SANTA CRUZ, CA--A study led by environmental toxicologists at the University of California, Santa Cruz, has confirmed what wildlife biologists have long suspected: Bullet fragments and shotgun pellets in the carcasses of animals killed by hunters are the principal sources of lead poisoning in California condors that have been reintroduced to the wild.
Lead poisoning is a major factor limiting the success of efforts to rebuild populations of the endangered California condor. Since the mid-1980s, condors have been bred in captivity and released back into the wild in California, Arizona, and Baja California. The birds, which feed on carrion, can ingest lead from ammunition in animal carcasses or gut piles left behind by hunters.
The UCSC researchers used a "fingerprinting" technique based on the unique isotope ratios found in different sources of lead. The technique enabled them to match the lead in blood samples from condors to the lead in ammunition. Their findings were published online today by the journal Environmental Science & Technology.
"There had been anecdotal reports for years about condors being exposed to lead from ammunition, but there was never enough clear evidence to document the extent of the problem. We knew that we could probably identify the sources of the lead using isotopic signatures," said Donald Smith, professor and chair of environmental toxicology at UCSC and a coauthor of the paper.
The study was spearheaded by Smith's graduate student, Molly Church, who is now at the University of Pennsylvania's School of Veterinary Medicine and is first author of the paper. Church worked with researchers at several institutions and organizations, including the Ventana Wildlife Society and the U.S. Fish and Wildlife Service, to collect and analyze blood and tissue samples from California condors. She also analyzed ammunition obtained from a variety of sources throughout central California.
"We sampled the ammunition that these birds are most likely to encounter in the remains of hunted game, and we found that lead from those sources can account for the lead in the blood of 77 percent of the birds tested," said Church, who undertook the study after spending a summer monitoring condors as a volunteer with the Ventana Wildlife Society.
The researchers obtained blood samples from 18 free-flying condors in central California and 8 birds that had been raised in captivity and were still in holding pens waiting to be released. At the time of the study, this sample represented 43 percent of the wild condor population in all of California.
The lead levels in the blood of prerelease condors were low, and the lead was isotopically similar to background lead in the California environment. In the free-flying condors, however, blood lead levels were higher and the lead had a different isotopic composition that approached the composition of the lead in ammunition. In the most severely lead-poisoned birds, the blood lead matched exactly the composition of the lead in ammunition, Smith said.
"We found that in the birds with elevated blood lead, the lead isotope ratios fit a mixing model in which one source of lead is the background environment and the other source is ammunition. The results show that lead ammunition in animals shot and left in the field is the predominant source of lead exposure in condors," he said.
Kelly Sorenson, executive director of the Ventana Wildlife Society, said the study provides a solid basis for efforts to resolve the problem of lead exposure in condors. Possible solutions range from legislation limiting the use of lead ammunition to voluntary programs to encourage the use of alternative ammunition in areas where condors are known to scavenge for food, he said.
"There are different ways to get at this problem, but no matter how it is resolved in California, this research was desperately needed not only to confirm the source of the lead but to demonstrate the extent to which ammunition is contributing to the problem," Sorenson said.
In another part of the study, the researchers showed that feathers can be used to monitor lead exposure in condors. They analyzed lead in tissue samples and a feather from a condor that had died of lead poisoning in Arizona. They found that the feather, sampled sequentially along its length, provided a record of the bird's history of lead exposure.
"This shows that we can use feathers to look at the history of lead exposure in individual condors, and we're just starting to do that now with birds that have been reintroduced in California," Smith said.
The elevated lead levels in reintroduced condors are cause for concern even when they are below the level that would cause death from acute lead poisoning, he added. Although it is very difficult to evaluate sublethal effects of toxic substances in wild animals, he said, lead is well known to cause neurological problems in vertebrate animals at relatively low levels of exposure.
"The lead levels that we commonly see in condors are well above the levels considered a concern for human health. If we saw those levels in children, they'd be diagnosed as lead-poisoned and medically treated," Smith said.
Church said she hopes the findings prompt greater efforts not only to reduce lead exposure in condors but to reduce lead contamination in California in general.
"Lead is a well-known toxin that should no longer be getting into the environment, and it would truly be a shame if lead poisoning negated the significant condor conservation efforts and achievements that have taken place over the past several decades," she said.
In addition to Church, Smith, and Sorenson, the coauthors of the paper include UCSC toxicologist Roberto Gwiazda, Robert Risebrough of the Bodega Bay Institute, Page Chamberlain of Stanford University, Sean Farry and William Heinrich of the Peregrine Fund, and Bruce Rideout of the Zoological Society of San Diego. The study was funded by a grant from the UC Toxic Substances Research and Teaching Program, an American Museum of Natural History Frank M. Chapman Research Award, the Bodega Bay Institute, and the Peregrine Fund.