A new study found that nearly half of bald eagles tested across the United States show signs of repeated lead exposure.
Scientists found harmful levels of lead in the bones of 46 percent of bald eagles in 38 states, from California to Florida.
The scientists recently reported their findings in the publication Science.
Researchers examined the blood, bones, feathers and liver tissue of 1,210 eagles between 2010 and 2018.
Todd Katzner is a wildlife biologist at the U.S. Geological Survey in Boise, Idaho.
He is also a co-writer of the study.
He said, "This is the first time for any wildlife species that we've been able to evaluate lead exposure and population level consequences at a continental scale."
He said it was "stunning," or very surprising, that nearly 50 percent of the birds showed signs of lead exposure.
Lead is a poisonous substance that affects the nervous system and interferes with the normal function of nerve cells.
Even in low amounts, lead can reduce bald eagles' ability to fly, hunt and reproduce.
In high amounts, lead causes seizures, breathing difficulty and death.
Similar rates of lead exposure were found in golden eagles.
Scientists say that means the birds likely ate animal meat containing lead from ammunition or fishing equipment.
The study estimated that lead exposure reduced the yearly population growth of bald eagles by 4 percent and golden eagles by 1 percent.
Bald eagles are one of America's most celebrated conservation success stories.
The birds were removed from the U.S. Endangered Species List in 2007.
But scientists say that high lead levels are still a concern.
Lead exposure also reduces their ability to face future difficulties, such as climate change or infectious diseases.
"When we talk about recovery, it's not really the end of the story--there are still threats to bald eagles," said Krysten Schuler.
She is a wildlife disease ecologist at Cornell University's College of Veterinary Medicine and was not involved in the study.
Earlier studies have shown high lead exposure in some areas of the U.S., but not across the country.
The blood samples from live eagles in the new study were taken from birds trapped and studied for other reasons.
The bone, feather and liver samples came from eagles killed by vehicles, power lines or other causes.
"Lead is…available to these birds more than we previously thought," said co-writer Vince Slabe.
He is a research wildlife biologist at the nonprofit group Conservation Science Global.
He added that a small piece of lead "the size of the end of a pin" is large enough to cause death in an eagle.
The researchers also found higher levels of lead exposure in fall and winter.
During these months, eagles eat animal remains left by hunters.
These remains often contain many pieces of lead ammunition.
Slabe said he hopes the findings provide a chance to "talk to hunters about this issue in a clear manner."
He added that he hopes more hunters will voluntarily begin using non-lead ammunition such as copper bullets.
I'm Jonathan Evans.