A team of scientists is proposing that endangered California condors raised in captivity be released near seal and sea lion rookeries so that the birds can once again feast on the carcasses of marine mammals as their ancestors did centuries ago.
The researchers from Stanford University and four other institutions base their conclusion on the feeding behavior of modern and early condors and their potential prey. Writing in the Nov. 7 online edition of the Proceedings of the National Academy of Sciences (PNAS), the research team concludes that whales and seals have been an integral part of the condor's diet since the last Ice Age, and that current efforts by the U.S. government to restore wild condor populations could be enhanced if captive-bred birds are released near marine mammal breeding grounds along the West Coast.
"Condors eat carrion; they don't kill for food," says C. Page Chamberlain, lead author of the PNAS study and chair of Stanford's Department of Geological and Environmental Sciences. "What we're proposing is a conservation strategy to train them to start eating dead seals and sea lions, whose populations are coming back along the California coast."
Standing nearly 5 feet tall and boasting a 9-1/2-foot wingspan, the California condor (Gymnogyps californianus) is the largest bird in North America. Thousands are believed to have inhabited the continent during the last Ice Age, their range extending from British Columbia to Baja California and east into Texas, Florida and New York.
Fewer than 300 are alive today, the majority recently bred and hatched in zoos as part of the U.S. Fish and Wildlife Service California Condor Recovery Program—a federally supported effort to establish self-sustaining condor populations in the wild. Since 1992, program affiliates have released a total of about 130 captive-bred birds in wild areas of California, northern Mexico and near the Grand Canyon in Arizona.
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, California, 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 the University of California-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 birds that died in the wild between 1993 and 2001; 60 historical birds that lived from 1797 to 1965; and 10 ancient condors that died in the Rancho La Brea tar pits in Los Angeles during the Pleistocene epoch between 11,000 and 36,000 years ago.
Bone or hair samples were also collected from potential prey, including Pleistocene bison and horses, as well as dozens of 20th-century mule deer, feral pigs, whales, seals, sea lions and domesticated cattle. "The California Department of Transportation even agreed to go around and snip off hair from dead deer killed on the highway," Chamberlain says. "Other people went around picking up samples from dead seals on beaches. We ended up getting an incredibly large database on hundreds of animals."
The isotopic data revealed that condors are likely to have undergone two major dietary shifts since the Pleistocene. "The first occurred from the end of the Ice Age about 11,000 years ago to historical times, and the second took place between the 18th century and the modern era," Chamberlain says, adding that both events coincide with the arrival of humans in North America.
"During the Pleistocene, condors were distributed across the continent," he explains. "There was plenty of megafauna to feed on, including mammoths, mastodons, camels and bison."
Isotopic analysis of ancient condor bones from the La Brea tar pits revealed that about 40 percent of Pleistocene birds fed primarily on marine species, the rest on a mixture of marine mammals and giant land mammals.
"When the Ice Age ended, most of the terrestrial megafauna vanished," Chamberlain says. "There's a theory that they were killed off by humans, and as a result, the condors that lived inland no longer had large prey to feed on."
The disappearance of terrestrial megafauna caused the first major dietary shift, according to the authors. "Our paper shows that condors on the West Coast survived the Pleistocene extinction by switching to marine mammals—large carcasses of whales, seals and sea lions that washed up on the beach. But the condors in Florida, Texas and New York died out because there was no fallback food source for them."
Condors remained plentiful along the West Coast for thousands of years until Europeans began settling the region in the late 1700s. "In 1806, Lewis and Clark found large condors feeding on whale carcasses near the mouth of the Columbia River in Oregon," Chamberlain notes. "But huge sealing and whaling industries started around that time, and the large seal rookeries and whale carcasses quickly disappeared. So the condors were once again stuck without a food source. Our isotopes show that the disappearance of the marine mammals coincided with the arrival of the large Spanish missions with their cattle, which they raised largely for tallow and leather. Many thousands of cattle were brought here, so the condors switched their whole diet to cattle."
The large cattle ranches disappeared within a few decades, once again leaving the condors without a reliable food source. "After the cattle were gone, condors starting getting rarer and hence more valuable as collector's items," Chamberlain says. "So people started shooting them for their feathers and stealing the eggs. That pushed them right to the brink of extinction."
By the 1980s, only two dozen condors remained in the wild, prompting the U.S. Fish and Wildlife Service to take the controversial step of rounding up the entire population and placing them in a captive-breeding program for future release.
"The decline of the condor was all man-induced," Chamberlain notes. "The megafauna are wiped out by early Ice Age humans, so the condors have to feed on marine carcasses along the coast. Then people come over again and killed seals, whales and sea lions, so the condors had to feed on cattle. Then we change our ranching practices so cattle disappear, and all the birds have left are lead-contaminated deer carcasses and the calves we supplement them with."
Chamberlain sees three options for the Condor Recovery Program: "They could feed them 100 percent on calves; then they're less likely to get lead poisoning. They could eliminate lead from the ammunition, which would require legislation or a major change in hunting practices. Third, and this is what we're suggesting, you could train them on another food source that reduces their chances of lead contamination, and that would be the marine mammals."
At least one group of condors appears to be heeding that advice on its own, says Kelly Sorenson, executive director of the Ventana Wildlife Society (VWS) and co-author of the PNAS study. Since 1998, VWS has released more than 30 captive-bred condors in Big Sur, a remote, mountainous region overlooking California's central coast. "We believe that marine mammals—primarily sea lion carcasses that wash up on the beach—now represent about 50 percent of the Big Sur population's diet," Sorenson says. "That's really exciting, because it means a much lower lead exposure for the condors."
He says that plans are under way to build a new holding pen along the coast south of Big Sur where seals frequently haul out. "Free-flying condors are just not finding these areas yet, so we need to encourage them to do so," he adds. Other major haul-outs can be found from Baja California to British Columbia.
"If we can train condors to find sites along the beach where there are seals and sea lions all the time, then they'll frequent those spots and it will get them off some of the terrestrial sources with lead poisoning," Chamberlain concludes. "It would be fabulous some day to see condors feeding all along the coast, from Mexico to Canada, the way they did when the Europeans arrived here three centuries ago."
Other co-authors of the PNAS study are former Stanford researcher Jacob Waldbauer; Kena Fox-Dobbs, Seth Newsome, Paul Koch, Donald Smith and Molly Church of the University of California-Santa Cruz; Sam Chamberlain of Reed College; and R. W. Risebrough of the Bodega Bay Institute. Research was supported by the David and Lucile Packard Foundation, the National Science Foundation and the U.S. Fish and Wildlife Service California Condor Recovery Program.
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