At least four freed birds have died of lead poisoning. Another 26 have needed treatment to reduce the amount of lead circulating in their blood, a stressful procedure that requires recapturing the birds and giving them several injections a day for up to two weeks.
"We think lead is the most limiting factor for their recovery," said Kelly Sorenson, director of the Ventana Wildlife Society.
The Salinas, California-based nonprofit group is helping to reintroduce the birds to California's Pinnacles National Monument and the Big Sur region of the state's coast (map of California).
"Virtually all condors in the wild are being exposed to lead," Sorenson said.
But in 2004 California's Fish and Wildlife Commission decided against requiring nonlead ammunition within the condors' range. The state government agency said at the time that the scientific evidence that bullet lead was the source of the poisoning was weak.
Condor recovery project managers recapture released birds at least once a year to check their health, which includes a field test measuring lead levels in their blood.
Molly Church, then a student at the University of California, Santa Cruz, helping to monitor the birds near the central coast of California, saw the regular blood tests as an opportunity to trace the source of lead in the condors' blood.
Church analyzed blood collected from 18 recaptured condors and 8 birds hatched in captivity and awaiting release. On average, birds that had been foraging in the wild had nearly ten times the amount of lead in their blood compared to captive birds.
But the lead in the freed birds' blood wasn't just higher, it also had a different chemical signature.
To track down the source of the lead, Church bought bullets and shotgun shells in three California counties within the condors' range.
She also sampled stillborn calves, mule deer, and a California sea lionsome common sources of food for the condorsto identify background levels of lead.
Church and colleagues then measured the ratios of different naturally occurring isotopes of lead in each sample.
The lead found in captive condors and released condors with low levels in their blood had isotope ratios similar to lead found in dead livestock and wildlife that had not been killed by hunters.
But the lead isotopes found in freed condors suffering from acute poisoning more closely matched that of ammunition.
"Ammunition isas we previously thoughtthe primary source of lead for California condors," Church said.
Church and her colleagues hope their findings will lead to new laws that prevent the use of lead ammunition within the condors' range.
The may already be having some success.
Steve Martarano, a spokesperson for the California Department of Fish and Game, expects renewed efforts to ban lead ammunition at the next meeting of the department's rule-setting commission.
"There is a very good chance there will be several proposals to the commission this round," Martarano said. The Fish and Game Commission will reconsider hunting regulations in February 2007.
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