Turns out being independent-minded is a double-edged sword for the critically endangered California condor. According to a study out of UC Davis published this month, condors that find their own food run a significantly higher risk of lead poisoning than those who prefer carrion provided to them by condor recovery teams.
Brought back from the brink of extinction by a captive breeding program that started in the late 1980s, California condors are slowly gaining in number due to releases of young condors produced by that breeding program. Biologists provide animal carcasses at release sites in order to give young, inexperienced condors a little bit of help with that scavenger learning curve. Ideally, adult condors will then graduate from relying on provided meals to finding their own dead animals out in the world.
But there's a problem. The most readily available sources of condor food aside from release site carcasses are carrion left by hunters, either whole or as body parts or gut piles. And despite a ban on most lead ammunition in condor country that's been in place since 2008, the study found that condors who found more of their own food in the years immediately following the ban didn't actually show lower levels of the heavy metal in their bloodstreams.
Lead poisoning, which is one of the biggest threats to free-roaming condors, results when the birds scavenge the remains of animals killed by lead shot. Condors ingest the lead, often in tiny shards created when the shot fragments on impact. Stomach acids dissolve the lead, which moves into the condors' bloodstreams rather effectively.
In a study published this month in the journal Conservation Biology, researchers from the UC Davis Veterinary School worked with the U.S. Fish and Wildlife Service, the California Department of Fish and Wildlife, the National Park Service, the Ventana Wildlife Society, and the U.S. Geological Survey to compare lead levels in blood samples drawn from California condors over a 15-year period.
Here's a brief video describing the work:
The sampling period studied lasted from 1997 through 2011. In each year, between 62 and 91 percent of condors sampled showed elevated levels of lead in their blood. Older condors had more lead exposure, suggesting that birds experienced enough to wander away from the release site handouts ended up ingesting more lead shot.
A broader statewide ban on lead ammunition was signed into law in 2013, but won't really begin to affect the amount of lead left in carcasses by hunters until 2019. And that ban affects only California: condor habitat in Arizona, Utah, and Baja California won't be affected. Which means that condors will be contending with lead in their food for some time to come across their expanding range, and that means that the fate of the species still depends on constant replenishment from the captive breeding program.
"Until we can ensure natural food sources are free from lead ammunition for the population, lead poisoning will threaten recovery of naturally sustaining populations of condors in the wild," said UC Davis' Terra Kelly, the study's lead author.