A new study headed by the University of California at Santa Cruz with contributions by the University of Colorado shows that the California condor is facing chronic endangerment due to lead poisoning resulting from exposure to used firearms ammunition.

According the study, the California condor, the largest land bird in North America, requires continuing human intervention if it is to achieve population stability and growth.

While the condor population has grown since facing near-extinction in 1982, increasing from 22 to roughly 400, this has only been accomplished through intensive management procedures. These include captive breading, veterinary care and monitoring.

But according to the study, one of the main threats to the condor's population recovery -- the birds' continued poisoning from lead-based bullet fragments -- has gone largely unmitigated.

"We will never have a self-sustaining wild condor population if we don't solve this problem," lead author Myra Finkelstein, a research toxicologist at UC Santa Cruz, said in a news release. "Currently, California condors are tagged and monitored, trapped twice a year for blood tests and, when necessary, treated for lead poisoning in veterinary hospitals. And they still die from lead poisoning on a regular basis."

The California condor, which once numbered in the thousands in the western United States and Mexico, is a type of vulture that regularly feeds on the carcasses of animals. At times these animals contain lead-based bullet fragments due to being killed by hunters. The condors ingest these fragments, and become ill.

According to the study, 20 percent of the California condors sampled annually from 1997 to 2010 suffered lead poisoning. The birds had to be treated with chelation therapy, a metal detoxification process. Over the 13 year period, almost half of the condor population tested was poisoned. Many of the birds suffered repeated poisonings within the same year, as well as over multiple years.

"Lead exposure and poisoning levels in condors continue to be epidemic," co-author Dan Doak, a new endowed chair and professor in CU-Boulder's Environmental Studies Program, said in the news release. "Despite the current efforts to help the species, the wild population will decline again toward extinction in a few decades unless these unsustainable and very expensive efforts continue in perpetuity."

While a partial ban on the use of lead-based ammunition in condor habitats was enacted in California in 2008, researchers found no evidence that this measure curbed the prevalence of lead exposure and poisoning in the birds. While substitutes to lead-based ammunition exist, researchers noted that regulations controlling the use of lead-based ammunition face firm resistance from gun-rights groups and hunting organizations.

According to the researchers, the anticipated cost of a multi-agency recovery program for the condor population is about $5 million per year.

"Integrating a study of the effects of toxic chemicals on individual wild animals into a modeling of whole-population impacts is very unusual," Doak said. "But the results provided a critical piece of our results in this study."

The study, published this week in Proceedings of the National Academy of Sciences, was supported by the National Park Service, U.S. Fish and Wildlife Service and the Western National Park Association.