A new study has found that nearly half of all bald and golden eagles in the USA suffer from chronic and/or acute lead poisoning, which the research team believes is the result of these birds scavenging the remains of animals shot with lead bullets.
The study evaluated signs of lead exposure in a total of 1,210 bald and golden eagles from 38 states across America, taking blood samples from live eagles, and examining bone, livers and feathers from dead birds.
Of 448 birds whose bone samples were analyzed using plasma mass spectrometry, 47 percent of bald eagles and 46 percent of golden eagles were found to have bone lead concentrations over the clinical thresholds for chronic lead poisoning, indicating repeated exposure to lead over a long term. Rates appeared to rise with age.
Furthermore, there was evidence of acute lead poisoning as well – suggesting a recent, short-term, high-exposure event – in nearly a third of bald eagles and around 7-9 percent of golden eagles. In some cases – 25.8 percent of bald eagles and 4.9 percent of golden eagles – lead concentrations in the liver were high enough to hit very conservatively set thresholds above which lead poisoning is "generally determined to be the cause of death."
If this was indeed the cause of death for this proportion of birds, the researchers determined that lead poisoning is suppressing population growth by some 3.8 percent in bald eagles and 0.8 percent in golden eagles. Bald eagles in particular have had a rough time in the last century or so, with populations dropping to less than 500 breeding pairs in the late 1960s due to hunting, DDT poisoning and habitat loss. Conservation efforts have seen great success, though; there are now more than 70,000 breeding pairs, and these iconic birds are now off the endangered species list.
The research team linked this lead poisoning problem back to commonly used lead ammunition.
"Elevated lead concentrations in predatory and scavenging birds are usually caused by primary lead poisoning, most frequently direct ingestion of lead fragments from ammunition," the study said. "Use of lead in ammunition during hunting seasons corresponds directly, both spatially and temporally, with the feeding ecology of facultative scavengers such as bald and golden eagles, a problem that has been studied extensively. Our data show a continent-wide temporal correspondence between acute lead poisoning of eagles and the use of lead ammunition."
“Medical science tells us that, for humans, there is no safe amount of lead," said Dr. Vince Slabe, lead author on the study and Research Wildlife Biologist for Conservation Science Global. "Today we also know that redistributed lead in our environment is harming eagle populations across North America.”
Lead-cored bullets are thought of as more humane by many hunters. When they hit a target, the lead can separate and shatter into tiny fragments that often move well beyond the bulk of the slug. These can cause enormous damage to muscles, organs and blood vessels, typically leading to a a quick loss of consciousness and death – but they often can't be recovered from the carcass.
Thus, once hunters field-dress the animal, there can be plenty of lead left in what's left on the ground. Eagles, being scavengers, love tucking into these entrails, and this appears to be where they're ingesting lead fragments.
One potential solution for hunters could be a switch to copper bullets, which some experts say have improved markedly in recent years to become nearly, or as effective, as lead-cored bullets. These do not fragment when they hit a target, but can be designed to mushroom out and deliver significant damage, with the added advantage of often passing right through the animal and leaving a blood trail that can easily be followed if it doesn't drop on the spot.
“With an ever-growing body of scientific findings, we hunters have an ever-improving understanding of the details of preventable exposure," said Chris Parish, President and CEO of The Peregrine Fund and co-founder of the North American Non-lead Partnership. "The hunting community has a long-standing tradition of conservation of wildlife in the United States, and we are the key to solving the problem. Through the efforts of the North American Non-lead Partnership, we are finding that with appropriately delivered information, hunters and anglers who are asked to help are eager to do their part in improving ecosystem and wildlife health. Wildlife agencies, sporting groups, and tribal communities, are coming together to increase awareness and solve this problem on behalf of wildlife across North America.”
The study was published in the journal Science.
Source: Conservation Science Global