From the viewpoint of a high-soaring California condor, it’s easy to overlook small dietary problems. But researchers on the ground have found that the accumulation of chemicals in the marine mammals the scavengers feed on in central California could be having devastating effects on their eggs.
“It turns out that marine mammals are filled with all kinds of contaminants that are passed onto the bird,” says Carolyn Kurle, assistant professor of biology at the University of California at San Diego and the lead author of a recent study published in Environmental Science and Technology. She says that these high levels of contaminants could be complicating the recovery of one of the rarest birds in the world.
Two toxic chemicals—the banned pesticide DDT and carcinogenic toxins called polychlorinated biphenyls (PCBs)—were dumped into the Los Angeles sewer system as recently as the 1970s by chemical and electronics companies, respectively. Much of this ended up in the ocean and sank to the seabed near the Channel Islands where California sea lions spend several months of every year breeding.
“After their breeding season they spread all up and down the coast all the way up to Vancouver and beyond,” Kurle says. When these marine mammals die, they form a major part of the diet of the central California condor population.
The largest land birds of North America haven’t had an easy time over the past few decades. California condors were declared extirpated, or locally extinct, in the wild in the late 1980s. But captive breeding and reintroduction projects worked to bring the birds back to areas of California, the U.S. Southeast and Mexico's Baja California. The U.S. Fish and Wildlife Service counts only 435 California condors left in the world and of these, only 155 are wild birds in California.
The California birds are more or less split evenly between two populations. The population that lives mostly along the central coast of the state were thought to be the healthier birds, Kurle says. Adult Southern California condors, which mostly live inland, have a lower survival rate than those of the central coast, because they eat animals that have been shot with lead bullets (California banned hunting with lead ammunition in 2013, a regulation that doesn’t fully go into effect until 2019). Some of Kurle’s coauthors on the recent research also conducted a 2012 study published in the Proceedings of the National Academy of Sciences that showed that this lead poisoning from ammunition may be a serious roadblock in the population recovery of those animals.
But the recent study used stable isotope analysis to find that the coastal condors had 12 to 100 times the levels of contaminants like mercury, PCBs, PBDEs (dangerous flame retardants) and some pesticides compared to their inland cousins. They also found high levels of DDE, a long-lasting byproduct of DDT. “The best predictor of how high DDT levels are going to be in a coastal bird is the number of years they’ve been feeding on marine mammals,” Kurle says.
Peter Cook, an assistant professor at New College of Florida who was not involved in the study, says any research that helps biologists working to connect the dots on how humans are impacting wildlife is worthwhile. “It’s always a complicated web of interactions,” he says.
Cook has done work in the past on how toxic algae blooms and other factors could be causing increasing levels of young sea lion strandings along the California coast. But he’s not entirely sure these strandings will be bad for the condors. Most of these recent sea lion strandings and deaths involve pups, who haven’t had as much time to accumulate toxins in their systems as adults.
“The pups may be safer forage in some ways for a condor than an adult,” he says, adding that acids sea lions absorb from toxic algae blooms aren’t as dangerous for condors, as they aren’t as persistent in tissue as chemicals like DDT, which infamously causes birds to lay eggs with thinner, more breakable shells.
Despite the challenges the coastal condors face, Cook is hopeful. He says that the adult survival rate of these birds is still higher than the inland California condors, though egg thinning could provide an obstacle for recovery.
Kurle and her coauthors ran a model that predicted that around 40 percent of breeding-age birds from the coastal population had levels of DDE that were high enough to cause egg thinning in bald eagles.
“If you have an eggshell that is too thin, you can’t sit on your egg until it’s time to hatch because it’s just going to crack,” Kurle says. “You get reproductive failure.”
A 2013 study published in BioONe showed that condors on the central coast of California had less hatching success due to eggshell thinning. While birds in the state’s south had 70-80 percent hatching success, Central California condors only had around 20-40 percent.
The paper noted that condor eggshells should recover their thickness as DDE contamination continues to decline, though. Kurle says the 435-odd California condors in the world has remained steady for the past few years and last year saw more live hatchlings (14) in the central coast population than deaths (12) for the first year in a long time.
“The hope is that that trend will continue,” she says. But, “the bottom line is there are only a little over 400 birds out there that exist and so any problems with their reproduction will just make it harder and harder for them to recover.”