Rare condors harmed by DDT in food chain, study discovers
SAN FRANCISCO — In the coastal redwood forests of central California, scientists trying to unravel the mystery surrounding the reproductive problems of dozens of endangered condors think they have uncovered the culprit: the long-banned pesticide DDT.
The soaring scavengers with wingspans wider than NBA players are tall were reintroduced to the rugged coast of Big Sur in 1997 after a century-long absence. Upon arrival, the birds found plenty to eat, with dead California sea lions and other marine mammals littering the craggy shoreline.
While a good food source, sea lion blubber often has high levels of DDT, a pesticide banned in 1972 that has proved to be a persistent pollutant because it accumulates in bodies of creatures throughout the food chain when animals eat one another.
Once used widely in agriculture, DDT was banned because it is a human toxin considered a possible carcinogen by the Centers for Disease Control and Prevention.
Kelly Sorenson, executive director of Ventana Wildlife Society and a co-author of a new study on condors, said researchers who spent six years studying their reproductive problems have “established a strong link” to DDT in the birds' food source.
The peer-reviewed paper written by 10 condor experts, including biologists from the Los Angeles and Santa Barbara zoos and the Fish and Wildlife Service, is being published this month in the University of California journal The Condor.
“In science, rarely is anything definitive, but we've established a strong link between ... DDT and eggshell thinning in California condors,” Sorenson told The Associated Press.
The eggshell mystery began in 2006, when a biologist inspecting a condor nest in the cavity of a redwood tree on the central California coast found the first thin shell.
Over the next six years, the scientists observed condors feeding on dozens of sea lions, and found that the Big Sur condor population had a low hatching success — just 20-40 percent — for 16 nesting pairs. In contrast, 70 to 80 percent of southern California condors in the Tejon area had hatched successfully over the same time. The southern California condors are inland, and sea lions are not a food source.
Biologists familiar with the ravages of DDT in bird populations immediately suspected the widely used pesticide as a factor.
Tests since the 1970s have found high levels of DDT in sea lions; and studies have linked DDT's metabolized version, DDE, to egg shell thinning in birds, including brown pelicans and bald eagles.