It’s illegal to shoot bald eagles but America’s national symbol is still dying as a result of hunting.

Researchers working at the Upper Mississippi River National Wildlife and Fish Refuge recently conducted autopsies on 168 dead bald eagles found in Illinois, Iowa, Minnesota, and Wisconsin. According to their tests, nearly half the birds had detectable levels of lead in their livers. Worse, 21 percent of the eagles most likely died from exposure to the toxic metal.

The source of this lead, according to the study, appears to be the gut piles left behind by hunters after they have shot deer and other animals. Lead bullets tend to fragment into tiny pieces when they strike a target and can spread through an entire carcass. The researchers collected the offal piles from 25 deer that had been shot with lead ammunition on the refuge. Radiograph tests showed that 36 percent of those gut piles contained lead, sometimes as little as one fragment and as many as 107 fragments.

Eagles in the winter “rely on carrion as a primary food source, especially deer carcasses and offal [gut piles] left in the field after hunting events,” wrote project leader Ed Britton in an earlier version of the study.

The FWS study does not say eagle populations are declining. The agency removed the bald eagle from the endangered species list in 2007 after 40 years of recovery efforts, but they remain protected under two other federal laws.

News of this bald eagle study came out the same day that the Humane Society of the United States, the Natural Resources Defense Council (NRDC), and 10 other conservation organizations filed a petition with the Department of the Interior to ban the use of lead ammunition on federal wildlife refuges and national parks.

The petition cites not just bald eagles but 130 other wildlife species that ingest fragments of lead bullets and shotgun pellets, often by feeding on prey that has been shot by hunters. The species affected range from California condors to grizzly bears. According to the petition, 10 to 20 million animals die from lead poisoning every year.

“There’s a reason we’re getting rid of all the other sources of lead in the environment,” said Andrew E. Wetzler, director of the Land and Wildlife Program at NRDC. “It’s a deadly toxin.”

Wetzler points out that the U.S. military is phasing out the use of lead ammo, as are shooting ranges, due to lead’s environmental impact. “There is plenty of more effective, alternative ammunition available,” he said. Nonlead ammunition is more expensive, but Wetzler estimates it only costs about $15 more per box. “Can’t we all agree that we shouldn’t be poisoning animals like bald eagles and California condors and grizzly bears so you can save $15 on a box of bullets?” he asked.

Hunting groups, not surprisingly, came out against the petition. A press release from a trade association called the National Shooting Sports Foundation (NSSF) decried the “anti-hunting news media spin” on the bald eagle study.

The press release stated that “no conclusive evidence exists that shows hunters and target shooters using traditional [lead] ammunition have caused a decline in the population of raptors.” NSSF also said the lead in bald eagles could have come from other sources, such as landfills, paint, or industrial activities.

The Wildlife Center of Virginia has documented many eagles with buckshot in their stomachs, and the Wildlife Rehabilitation Center of Northern Utah took in three lead-poisoned bald eagles this past February, all of which died. Several scientific studies have also examined the threat that lead-tainted carcasses and gut piles pose to bald eagles and other birds.

“Conservation-minded hunters know this isn’t about hunting,” noted Wetzler. “There’s no safe amount of lead. If it’s ingested by kids, it can cause irreversible brain damage. And it’s deadly to wildlife.”

Wetzler said the federal government has the authority to ban the use of lead bullets on all federal land—it had banned such ammunition for hunting waterfowl in 1991. “The clearest, cleanest, and easiest way to begin with getting rid of lead is with the few places left in the country that are supposed to be havens for wildlife,” he said.