The Surprising Way Marine Mammals Are Poisoning California Condors

More than 40 years after being banned, DDT has reared its ugly head again.

Somewhere along the beautiful coast of Big Sur, a California Condor sinks its beak into the decaying flesh of a dead sea lion. It’s a gruesome sight, but it's not unusual—even 10,000 years ago condors were feasting on the carcasses of washed-up marine mammals. The difference between then and now is that today's deceased pinnipeds come chock-full of harmful toxins, the rotting legacy of decades of poor environmental regulations in the United States.

It's theorized that beached mammals might have actually kept condors from going extinct at the end of the Pleistocene, when they were steadily deprived of the corpses of prehistoric land animals such as mammoths, sabercats, and giant sloths. But new research published last month in Environmental Science & Technology suggests that those marine scraps could now be putting the endangered vultures at greater risk by exposing them to an old, familiar foe for conservationists: DDT.

Because sea lions are at the top of the marine foodchain, they’re often tainted with toxins that are found in fish, cephalopods, and other aquatic prey. Among those contaminants is the pesticide DDT,  which was dumped in large quantities off of the Los Angeles coast between the 1940s and the 1970s. Scientists think that all California sea lions may be vectors of the poison, given that they swim these waters as they head to the Channel Islands each year to breed.

To see if condors were ingesting DDT and other contaminants associated with marine mammals (like fire retardants), UC San Diego ecologist Carolyn M. Kurle and her team analyzed and compared the blood collected from coastal and inland condors during routine health work-ups. They also checked the blubber of dead pinnipeds and whales in Monterey County, California.

As expected, the coastal birds posted significantly higher levels of contaminants than the inland individuals. Another recent investigation revealed that the coastal condors are laying eggs with thinner shells than their inland counterparts, causing them to be 20 to 40 percent less successful reproducing. 

This development is just another setback in the efforts to restore the large, charismatic bird's populations. In 1987, there were only 22 California Condors left alive in the world. But by the mid-1990s, scientists were releasing their captive-reared descendants back into the wild. Today there are more than 400 of the birds in captivity and the wild. The species’ range now extends throughout California, as well as Arizona, Utah, and Baja California, Mexico.

Despite this success, there are still many roadblocks to the condor’s comeback—the main one being lead-based ammunition, which the birds consume while scavenging on bullet-ridden carcasses. Lead wreaks havoc on the vultures' digestive system, causing the animal to slowly starve to death. In the past, condors living near Big Sur were thought to be at lower risk of lead poisoning because they have the advantage of being able to feed on marine mammals. Indeed, a study published earlier this year confirmed that individuals found on the coast tend to live longer than those that stick to inland habitats. 

While lead ammunition can at least be eliminated from the ecosystem (California, for example, is slowly working up to a ban), there's not much to be done about the pollutants that persist in the waters long after they've been restricted. Kurle hopes that this research will underscore the long-term consequences of harmful pesticides by exposing their effect on a beloved and imperiled species. "These things have been banned since the 1970s, but they're still wreaking havoc environmentally,” she says. “What are we doing now that we're going to be paying for in the next 50 years?"


Stay abreast of Audubon

Get updates about our conservation work and how to help birds.