Gulls off the coast of Argentina have developed a peculiar food habit: pecking at whales' back fat.
The birds used to feed on skin shed by the whales, but they figured out how to go straight to the source.
This can threaten the survival of calves who are too young to know how to protect themselves,
"The attacks are very painful and cause large, deep lesions, particularly on the backs of young calves," Mariano Sironi, scientific director of the Instituto de Conservación de Ballenas in Argentina and co-author of the study, told The New York Times.
The birds — kelp gulls — used to feed off sheets of skin from right whales, which the animals naturally shed.
But about 50 years ago, some of them figured out they could tap the whales' back fat straight from the source, The Times reported.
The bizarre sight is seen on the Peninsula Valdés on the east coast of Argentina. The birds flutter about waiting for the whales to surface, then they pounce, pecking at the whale's hide to get to the blubber.
The wounds left on the babies — known as calves — can range from scratches to bigger wounds covering "a big portion of the calves' back and can be one meter long or even bigger," said Sironi.
The gulls aren't the only ones to blame. Scientists said that humans are contributing to an increase in the gull population with mismanaged landfills and fishing waste, per The Times.
The gull's pecking is the straw that breaks the calves' back
Calves are particularly vulnerable before their first birthday, said the study, which was published in the peer-reviewed journal Biology Letters.
That's because they haven't had time to learn protective behavior. Adults know to arch their backs when surfacing for air so that only their blowholes pierces the surface. But the calves expose themselves more, and pay for it.
More and more birds are learning the behavior, and that's a problem.
Looking at records from aerial photographs and sightings between 1970 and 2017, the researchers discovered calf injuries from gulls had doubled, The Times reported. At the same time, more calves were dying before their first birthday and the birds may be the contributing factor, per the study.
"Our analysis supports recent studies indicating that gull harassment at Peninsula Valdés may impact southern right whales population dynamics," the scientists said in the study.
The calves undergo a strenuous migration with their mothers to their feeding areas over the summer. The added stress of the wounds on the calves already facing pressures from fishing lines and warming climates is too much for many.
This is particularly bad news for this population of whales, who were brought back from the brink of extinction when hunting was banned in 1935, per The Times.
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