Deer hunters, health officials and wildlife managers were shocked last spring when lead bullet fragments were discovered in venison donated to Minnesota foodshelves.
Dr. Pat Redig and Dr. Luis Cruz-Martinez weren't.
The University of Minnesota veterinarians and researchers say bald eagles in Minnesota have been dying of lead poisoning for years. And they say the evidence now is overwhelming that the source of that poison is fragmented lead bullets fired by Minnesota hunters.
Bald eagles are ingesting lead when feeding on deer entrails, carcasses or wounded animals that later die, they say. Of the 100 to 125 eagles brought to the University of Minnesota's Raptor Center each year, 80 percent have elevated lead levels. And each year, at least 20 bald eagles die from lead poisoning.
"We're poisoning eagles, our national symbol," said Redig, a professor at the university's College of Veterinary Medicine and former director of The Raptor Center.
And those are just the eagles that are recovered. It's unknown how many others might be succumbing to lead poisoning and are never found, or what effects lower lead levels are having on eagles.
Cruz-Martinez, a graduate student, has analyzed research conducted by Redig and others, and both say there are four smoking guns implicating lead bullets:
• The incidents of lead poisoning in eagles correspond to the deer hunting season.
"About 10 days to two weeks after the deer season, we start getting an influx of lead-poisoned eagles," Redig said.
Poisonings are much rarer other months.
"There's carcasses and gut piles laying out there with lead fragments, and eagles are chowing down," he said. About 10 eagles with lead poisoning have been brought to the center so far this fall.
• More poisoned eagles come from northern Minnesota, where rifles are allowed for deerhunting.
"We know from previous studies that lead rifle bullets fragment a lot," Cruz-Martinez said, and lead shotgun slugs -- required in southern Minnesota -- fragment less.
A Department of Natural Resources ballistic study this year that used sheep carcasses as stand-ins for deer confirmed that bullets fired from high-powered rifles leave dozens of fragments when they hit an animal. Those lead fragments, often too small to see, spread up to 18 inches from the wound.
Other studies have shown similar results. And when Cruz-Martinez examined the entrails from the 72 sheep carcasses shot in the DNR study, about 60 percent contained lead particles.
• Many lead bullets have copper jackets, or coverings, and eagles found with lead poisoning often had higher copper levels in their kidneys than other eagles (The copper levels weren't toxic.) "We found a direct correlation," Redig said.
• Different sources of lead have different isotope ratios that can be compared to help pinpoint a source of the lead. Lead found in the majority of Minnesota eagles had isotopes similar to the lead in bullets tested in a California study.
Banning lead bullets?
Those four pieces of evidence are compelling, the researchers say.
"We have strongly felt and believed that lead from ammunition, particularly rifle bullets, is the main source of lead for bald eagles," Redig said. "This is a strengthening of a case we've been building for a long time."
That suspicion goes back several decades. Officials knew eagles were being poisoned by eating waterfowl that had been shot with lead shot; they found the shot in eagle droppings. When lead shot was banned for waterfowl hunting in 1991, "we expected to see an immediate turnaround," Redig said.
It didn't happen.
The possible impact of lead bullet fragments on wildlife isn't just a local issue.
In July, California banned the use of lead bullets for hunting in the state's condor range because of concerns that lead bullet fragments have been poisoning the endangered bird. And Arizona officials say lead is the leading cause of death of condors reintroduced into that state. A study there also implicates lead bullets.
But it's the possible impact of lead ammunition to human health that has highlighted the issue, Redig said. He and Cruz-Martinez say the eagle poisoning is a concern for humans, because it confirms that lead bullet fragments are a real issue. Lead particles were discovered earlier this year in venison donated to foodshelves in Minnesota, North Dakota and Wisconsin. And a study in North Dakota showed that people who eat wild game killed with lead bullets tend to have higher levels of lead in their blood than people who don't.
Redig and Cruz-Martinez say there is only one solution: ban lead bullets.
"We know lead is a toxin," Cruz-Martinez said. "It was taken out of gasoline and paint. We need to get rid of lead, one way or the other."
Non-toxic bullets, such as ones made from copper, are a viable option, he said. They hope hunters, once they learn more about the problem, will embrace such alternatives.
Both say the lead issue isn't being driven by an anti-hunting agenda, as some have suggested.
"There is absolutely no anti-hunting intent or hidden agenda at all," Redig said. "I grew up on the Iron Range, and from the time I was 9 I had a rifle in my hand."
Eagle population grows
Despite the poisonings, the eagle population in Minnesota has grown dramatically since the mid 1970s, when there were an estimated 100 pairs of eagles. In a 2005 DNR survey, there were more than 1,200 pairs.
So why worry about lead poisoning a few eagles every year?
"There's an ethical issue here," Redig said. "We don't know long-term what this is doing to the eagle population."
Meanwhile, another deer hunting season is nearly over, discarded gut piles and carcasses are in the woods, and Redig and Cruz-Martinez expect to see more lead-poisoned eagles this fall and winter. Two were brought in Tuesday.
"It's a heart-wrenching thing to watch," Redig said. "They can't breathe, they can't see, they have seizures."
Doug Smith • firstname.lastname@example.org