Tens of thousands of dead birds are washing up on the beaches of Alaska’s Prince William Sound, an unexplained mass die-off that some experts say may be related to the changing climate.

The birds, all of a species known as the common murre, appear to have starved to death, federal wildlife officials say, suggesting disruptions to the supply of herring and other fish that make up the birds’ diet.

A survey by wildlife officials earlier in the month  counted more than 8,000 dead murres on the shores of one beach near Whittier, about 60 miles southeast of Anchorage. Local news video showed bodies of the black-and-white birds scattered on the beach and floating in the water offshore.

Biologists who have examined the birds say they appear emaciated.

“We know they are starving. Their stomachs are empty,” said Robb Kaler, a seabird biologist with the U.S. Fish and Wildlife Service in Anchorage. “But we don’t yet know what the mechanism is.”

New surveys over the weekend found large numbers of bird carcasses in other parts of the sound, though not as many as were discovered near Whittier, Kaler said. “It does seem to be widespread in Prince William Sound,” he said.

Wildlife officials say it’s not yet known why the birds are starving. One possible explanation is that the birds’ usual food source — herring and other small fish usually found near the coast in dense schools — are either not as plentiful or are behaving differently, perhaps because of unusual weather the region has experienced in recent years. The waters off Alaska’s southern coast have been unusually warm since 2014, and this year El Nino weather pattern has led to further warming. A 2008 scientific study  found that mortality rates for the murre tend to increase in years when water temperatures are even slightly above normal.

Murres are diving predators who plunge into schools of small fish, with adults consuming up to 300 fish a day. If the schools are staying further off the coast, or in deeper waters, it may be harder for murres to fish efficiently, Kaler said.

While generally plentiful elsewhere in Alaska, herring populations have been depressed in the Prince William Sound, scene of the 1983 Exxon Valdez oil spill.

Alaska is home to an estimated 2.8 million murres scattered among about 230 primary nesting grounds, and the region has seen major die-offs before. Still, local officials say this year’s event is exceptional.

“Scientists tend to get blasé about this, but this is bigger than I’ve ever seen,” David Irons, a biologist who recently retired from the U.S. Fish and Wildlife Service’s migratory bird division, said in a video interview with KTVA-TV as he assisted with a bird count.

“Seabird biologists say seabirds are indicators of the health of the ecosystem. Now they’re dying, and that is telling us something,” he said.

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