When biologists first spied captive-bred California condors eating a dead sea lion that had washed up on the Big Sur coast, they were thrilled.
"They were foraging on their own, which was a big step in the right direction for recovery of this species," said Joe Burnett, senior wildlife biologist with Ventana Wildlife Society.
Burnett has spent many a night hiking into the backcountry of Big Sur, carrying carcasses to leave for these endangered scavengers in an attempt to mimic natural conditions. Following this initial feast in 1999, Big Sur condors began feeding on beached marine mammals more and more each year. By 2006, their most common food source was California sea lions.
"In the mid to late 1800s, condors were documented in Monterey Bay foraging on marine mammals," said Burnett. "So we suspected and hoped they'd develop the same pattern."
But what they didn't count on was that, along with nutrients and energy, condors would ingest harmful levels of marine contaminants. Now, preliminary reports suggest that these contaminants — including residues from the banned insecticide DDT — are affecting the already tenuous reproductive success of the flock.
"We had our first known nesting in 2006 in the burned out cavity of a redwood tree 60 feet above the ground," said Burnett. Though researchers initially observed characteristic nesting behavior, there was no sign that a baby had hatched. "We suspected the birds had aid an egg and something had Advertisement happened ... (so) a biologist from the U.S. Fish and Wildlife Service climbed up in the tree. He sifted through the substrate in the bottom of the nest and found egg shell fragments. This put it on our radar."
It's been well documented in scientific literature that DDE, the metabolic breakdown of DDT, which was banned by the FDA in December 1972, causes egg shell thinning in many birds species, including bald eagles, ospreys, peregrine falcons and brown pelicans. So when Burnett saw the shell fragments, his suspicions were immediately aroused.
"When you see egg shell thinning, all scientific data points to DDE. So that threw up a red flag," said Burnett.
The following year, biologists began to collect all the wild-laid eggs they could find and swap them out with captive-bred eggs from the Los Angeles Zoo. While unsuspecting wild condors raised chicks hatched from captive-bred eggs, the wild-laid eggs were closely monitored to ensure survival. Working in conjunction with USFWS, Ventana began to collect data on these wild-laid eggs, comparing egg shells and tissues from this coastal population of condors to those of condors from the Southern California flock. Unlike the Big Sur birds, members of the Southern California flock are released more than 40 miles inland near the foothills of the southern Sierra Nevada mountains, where they don't encounter marine mammals as a food source.
Although the study is ongoing, Burnett said that preliminary results suggest that eggshells from the Big Sur flock were substantially thinner than those found down south. Early indicators suggest DDE as the principle cause.
More:
http://www.mercurynews.com/breaking-news/ci_15524018?nclick_check=1
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