The study, which analyzed nearly three decades of almost-daily observational data, found that condors are eating less of the lead-free carcasses provided by conservation workers. Instead, they are foraging farther from their release sites into areas where the impact of lead-ammunition bans and education campaigns might not be as strong.

Another likely reason for the increase in lead poisoning is that more wild pigs are being shot in California due to their expanding numbers. The authors of the new study, published in the journal Nature Communications, say these dynamics are masking the continued effectiveness of the lead-bullet bans and outreach—a clear pattern that emerged after accounting for foraging and hunting changes.

"We feel this is critically important research that reveals the lead bans and outreach in California have been effective at reducing condor lead poisoning risk," said the study's senior author, Myra Finkelstein, an adjunct professor of microbiology and environmental toxicology at UC Santa Cruz. "Together, our results show that multiple factors have influenced condor blood-lead levels, including changes in wild behavior by condors and shooting behavior by humans."

Finkelstein and her team were the first to definitively link lead bullets to the poisoning that slowed the recovery of condors in California. Her pioneering research on lead poisoning in California condors helped bolster a 2019 state ban on the use of lead ammunition in hunting. A partial ban within the vulture's geographic home range was enacted 12 years earlier.