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Almost half of all American bald and golden eagles are being poisoned by lead.
The impacts of the toxic metal are holding back the recovery of the eagles in the USA, with its effects becoming more pronounced in winter.
The effect of lead poisoning could be enough to tip American golden eagle populations into decline.
A study published in the journal Science found that long-term, chronic lead poisoning in the eagles was as high as 46%, while evidence of very high short-term exposure was found in 9%. In bald eagles, the latter figure was 28%.
A significant amount of the lead poisoning is believed to result from the use of lead ammunition by hunters.
Co-author Todd Katzner says, 'This is the first study of lead poisoning of wildlife at a nationwide scale, and it demonstrates the unseen challenges facing these birds of prey. We now know more about how lead in our environment is negatively impacting North America's eagles.'
The findings reveal more information about how lead pollution both past and present can affect the environment.
Lead is a metal which has been used by people for thousands of years. Its ease to work with, and relative abundance, led to it being used for many purposes, including paint, makeup and plumbing.
Despite the toxic impacts of lead having been recognised millennia ago by the Romans, it was not banned as a fuel additive until 2000, while it is still used in batteries, radiation and gun shot to this day.
Once in the environment, lead can persist for decades in soils and sediments, meaning that the use of lead in the 1980s is still having impacts to this day. It was only in 2016 that contamination of anthropogenic lead became low enough that natural levels could be detected in Atlantic seawater.
In birds of prey such as bald and golden eagles, lead poisoning is more common due to the continuing use of lead ammunition. When they scavenge on animals killed by lead shot, they can consume the metal. If enough is consumed then the eagles can die.
While action has been taken to protect these eagles, no large study has been undertaken to show the impact across a wide area. The results of the recent research show that lead poisoning is widespread.
'Studies have shown lethal effects to individual birds, but this new study is the first to show population-level consequences from lead poisoning to these majestic species at such a wide scale,' said Anne Kinsinger, associate director for ecosystems at the United States Geological Survey which helped carry out the research.
Over 1200 eagles from 38 US states were sampled to test their bodies for signs of lead poisoning. Blood samples were taken from living eagles, while bone, liver and feathers were taken from those found dead.
Of the dead birds, almost half of bald and golden eagles were found to have levels of lead indicative of chronic lead poisoning, with adults more likely to be poisoned than juveniles. Almost 30% of bald eagles showed evidence of acute poisoning, as opposed to around 10% of golden eagles.
This short-term poisoning was more common in the winter months, when eagles are more reliant on scavenging for prey as food is scarcer. This means that they are more likely to eat carcasses or butchered organs that contain lead.
While the level at which lead becomes lethal to these eagles is unknown, it is commonly assumed that dead birds with concentrations of lead above six milligrams per kilogram in the liver have died of lead poisoning when other symptoms are present.
Based on this threshold, it was estimated lead poisoning causes the population growth of bald eagles to be reduced by 3.8%, and by 0.8% in golden eagles.
Brian Millsap, another co-author, says, 'The study's modelling shows that lead reduces the rate of population growth for both of these protected species.
'That is not as impactful for bald eagles since this endemic species population is growing at 10% per year across the U.S.
'In contrast, the golden eagle’s population is not as stable, and any additional mortality could tip it towards a decline.'
The scientists hope their work will allow conservation efforts for these species to tackle the issue of lead poisoning in the future.