In 1995, as members of the California Condor Recovery Team approved a proposal to release condors in California’s Big Sur area, they noted two things:
First, the central-coast birds would probably feed on carcasses of marine mammals washed up on shore. After all, the first record of the species, in 1602, was of condors seen near a dead whale in Monterey Bay. Second, the condors might in turn accumulate organochlorine contaminants that could harm their ability to reproduce.
Decades earlier, in the 1960s, the southern California coastline was one of the most contaminated in the world. Derivatives of the insecticide DDT, principally DDE, had been discovered in northern anchovies from Los Angeles Harbor and in Brown Pelican eggs on Anacapa Island, between Santa Monica and the Channel Islands. And sky-high concentrations of several DDT compounds had turned up in sea lions, a likely condor food item, in both southern and central California.
The chemicals have been shown to cause thinning and structural abnormalities in the eggs of many fish-eating birds and raptors, including not only Brown Pelican but Double-crested Cormorant, Bald Eagle, Ashy Storm-Petrel, and Peregrine Falcon.
Now researchers say it’s all but certain that the condors of the central coast have been affected, too. The birds were first released in Big Sur in 1997, and have been observed feeding on sea lion carcasses since 1999.
Between 2006 and 2010, the investigators monitored 16 nestings and compared eggs and shell fragments recovered from 12 nests with shell pieces from nests in interior southern California, where condors eat no sea lions. Not only were the central California fragments 34 percent thinner, but the hatching success in central California (only 20-40%) was significantly lower than that recorded in southern California (70-80%).
Because the contamination of the southern California marine environment has been decreasing since 1971, the researchers predict that eggshell thickness and productivity will increase to normal levels “but at a rate that cannot be estimated with any certainty at this time.”
The researchers published their findings in a recent issue of The Condor, the quarterly journal of the Cooper Ornithological Society.
Read the article
L. Joseph Burnett, Kelly J. Sorenson, Joseph Brandt, Estelle A. Sandhaus, Deborah Ciani, Michael Clark, Chandra David, Jenny Theule, Susie Kasielke, and Robert W. Risebrough, 2013, Eggshell Thinning and Depressed Hatching Success of California Condors Reintroduced to Central California, The Condor 115 (3): 477-91.
A version of this article appeared in the December 2013 issue of BirdWatching magazine.
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