Bullet fragments left in carcasses of deer and other animals killed by hunters are poisoning endangered California condors with lead.
The blood of wild condors that feed on animals wounded by hunters or killed and left behind contain nearly ten times the lead concentration of captive birds, a new study suggests. The type of lead found in elevated concentrations in the wild birds were either close to or matched that found in bullets and shotgun shells used by hunters.
The findings are detailed online in the journal Environmental Science and Technology.
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Dangerously high levels
The researchers measured lead concentrations in the blood of 18 wild condors living in central California and compared it to samples taken from eight captive birds. Blood lead levels in the captive condors were low, about 28 parts per billion (ppb), and were what would be expected based on natural background lead levels in California.
The wild birds, in contrast, had blood lead levels nearly ten times higher—about 246 ppb.
"If these birds were children, they would be entered into a clinical setting to be chelated or managed by public health," study leader Donald Smith of the University of California, Santa Cruz said in a telephone interview.
Chelation is a procedure for removing heavy metals from the bloodstream.
To determine the source of the extra lead, the researchers compared lead found in the condors' blood with lead contained in bullets and shotgun shells purchases from stores close to the condor range. They found that the form of lead from the two sources either matched or were close. Tissue and feather samples taken from a lead-poisoned condor in Arizona also revealed that the bird was exposed to a version of lead not commonly found in the bird's natural setting.
The birds are likely being poisoned after eating carrion laced with the toxic metal, and not from eating the bullets themselves, scientists say.
In another recent study, detailed in Wildlife Society Bulletin, Lindsay Oaks at Washington State University and colleagues X-rayed carcasses of deer killed by hunters. They found that bullets explode into dozen of tiny pieces after piercing the animals, leaving trails of lead fragments along their paths. Of the 38 carcasses examined, half carried at least 100 bullet fragments. Lead fragments were also found in the gut piles hunters leave behind after cleaning out a deer.
The effects of lead poisoning
Kelly Sorenson, a co-author of the current study and executive director of the Ventana Wildlife Society in California, said lead-poisoned condors often die after their crops—a "pre-stomach" that the birds use to store food before digestion—freeze up, effectively shutting down their digestive system.
"They starve to death," Sorenson told LiveScience.
In some cases, Sorenson said, the birds can go blind, lose feathers and become so affected that they can no longer fly and walk.
Condors are not the only birds to feed on the tainted carcasses, scientists say. Evidence suggests bald eagles, golden eagles and other avian scavengers likewise feed on the remains and are also affected.
Since 1995, at least four condors have died from lead poisoning. An additional 26 have received emergency chelation therapy to reduce blood-lead levels, but such therapy can costs up to $10,000 per bird, Sorenson said, adding that it costs $5,000 to $10,000 to bring a condor from an egg all the way to release on an annual basis.
"If we didn't have that lead out there, we could just spend all the money on the new birds going out," he said.
California condors are the largest flying land birds in the Western Hemisphere. By the end of 1982, there were only 22 of the birds left in the wild. Now, after years of intense recovery efforts, about 135 captive-bred descendents of those birds now fly free throughout the southwestern United States. The U.S. Fish and Wildlife Service estimates that up to 40 million dollars has been spent on condor recovery efforts over the past 20 years; the federal government continues to spend $900,000 annually on the program.