The last few decades have been good to California condors, which were once near extinction but have surged in population due to a concerted conservation effort across the country. But now that their population no longer seems doomed, writes Mary Beth Griggs for Popular Science, another threat has surfaced.

Contaminated food is to blame, writes Griggs, and humans are at least indirectly responsible for the threat. In a new study published in the journal Environmental Science and Technology, researchers studied the diet of current-day condors. Though the birds are partial to carrion (think: the carcasses of dead mammals like deer and cattle), they also eat marine mammals like sea lions.

That’s a problem, says the study: When researchers assessed the diets of condors in their favorite coastal habitats, they learned that they largely eat marine mammals that have been contaminated by pesticides that could endanger condors’ reproduction and future survival. Coastal condors had blood concentrations of contaminants like mercury, chlorinated pesticides like DDE (which is formed when DDT breaks down), industrial products like PCBs, and other chemicals that were between 12 and 100 times higher than those of their non-coastal cousins.

All of those contaminants are associated with marine mammals, which chow down on fish and other lower-food-chain animals that in turn absorb contaminants in their fatty tissues as they eat other contaminated food and swim in contaminated ocean bottoms. And every single one is associated with human activity.

The research team concluded that in order to continue to support condor recovery, contaminants must be reduced in the ocean. Though condor conservation has been a runaway triumph for conservationists (a major conservation project saved the animals from extinction), humans could inadvertently be threatening the lives they have worked so hard to save.

Condors are still very susceptible to things like lead poisoning from leftover ammunition, which threatens the birds further inland. And as Griggs reports, it’s unclear just how humans can ensure condor diets’ safety. But reducing contamination in ocean waters could be a good first step for the majestic, bizarre birds who live closer to the sea.