Analyzing the carcasses of rare birds of prey, European researchers found significant lead contamination in the raptors' bodies, including in soft tissues, in bones and as solid matter in the digestive tract.

Their study, published March 2 in Science of the Total Environment, suggests that the source of the harmful contamination is hunting ammunition, demonstrating a need for stricter regulation of these bullets and shotgun shells in Europe and throughout the world.

The laws around lead ammunition vary by country, and are a contentious issue in some parts of the world. For example, in the United States, lead ammunition has been banned for use in waterfowl hunting since 1991. The Obama administration implemented a ban on all lead ammunition in national park hunting of any type, but it was revoked in the early days of Donald Trump's presidency.

In Europe, where the study was conducted, lead ammunition has been restricted in 23 countries but only banned in two, Denmark and The Netherlands.

Proponents of such ammunition tend to blame industrial lead use for any ecological contamination, claiming that ammunition does not pose a threat. However, there is a large body of scientific research suggesting that ammunition does play a significant role in lead poisoning of wildlife.

"The first evidence of the impact of lead from hunting ammunition on large raptors came from the California condor, one of the most endangered birds in North America," said senior author Alessandro Andreotti, an ornithologist at the Italian National Institute for Environmental Protection and Research. She said that since the late 1980s, "A number of studies carried out all around the world revealed that raptors are exposed to lead when feeding on unretrieved game or gut piles left behind by hunters to prevent the spread of intestinal bacteria into the meat of their prey."

Lead pollution is only one type of pollution that birds are faced with, but it is a dangerous one because birds are particularly susceptible to lead toxicity. Lead poisoning can cause gastrointestinal issues, neurological impairment, weakness and paralysis.

The researchers focused on European raptors, picking four rare species: the golden eagle, griffon vulture, cinereous vulture and bearded vulture. Andreotti explained to The Academic Times that though they are rarer than other birds, predatory ones make great "biological sentinels" because they are extremely sensitive to contaminants and are unlikely to be hunted themselves as apex predators.

The researchers gathered 252 total carcasses, all birds that had died in the wild, and took 595 tissue samples from the bones and organs, using these samples to determine the degree of lead contamination.

"Results were striking: 44% of the birds [were] lead-contaminated and more than a quarter had lead concentrations revealing clinical lead poisoning," Andreotti said. "These figures show that the threat of lead poisoning from hunting ammunition [is] more relevant than we thought all across our study area."

The team was able to confirm that ammunition was a major source of this contamination by comparing the levels of lead in birds with and without lead particles in their digestive tract. The higher levels of lead in tissue of birds that also had lead in their digestive tract suggests that the lead from these particles permeates the gut and diffuses into other tissues.

In the short term, the researchers hope that their findings will encourage governments to adopt stronger regulation for lead-based ammunition, as well as better monitoring efforts for birds of prey worldwide.

"A procedure to introduce a ban on the use of lead ammunition on all terrestrial habitats has been recently launched by the European Commission," Andreotti said. "Our results add new evidence on the effects of lead ammunition on wildlife at European scale, supporting decision-makers to adopt the ban."

More broadly, the researchers maintain that studying this type of environmental toxicology is important for human health as well, because toxins that work their way into apex predators are present in the animals that hunters eat.

"In a way, we can argue that studying raptors allows us to detect toxic substances potentially dangerous for ourselves," Andreotti said. "Therefore, it is in our own interest to preserve a well-structured community of raptors around us and to monitor it constantly."

The study, "Lead contamination in tissues of large avian scavengers in south-central Europe," published March 2 in Science of the Total Environment, was authored by Enrico Bassi, Regional Authority for Services to Agriculture and Forestry; Roberto Facoetti and Maria Ferloni, independent researchers; Alberto Pastorino, Sondrio District Administration; Alessandro Bianchi and Giorgio Fedrizzi, Experimental Zooprophylactic Institute of Lombardy and Emilia Romagna; and Alessandro Andreotti, Italian National Institute for Environmental Protection and Research.