The pesticide known as DDT may have been banned in the United States decades ago, but it might still be hurting the California condor.
In a study pending publication in the journal The Condor, researchers argue that DDT is responsible for the thinning of condor eggs, which in turn is leading to the failure of several nests in the Big Sur region in California. The pesticide has been linked to thinning egg shells in other birds, such as bald eagles and peregrine falcons.
The Ventana Wildlife Society, the group that manages the condor reintroduction program in the Big Sur region, had suspected DDT was affecting condor eggs back in 2006 when a failed nest was found with eggs with shells so thin they didn’t resemble normal condor eggs. Between 2007 and 2009, 12 out of 16 condor nest sites failed. Thin egg shell fragments were recovered from the nesting sites. The success rate in Big Sur – 20 to 40 percent – is strikingly low compared to another population of California condor in the Tejon area, who have a success rate of 70 to 80 percent.
What’s different between the populations? Diet. Big Sur is coastal, so the condor’s diet consists of a lot of marine mammals, including sea lions. The animals that condors in this particular population eat live near the Palos Verdes Shelf Superfund site, an underwater region that was contaminated by an estimated 1,540 metric tons of DDT discharged by the Montrose Chemical Corp. A study conducted between 1994 and 2006 found DDT and related compounds in the blubber of California sea lions. Because the condors feed on sea lions, the DDT makes its way through the food chain.
“A vast majority of California sea lions have spent at least a portion of their lives in the waters of southern California, which is the most DDT-contaminated coastal environment in the world,” [Kelly] Sorenson [executive director of Ventana Wildlife Society and a co-author of the study] said. “Northward movements of sea lions provide a pathway of DDT to condors in central California.”
The authors think that, while the study isn’t conclusive, they have made a strong link between DDT and the thinning condor eggs. However, not everyone is convinced. Noel Snyder, a former condor program manager, thinks that the study only closely looks at one potential cause of egg shell thinning and doesn’t properly evaluate other factors. He says that, “DDT is not the only thing that causes eggshell thinning, and the authors of the paper don’t present a significant correlation of DDT with the thinning found, and thus do not do a convincing job of linking DDT with the happenings in Big Sur.”
Regardless of the actual effect DDT currently has on the condor population, it should decrease over time as the prevalence of the compound decreases naturally.
One thing everyone can agree on is that the major threat to California condor survival now isn’t DDT; it’s lead ammunition. California has recently voted to ban lead in hunting ammunition, and it awaits the governor’s signature.
Photo Credit: George Kathy Klinich
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