To find out, the Ventana Wildlife Society, which manages the Central California condor releases, has collected as many subsequent wild-laid eggs as possible. The handful of Big Sur breeding pairs lay a single egg once every other year. Ventana biologists brave the region’s trackless terrain to exchange a wild-laid egg with one from the zoo-based captive-breeding program. The unsuspecting condor pair then hatches the substitute egg as if it is their own.

In addition, Ventana biologists began to look for possible sources of DDT. Condors are carrion eaters, and in recent years the Big Sur birds have turned to what was historically a major food source: marine mammals. Mr. Burnett now suspects that animals like California sea lions may present a hidden danger to condors. Even today, sea lion blubber contains high levels of DDE, a toxic metabolic breakdown product of DDT.

Ventana biologists have been comparing the thickness of the eggshells recovered from the Big Sur birds with those produced by the Southern California condor flock that lives many miles from the coast. The Southern California birds do not feed on marine mammals, and their eggs are normal. Mr. Burnett says that preliminary results from Ventana’s study suggest that the Big Sur eggs are “substantially thinner” than those from the inland birds, and that early indicators point to DDT as the principal cause of the thinning.

Although no known source of DDT exists near Big Sur, a large DDT hot spot in the marine sediments off the Southern California coast called the Palos Verdes Shelf has attracted Mr. Burnett’s attention because it is near a breeding ground for California sea lions that eat the area’s fish. The sea lions then migrate up the coast. Hundreds of these sea lions use a rocky beach near Big Sur as a stopping point on the trip north. In recent years, this sea lion “haul-out” has become a favorite feeding spot for the Big Sur condors.

The DDT that pollutes the Palos Verdes Shelf originated half a century ago with the Montrose Chemical Corporation. At the time, Montrose was the world’s largest producer of what was once hailed as a “miracle pesticide.” According to Carmen White, the Environmental Protection Agency’s remedial project manager for the site, in the 1950s and ’60s Montrose discharged its untreated DDT waste directly into the Los Angeles County Sanitation District’s sewer system. An estimated 1,700 tons of DDT settled onto the seabed, where it continues to contaminate Pacific Coast waters. The E.P.A. has declared the area a Superfund site, and Ms. White is coordinating a plan to cover the most contaminated parts with a cap of sand and silt in 2012.