Just one week after the California legislature voted to ban lead ammunition to protect California condors from the toxic substance, which they can consume via carcasses shot by hunters, new research shows that the critically endangered birds are also still at risk from another long-banned toxic substance: the pesticide DDT.

The U.S. banned the use of DDT (dichlorodiphenyltrichloroethane) back in 1972 after studies linked it to the thinning of eggs in bald eagles, peregrine falcons and other species. The pesticide has also been linked to other health hazards in wildlife and humans. But even though it is no longer employed in this country, DDT persists for a long time in the environment, and its effects are still being felt today.

The Ventana Wildlife Society (VWS), which manages the California condor (Gymnogyps californianus) reintroduction program in the coastal Big Sur region, first began to suspect in 2006 that DDT was affecting the big birds. Two captive-born condors successfully nested in the wild then for the first time in that region. The birds mated and laid eggs, but they soon cracked and the nest failed. An examination revealed that the shells were so thin that they didn't even resemble normal condor eggs.

Since that time many more eggs have been laid in the region but 12 out of 16 condor nest sites failed between 2007 and 2009. Fragments of shells—all visibly thin—were recovered from those sites. Meanwhile, the condors released 650 kilometers farther south have enjoyed a 70 to 80 percent hatching success rate.

Now, research pending publication in the journal The Condor reveals that the egg fragments recovered in the Big Sur region were 34 percent thinner than eggs laid at the same time in the southern reintroduction zone. Many of the latter shells lacked a normal external crystalline layer. The researchers link the thinness and malformations to DDT and the compound DDE (dichlorodiphenyldichloroethylene), which is formed when the pesticide breaks down.

How did the condors end up with DDT and DDE in their systems? The birds in Big Sur have been observed dining on the carcasses of sea lions, sea otters and other marine mammals, animals the inland southern population lacks the opportunity to eat. Previous research into California sea lions (Zalophus californianus) from 1994 to 2006 found high levels of DDT and related compounds in their blubber, especially in the males. The marine mammals live near the 54-hectare Palos Verdes Shelf Superfund site, an underwater region contaminated by an estimated 1,540 metric tons of DDT discharged by the Montrose Chemical Corp. DDT manufacturing plant between the 1950s and 1970s. Earlier this year new tests estimated that the DDT at the site had somehow shrunk to just 12.7 metric tons; it is not yet known what happened to all of those missing chemicals.

The new study, available on the VWS's site, does not conclusively prove that DDT or DDE caused the condors' eggs to thin, but it does say the data is good enough to support their conclusion that it was the likely cause. The study also says the condors will likely do better in the future as DDT and DDE levels continue to decline naturally.

Meanwhile, the society lent its support to the push to ban lead ammunition, which now awaits the governor's signature. In a press release VWS's executive director, Kelly Sorenson, said, "lead trumps everything in condor recovery."

Photo: California condor photographed in Big Sur by Gregory Smith, via Flickr. Used under Creative Commons license

Previously in Extinction Countdown: