Lead poisoning in condors has reached "epidemic proportions," said Myra Finkelstein, a research toxicologist at the University of California, Santa Cruz.
The condor's feeding habits contribute to the problems, researchers said, estimating an 85 to 95 percent likelihood that a condor will feed on a lead-contaminated carcass during its lifetime.
"We will never have a self-sustaining wild condor population if we don't solve this problem," Finkelstein told the San Francisco Chronicle.
Condors, with their 10-foot wingspans, have been on the federal endangered species list since 1967, hunted almost to extinction by hunters and trappers as the western United States were settled.
Only 22 condors remained 30 years ago, but recovery efforts by conservationists have seen the condor population return to more than 400.
However, nearly half the California free-flying condors have suffered from chronic lead poisoning in the past 10 years, many of them poisoned repeatedly, researchers said.
A population study of California's remaining wild condors found that without continued treatment for lead poisoning, the population will never increase and probably even decline, they said.