge gut piles left behind by hunters.
"Eagles target this food source during the fall and winter seasons, when big game hunting occurs," said Slabe, who is a hunter.
He added that hunters can help by switching to non-lead ammunition.
“Use a nontoxic, nonlead alternative to harvest the animal, gut the animal and walk away, and feel safe that you're leaving a much safer food source for eagles on the landscape,” said Slabe.
Slabe has helped develop nonlead ammunition programs in Wyoming in Montana, with plans to expand out to the East Coast.
