Thousands of dead common murres are washing up on the beaches of Whittier, an unprecedented die-off that has scientists wondering how many more thousands remain uncounted throughout Prince William Sound.

A recently retired federal biologist doing beach surveys in Whittier over the weekend estimated there were more than 7,800 dead murres along a little over a mile of beach. That's nearly five dead birds per meter of beach, officials say.

The scale of the die-off is unprecedented along the Sound, longtime residents say.

David Irons, a former U.S. Fish and Wildlife Service biologist, made the grim discovery in Whittier and sent alerts to colleagues. The beach survey probably signifies an even larger loss -- maybe tens of thousands -- of the large black-and-white seabirds, Fish and Wildlife Service officials say.

To get a better picture what's happening throughout the Sound, the U.S. Geological Survey has chartered a boat out of Whittier to explore more remote beaches on Thursday, according to John Piatt, a research biologist at the USGS Alaska Science Center.

There have been other seabird die-offs in Alaska -- most recently in 1993 -- and along the West Coast. But the current event, which began this spring from California north, is the worst Piatt has seen in 40 years of research in both the Atlantic and Pacific oceans.

"It's a regular part of their life history, but I would say this is the most extreme I have ever seen or heard of," he said Tuesday by phone from Newfoundland, where he is on leave and monitoring the situation in Alaska.

Scientists don't know exactly what's causing the die-offs and associated murre strandings inland, far from the North Pacific, where they normally spend the winter. But they know the birds are dangerously underweight and emaciated.

It's possible that warmer ocean conditions have pushed prey fish deeper, beyond the murres' diving range of about 600 feet, or that there's a disease or some other medical condition that's causing them to starve. It's also possible that strong winds from recent storms pushed the birds off course, or stressed the already starving murres to the point of exhaustion and death.

Murres, which weigh a little more than two pounds, need to eat half their body weight every day, Piatt said.

"These birds are wicked skinny -- no fat reserves," he said of the murres being recovered this year. "It's an awful way to die, and they're dying en masse."

The dead birds are getting the public's attention. A Fish and Wildlife Service seabird expert got 50 calls over the weekend from concerned people finding dead murres on beaches, said Andrea Medeiros, a spokeswoman for the agency.

"It's turning out to be something that does have the potential for population-level effects," she said. "It is just off the chart as far as what we typically see with these events."

The estimated world breeding population of murres is between 13 and 20 million birds, according to the USFWS. Alaska's population is harder to count but the state's common murre population is estimated at 2.8 million breeding birds at 230 colonies.

Murres dive for their food, mostly small fish, using small wings that propel them underwater. Winter usually finds them at sea or on small islands, often forming large rafts.

Built for marine life and running atop water to launch into the air, murres struggle to take off from land.

The recent discovery of the Prince William Sound die-off follows a highly unusual and sudden influx of disoriented, starving murres getting stranded inland over the past few weeks, a phenomenon that began in late fall but spiked in late December.

Hundreds of birds seen or picked up by people from Homer to Two Rivers and Glennallen have overwhelmed wildlife and rescue agencies that are trying to rehabilitate the birds for release back to the North Pacific.

Rescuers say they don't know what will happen to them once they get there.