It may not be the most charismatic species, but the endangered California condor, a bird with an incredible wingspan of nearly 10 feet, has received a lot of attention over the years. After dropping to a population of just 22 individuals in the early 1980s, captive breeding programs have boosted their numbers to around 400, with some 200 living in the wild. Even that small population is extremely high-maintenance, however. All the birds are tracked by radio or GPS tags, and are frequently caught for medical examination and treatment.

As is the case with most birds that eat carrion, lead poisoning has long been a concern for the condors. When large animals are killed by a hunter’s lead bullet but not harvested, they can become a dangerous meal. When the birds eat the meat, they can ingest lead particles along with it.

When the condors get their medical checkup, a blood sample is analyzed for lead. If the levels are dangerously high, the birds are given treatment. The impact of lead poisoning on the condor population has long been debated, but a study published this week in the Proceedings of the National Academy of Sciences provides some clarity. It shows that many condors are suffering the effects of lead poisoning, suggesting the population will continue to struggle as long as lead ammunition remains in use.

Researchers compiled the results of over 1,100 blood samples taken from 150 California condors between 1997 and 2010. The US Centers for Disease Control and Prevention recommends that children with blood lead levels above 450 ng/ml undergo treatment, and this is roughly the level at which the condors are taken in for treatment as well. About 20 percent of the birds sampled each year exceeded this level, and 48 percent of individuals living in the wild exceeded it at some point during the 14-year period. Blood lead levels as high as 6,100 ng/ml were seen.

Because blood samples only provide a snapshot of lead levels, the group also analyzed feathers from 18 condors. The concentration of lead in each segment of the feather depends on the concentration in the body at the time that segment formed. That means that each feather records a few months of exposure history. From comparisons of the most recent segment to blood levels, they were able to estimate blood lead levels over the duration of the feather.

The feathers showed that after ingesting lead, the birds’ blood concentrations exceeded 450 ng/ml for about a month. Unsurprisingly, blood samples often miss the peak lead concentrations, which were 1.4x to 14.4x higher. About 34 percent of the average feather history was higher than the level at which the condors are treated.

Lead poisoning has consequences at concentrations lower than those that are lethal, but they are difficult to determine in wild populations. To get a handle on how sensitive California condors were to lead poisoning, the researchers measured a biomarker (δ-aminolevulinic acid dehydratase, or ALAD, enzyme activity) in 60 blood samples from 34 birds. That enzyme is important in a number of biochemical pathways, and its activity is strongly inhibited by lead, making it a great indicator of the effect of lead in the body. At blood lead levels of 450 ng/ml, the activity of ALAD was suppressed by 90 percent. Even at 200 ng/ml (a level exceeded by about 30 percent of blood samples each year, and for more than half of the duration of the feather records), activity was down 60 percent.

But how can we be sure the lead in those condors came from ammunition in carcasses and not some natural source? To see, the researchers measured the lead-207/lead-206 isotopic signature in 132 blood samples. Of those samples, 79 percent were consistent with lead ammunition and 27 percent were within the range of “background” ratios in captive birds (there’s some overlap). Several birds had isotopic signatures similar to lead-based paint, and had been observed roosting in an old fire tower with peeling lead paint.

If lead ammunition is a real problem for California condors, where does that leave the effort to restore the population? To answer that question, the group used population models and several scenarios. If present conditions continue, with the same lead exposure and active care of the birds, the wild population would just barely grow. The authors write, “without future releases of captive-reared birds, the population would take ∼1,800 [years] to meet the recovery goal of a noncaptive population of 150 individuals within California.”

If, instead, we gave up on the expensive work of capturing birds and treating those with high blood lead levels, the population would decline back to 22 in one to six decades. Finally, if lead exposure was eliminated, the wild population would grow at a rate of about 2 percent per year. And that’s a conservative estimate, the researchers say, because their “estimated rate of lead-caused mortality is based on the actual deaths that occurred despite intensive management interventions to mitigate lead poisonings; if lead was truly removed as an environmental hazard, the increase in condor health and survival should be substantially greater than modeled here.”

That is likely the outcome that California authorities were hoping for when, in 2008, they instituted a ban on lead ammunition for hunting many species within the condor’s range in southern California. But when the researchers compared blood lead levels in condors before the ban (2006-2007) and after (2009-2010), they found no improvement.

The researchers are currently evaluating the ineffectiveness of the ban so far, including a look at whether hunters are fully complying with the new rule. Myra Finkelstein, a University of California-Santa Cruz researcher involved in the project, told Ars that “even if only a few people are still using lead ammunition, there will be enough contaminated carcasses to cause lead poisoning in a significant number of condors. We found that over the course of ten years, if just one half of one percent of carcasses have lead in them, the probability that each free-flying condor will encounter a contaminated carcass is 85 to 98 percent, and one exposure event could kill a condor.”

On the national stage, 100 conservation groups recently filed a lawsuit against the US Environmental Protection Agency for denying petitions asking it to regulate lead ammunition used for hunting. The agency says it has no authority to do so, a fact the groups are challenging in court.

PNAS, 2012. DOI: 10.1073/pnas.1203141109 (About DOIs).