Seven endangered California condors — about 20 percent of Southern California's population — have been found with lead poisoning.
The birds started turning up sick about a month ago during random trappings at Bitter Creek National Wildlife Refuge in the San Joaquin Valley.
One of the birds died during treatment at the Los Angeles Zoo and six others, among them a chick and its mother, are still being treated there.
Three have recovered and are ready to be picked up by wildlife officials for release back to the wild, Curtis Eng, director of health services at the zoo, told msnbc.com. That process could take a week or two, he said.
Three others are still being treated with injections of a calcium compound that binds the lead and removes it from the body, Eng said. "They're still showing elevated lead levels," he added. "I would not say they're out of the woods."
The zoo has treated condors in the past, including a few last summer, but never this many at once. "It's definitely the most animals that we have seen at one time," Eng said. "That's what's most disconcerting to us."
Officials don't yet know the source of the contamination, but a U.S. Fish and Wildlife official said the birds were likely poisoned by eating the carcasses of animals that had been shot by hunters.
Lead poisoning is a known threat to the majestic birds and the main reason the state is about to ban hunting with lead bullets.
Jesse Grantham, a U.S. Fish and Wildlife condor coordinator, called the poisonings alarming and said the agency was in "crisis mode."
The California condor nearly went extinct in the 1980s, but a trapping and breeding program has helped restore the species. There are only about three dozen of the endangered birds in Southern California, and about 200 in the wild overall.
Experts believe lead poisoning is a major factor in preventing the species' recovery.
Under a ban that takes effect July 1, it will be illegal for California hunters to possess or fire lead ammunition when they are in the birds' habitat.
Arizona has also seen the problem with its condor population.