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Scavenged Bullets Dooming Condors

25 June 2012 4:12 pm
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Joe Burnett

In danger. The California condor is threatened by lead poisoning from bullets in scavenged carcasses.

Spreading its wings to a 3-meter span, flying at a speed of up to 96 kph, and living as long as 60 years, the California condor (Gymnogyps californianus) is one of the world’s most magnificent birds. It’s also one of the rarest. Only 22 condors were alive in 1982, due to poaching, habitat destruction, DDT poisoning, and shooting by cattle ranchers who mistakenly believed that the carrion-eating birds were killing young calves. An intensive captive breeding program has increased the condors’ numbers to almost 400, about half of which are free flying. But the birds still suffer from lead poisoning caused by scavenging the carcasses of animals shot with lead-based ammunition. Unless this source of lead is eliminated, the birds will never survive without human help, a new study finds.

Unlike many other carrion eaters, condors feed predominantly on the remains of large animals, which are more likely to have been shot than roadkill or small animals partially eaten by larger ones. Even small amounts of lead can affect the nervous system and kidneys of birds, mammals, and humans. In condors, high levels of lead can shut down the digestive system, causing the birds to starve.

Ironically, the success of captive breeding and releasing programs may be masking the continuing danger of lead, says research toxicologist Myra Finkelstein of the University of California, Santa Cruz. Condors’ numbers have increased because captive birds have been released into the wild; only a few chicks have been “born free.” And even after release, the birds receive massive assistance from biologists, she explains. For example, condors are caught twice a year and tested for lead; those with excessive lead levels are taken to one of several zoos, treated, and released again. “This revolving door effect keeps us from seeing whether condors can survive without human help,” she says.

To get a better handle on the impact of lead poisoning, Finkelstein and colleagues checked over 1000 blood samples taken from 150 condors between 1997 and 2010. About 70% of birds had telltale signs of lead exposure, while nearly half of free-flying condors met the standard for lead poisoning, the researchers report online today in the Proceedings of the National Academy of Sciences.

By checking the lead levels in the birds’ feathers, which grow over a period of several months, the researchers also got an idea of how long the condors had been exposed. Analysis of 18 feathers showed the birds had lead exposure for 75% of the feather-growing period and lead poisoning for 30% of that time.

Finkelstein and colleagues verified that bullets were indeed the source of the lead. By analyzing “isotope ratios,” a sort of mineral fingerprint made of different forms of lead, the team found that the majority of free-flying condors had ratios that matched those found in ammunition. The same technique is used to identify sources of lead poisoning in humans, Finkelstein says.

To see whether the birds could cope with lead poisoning unassisted, the researchers constructed a population model that factored in various scenarios of lead exposure. “If current management efforts go on basically forever, the condor population will remain stable,” says Finkelstein. If lead ammunition is eliminated, she says, the birds can also survive without human help. On the other hand, with no human intervention and with lead exposure as it is now, the birds will go extinct, the model predicts.

“The science is solid and the conclusions inescapable—the condor will never be a free-living species as long as exposure to lead from ammunition continues,” says raptor biologist Patrick Redig of the University of Minnesota, St. Paul, who has served on condor recovery programs but was not involved in the new study.

In 2008, California banned the use of leaded bullets in the eight counties in which condors are found. But although lead levels have since declined in other birds with smaller territories and more varied diets, such as golden eagles and turkey vultures, the condors have not yet benefited from the limited ban. Condors can fly 240 kilometers in a day (far beyond the counties in which lead is banned). Throughout their long lives they eat 75 to 100 carcasses per year, and even a single exposure to lead is potentially fatal.

Efforts to ban lead-based ammunition throughout the United States have met with stiff resistance from gun users, according to toxicologist Michael Fry, who is now with the U.S. Fish and Wildlife Service in Honolulu but previously worked on condor issues with the American Bird Conservancy in Washington, D.C. In April, the U.S. House of Representatives passed a bill that prevents the Environmental Protection Agency from regulating toxic components of ammunition. “Lead-free versions are available for just about every type of ammunition, and many gun users like them better,” says Fry. The bullets are more expensive, he admits. However, condor-protection programs cost an estimated $5 million per year, most of it paid for by state and federal taxes, according to the study authors. Fry notes that although lead alternatives cost just two or three cents more, a box of lead-free bullets is currently 50% higher, probably more expensive because they’re not yet in widespread use.

Finkelstein is hopeful that communication may succeed where legislation fails. Some condor-protection groups are conducting outreach programs that invite hunters to use nontoxic bullets, and Finkelstein says the hunters’ responses are encouraging.

*This item was updated to add that Michael Fry previously worked at the American Bird Conservancy in Washington, D.C, and to provide further explanation about the price of lead-free bullets.

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