Pity the fur seal pups of Guafo Island. If they can evade being battered by the storms that pound the island, and avoid being trampled by the huge adult males that periodically charge across the beach, and dodge the killer whales that patrol the surrounding waters, then they might just live to get their perineums pecked raw by seagulls.

Guafo, sitting just off the coast of Chilean Patagonia, is home to the world’s largest colony of South American fur seals. Mauricio Seguel from the University of Georgia has been studying these animals since 2003. Every year, he’d capture a few of the pups from the huge colony, check them for parasites, and monitor their health. And in 2008, he noticed that some of them had large wounds on their perineums—the areas between their genitals and anuses.

Seguel discovered the culprits in 2012. While watching the colony through his binoculars, he happened to catch a pup pooping. “As soon as he did that, all the seagulls formed a line behind him and ate the feces,” Seguel says. “And some would go up and peck his butt.”

Fur seals aside, Guafo Island is also home to kelp gulls—formidable birds with four-foot-wide wings and sharp, yellow beaks. Since 2012, Seguel and his colleagues have seen these birds aggressively pecking poop from the pups’ nether-regions on at least 20 occasions. And once the kelp gulls have had their fill, another smaller species—the dolphin gulls—swoop in to continue the harassment.

To be clear, the gulls aren’t eating the fur seals. It’s the poop that attracts them. Seguel also found that the pups are more likely to be attacked if they’re infected with hookworms and other intestinal parasites. The gulls might be drawn to the blood in infected pups’ stools, or to the worms that they discharge. The pups, and their perineums, are just caught in the crossfire.

The pups aren’t fast enough to escape, and they aren’t agile enough to bite the gulls, although they sometimes try. As a result, between 5 and 9 percent of the pups that Seguel catalogued ended up with perineal wounds. None of these wounds were fatal in themselves. But many were so bad that they had become badly infected, and the pups that bore them had become slow-moving. The gulls might not be out to harm them, but they are doing so nonetheless.

These pups of Guafo are the fortunate ones. Gulls are opportunistic animals that will take advantage of whatever sources of food they can find, even when food is already plentiful. And in other parts of the world, they take things much further. In Baja California, western gulls that scavenge on the placentas of newborn sea lion pups will sometimes kill the pups too by pecking at their eyes and bellies. In Namibia, kelp gulls also eat the eyes of fur seal pups. And in perhaps the most surprising case of all, the kelp gulls of Península Valdés in Argentina have taken to flaying whales.

Between June and December, thousands of southern right whales amass off Península Valdés. And as they surface for air, kelp gulls land on their backs and tear off bits of skin and blubber. They target whale calves in particular, and one poor youngster had gull-inflicted wounds over a fifth of its back. Gulls frequently feed on rubbish heaps, so their unsanitary beaks are not what you want probing around an open wound. Their antics could also force the whales to waste a lot of time and effort on fleeing and evading, when they should be conserving their energy for playing, resting, feeding, or nursing.

“Kelp gull harassment has clearly become a significant threat to the health and welfare of the Península Valdés right whale population,” wrote Carina Marón from the University of Utah in 2015. Local authorities started trying to cull the birds in 2012. And the whales themselves have developed a new breathing technique that minimizes the amount of body they expose at the surface.

For now, the gull problem is here to stay. Scientists first noticed their behavior in 1972, back when just 1 percent of the local right whales carried wounds from marauding beaks. But the gull population has since exploded because human dumps and fisheries provide them with so much food. By 2008, there were so many of them that they had injured 77 percent of the whales.

That could be a sign of things to come in other parts of the world, like Guafo Island. There, kelp gull populations haven’t ballooned, which might explain why only 1 in 9 fur seal pups at most suffer from perineal wounds. “We don’t have a big problem now, but watch out,” says Seguel. “Human activities around coastal areas provide sources of food for the seagulls. And if you do anything that affects seagull populations, it could also affect other species.” If yesterday’s metaphor for impending doom was the canary in the coal mine, then tomorrow’s might be a seagull on a seal pup’s perineum.