Newly released condors are fed stillborn calves donated by local cattle ranchers. Eventually, the birds supplement their diet with carcasses of deer, boar and other terrestrial wildlife they happen to find. Unfortunately, many of these animals are killed by hunters using lead shot, which is highly toxic to scavengers. "Condors are obligate scavengers, so they'll feed on anything that's there," Chamberlain says. "If they consume just a small lead fragment, it can kill them."
For the PNAS study, the researchers focused on the dietary habits of condors, past and present, to determine if their food sources had changed over the centuries.
"We wanted to find out what they fed on in historical times and what they feed on now," Chamberlain says, noting that an animal's diet can be determined by measuring the ratio of carbon and nitrogen isotopes that accumulate in its body. For example, seals and sea lions are more enriched in heavy nitrogen-15 and carbon-13 than terrestrial wildlife, such as deer.
Feathers and bones
In 2002, Chamberlain and his colleagues embarked on a worldwide quest for feather and bone samples that would reveal the body chemistry of condors through the ages. "This work was fun," Chamberlain recalls. "First we got modern feathers from the Ventana Wildlife Society in Big Sur, Calif., and other groups involved in the federal Condor Recovery Program. Then I started writing letters to all the museums that had condors, and they were incredibly cooperative."
Among those providing samples were the Smithsonian Institution, the American Museum of Natural History, the California Academy of Sciences, the Natural History Museum of London and the Museums of Paleontology and Vertebrate Zoology at Berkeley.
Using a special laboratory in Stanford's School of Earth Sciences, the research team conducted an isotopic analysis of bones and feathers from 92 individual condors--22 modern