At 5 feet tall, with a wingspan of more than 9 feet, California condors are the King Kongs of North American birds. But the enormous, high-flying gliders can be felled if they eat the amount of lead in a single shotgun pellet.
The scavenging birds ingest lead when they feed on the carcasses of deer and boar that have been killed by hunters using lead shot. “All it takes is a small fragment to push [the accumulated lead] to a critical level,” says Page Chamberlain, chair of the department of geological and environmental sciences. “Lead poisoning is what’s killing them off.”
Condors have been on the endangered-species list since 1967. In the past 20 years, careful breeding has increased their numbers from nine to almost 300. Since 1992, some 130 captive-bred condors have been released into the wild in California, Arizona and northern Mexico. The trick is making sure they don’t fall prey, yet again, to human interference with their habitat.
First priority: making sure the condors have something to eat. Chamberlain and scientists at four other institutions suggest the birds’ diet could include washed-up carcasses of seals and sea lions—as it did long ago.
The researchers examined the dietary habits of condors dating back to the Pleistocene era, between 11,000 and 36,000 years ago. Isotopic analysis of Pleistocene condor bones from the La Brea tar pits in Los Angeles revealed that the birds fed on a mixture of giant land mammals—bison, sloths and mastodons—as well as marine species. By the end of the Ice Age, the terrestrial megafauna had disappeared and condors survived by eating dead whales, seals and sea lions. Only when Russian fur traders depleted those populations in the 18th century did condors turn to eating cattle carcasses at the Spanish missions that dotted the state.
In a paper published in the November 15 online edition of the Proceedings of the National Academy of Sciences, the researchers recommended that captive-bred condors be released near marine-mammal breeding grounds along the West Coast.
That’s already happening in California’s Big Sur area, where the Ventana Wildlife Society has reared and released more than 30 condors. VWS officials believe the big birds are getting half of their nutrition from sea lion carcasses that wash up on the beaches. This means they’re ingesting less lead.
An isotope geochemist who likes to work at the intersection of geology and biology, Chamberlain traces dietary history in condors by measuring the ratio of carbon to nitrogen isotopes in their feathers. VWS and Forest Service staff “periodically recapture birds and either remove a feather or take small clippings just before molt,” he says. “We can do so much with one single feather.”
“Because the birds in Big Sur have been outfitted with GPS recorders on their wings, we can go back into the computer history of how these birds are moving,” Chamberlain says. “We can say, ‘Well, we have that feather from that period of time, and it was eating that, and that’s how much lead there was.’ ”
The next phase of study will be to look at concentrations of toxins in seals, to see how clean they are as a food source. Although they don’t ingest lead, some seals on islands off the coast of Los Angeles have accumulated DDT in their tissues. “Man plays into every bit of condor history,” Chamberlain says.
