For the last year and a half, Americans have watched and worried as H5N1 bird flu racked dairy herds and killed hundreds of millions of commercially raised chickens, turkeys and ducks.

But far less widely known is that the virus has devastated wildlife across the globe, killing millions of wild birds and mammals.

Few animals have been harder hit than elephant seals, sea lions and fur seals in the Southern Hemisphere. In some places thousands of carcasses and orphaned pups have littered the beaches.

On Thursday, a research team led by Connor Bamford, a marine ecologist with the British Antarctic Survey, reported a 47% drop in breeding females between 2022 and 2024 in the three largest elephant seal colonies on South Georgia Island.

The elephant seals of South Georgia Island, located between South America and Antarctica in the South Atlantic, are the largest breeding colony on the planet.

The virus hit there in 2023, Bamford said, and researchers were there to see it. But it was their visit in 2024 that really drove the devastation home.

"Normally there's about 6,000 seals on St. Andrews Bay," he said, describing a two-mile strip of beach along the northeastern side of the island. Usually it's hard to make your way through the animals, it's so jam-packed.

But in 2024, "it was easy. There were massive gaps. There were so few of them," he said.

Other large breeding colonies — including along the Argentinian coast, as well as several other islands north of the Antarctic circle — have also been hit. In 2023, UC Davis researchers reported that nearly 97% of elephant seal pups died at Argentina's Peninsula Valdes, the most deaths ever recorded for this species.

According to Ralph Vanstreels, a marine ecologist with UC Davis who is researching the animals in Argentina, two-thirds of southern elephant seal colonies are now infected. Only those near New Zealand and Australia have been spared.

"We're just holding our breath," in hopes the virus doesn't get there, he said.

Vanstreels said genetic analyses show the strain of virus circulating in Argentina acquired mutations allowing it to pass easily between mammals. He said it's not yet clear whether the virus that has hit other elephant seals and pinnipeds in the region carries the same mutations.

Nor does anyone know whether the virus will move north to populations along the California coast — or into people.

But it's left a deadly wake.

Reports of southern sea lions, fur seals and crabeater seals dying en masse have come in from across the region.

Vanstreels and Bamford say there's no way to know the full extent of the virus' toll on these animals. Many of these species, such as crabeater seals, are so remote that there are few, if any human observers to witness the devastation.

More than 30,000 sea lions in Peru and Chile died between 2022 and 2024. In Argentina, roughly 1,300 sea lions and fur seals perished.

Vanstreels said researchers don't yet have any clear idea about why northern elephant seals and marine mammals in the north Pacific, including those that breed along the California coast, have been spared.

He said the strain circulating off the North American Pacific coast doesn't carry the mutations seen in South America, so that may be why. There may also be differences in population densities or in the local marine ecosystem.

"We think the South American sea lion played a big role in transmission, carrying the virus along the coast and perhaps introducing it to the elephant seal population," he said. "Maybe the areas where the Northern elephant seal lives don't have as good a vector for the infection to be spread."

Bamford and Vanstreels say the loss of this many animals will probably affect the broader ecosystem as well.

For example, elephant seal placentas are a major source of food for a variety of coastal animals, such as birds and crabs. In addition, the seals' deep-sea foraging brings nutrients to the ocean surface, where fish, kelp, shrimp and other sea life depend on their waste and refuse for sustenance.

"You get rid of half of their population, that's going to have an impact," Vanstreels said.

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