Zoo-funded study cracks mystery of broken condor eggs

September 20, 2013 - 10:20am

Researchers attribute failed eggshells to decades-old toxic dumpsite

What do California condors have in common with great white sharks and killer whales? They like to dine on marine mammals. Unfortunately for the condors, being in the gastronomic company of such fearsome predators has left them particularly vulnerable to toxins.

A new study by the Ventana Wildlife Society, supported in part by the Oregon Zoo Foundation, has revealed that some condors feeding on contaminated sea lion carcasses are laying eggs that don't hatch.

Researchers linked the reproductive problems to DDT, a banned carcinogenic pesticide known to thin and weaken bird eggshells. Birds that have ingested DDT have been known to crush their own eggs when they attempt to incubate them. The chemical can also allow bacteria to penetrate the shell walls.

The source of the DDT was traced to a 40-year-old underwater Superfund site in Southern California.

"The takeaway message of this study is that any environmental catastrophe, like the recent Gulf oil spill, can have longer-lasting impacts than people think."

—Joe Burnett, Ventana Wildlife Society

"This was where the world's largest DDT factory discharged its waste and produced one of the world's most toxic dump sites," said Joe Burnett, co-author of the study. "Most California sea lions spend part of their lives breeding and feeding near that site and when they disperse north, they can pass those accumulated toxins directly onto scavenging condors."

For the study, researchers located and measured the thickness of condor eggshells in the coastal redwood forests of Central California. At least half of the failed eggs they found were attributed to DDT.

Condors are the latest in a long list of species affected by DDT contamination over the past decades. Studies in the 1970s and '80s found high levels of the toxin in Southern California mussels, anchovies, pelicans and eagles. Because coastal condors feed at the highest level of the food chain — on marine mammals — they are susceptible to magnified levels of the toxin. DDT has not been found in inland condors, which do not consume marine mammals.

The good news, according to the study's authors, is that levels of DDT found in the food web have been in steady decline since the factory stopped discharging waste in 1971. Condor eggshells are expected to return to normal thickness, although the authors can't predict when.

Comparison of a failed condor eggshell from central California (left) with a normal eggshell from southern California (right). Photo: Ventana Wildlife Society.

"Most people know something about the effect of DDT on birds based on Rachel Carson's book Silent Spring," Burnett said. "The takeaway message of this study is that any environmental catastrophe, like the recent Gulf oil spill, can have longer-lasting impacts than people think."

In 2014, the zoo will support two studies investigating toxins in marine mammals in Oregon and Washington. With a grant from the Oregon Zoo Foundation's Future for Wildlife program, the conservation group Coastal Raptors will take blood samples from vultures, ravens and eagles feeding on marine mammal carcasses along the Oregon and Washington coasts.

Pacific University and Portland State University will use their grant to directly examine stranded sea lions and harbor seals for toxins.

"Marine mammals can act as vectors for a variety of contaminants," said Oregon Zoo deputy conservation manager David Shepherdson. "The results of these studies will provide important data for ocean health and the Oregon Zoo condor program."

The California condor was one of the original animals included on the 1973 Endangered Species Act and is classified as critically endangered. In 1982, only 22 individuals remained in the wild and by 1987, the last condors were taken into captivity in an attempt to save the species. Biologists decided to place the remaining condors in a captive-breeding program. With the help of breeding programs like the Oregon Zoo's, condor numbers now total around 400, counting those in breeding programs and in the wild.

More than 40 condor chicks have hatched at the zoo's Jonsson Center for Wildlife Conservation since the program began in 2003, and more than 20 Oregon Zoo-reared birds have gone out to field pens, with most released to the wild. In addition, several eggs laid by Oregon Zoo condors have been placed in wild nests to hatch.

California condor breeding programs are also operated at San Diego Zoo's Wild Animal Park, the Los Angeles Zoo and the Peregrine Fund's World Center for Birds of Prey in Idaho. The Oregon Zoo received The Wildlife Society's conservation award in 2005 for "creating the nation's fourth California condor breeding facility."