The mass of dead seabirds that have washed up on Alaska beaches in past months is unprecedented in size, scope and duration, a federal biologist said at an Anchorage science conference.

The staggering die-off of common murres, the iconic Pacific seabirds sometimes likened to flying penguins, is a signal that something is awry in the Gulf of Alaska, said Heather Renner, supervisory wildlife biologist at the Alaska Maritime National Wildlife Refuge.

"We are in the midst of perhaps the largest murre die-off ever recorded," Renner told the Alaska Marine Science Symposium on Thursday. While there have been big die-offs of murres and other seabirds in the past, recorded since the 1800s, this one dwarfs most of them, Renner said.

"This event is almost certainly larger than the murres killed in the Exxon Valdez oil spill," she said.

After that spill -- at the time, the nation's largest -- about 22,000 dead murres were recovered by crews conducting extensive beach searches in the four months after the tanker grounding, according to the Exxon Valdez Trustee Council, the federal-state panel that administers funds paid to settle spill-related claims for natural-resource damages.

Now, hundreds and thousands of dead murres are turning up on a wide variety of Alaska beaches, including nearly 8,000 discovered this month on a mile-long stretch in Whittier, she said. A preliminary survey in Prince William Sound has already turned up more than 22,000 dead murres there, she said. Starving, dying and dead murres are showing up far from their marine habitat, in inland places as distant as Fairbanks, hundreds of miles from the Gulf of Alaska coast, making the die-off exceptionally large in geographic scale.

Even if she weren't an expert, the bird die-off would be obvious to Renner. She lives in Homer, where the beaches are "littered" with murre carcasses, she said.

"You can't walk more than a few feet without finding murres," she said.

Since only a small proportion of those killed ever show up as carcasses on the shore -- past studies put that proportion at 15 percent -- the actual death toll is likely much higher, Renner said.

The murre die-off began last spring, making it an especially long-lasting event. It coincides with widespread deaths of other marine animals, from whales in the Gulf of Alaska to sea lions in California. The die-off is overwhelmingly affecting common murres rather than thick-billed murres, which are closely related but tend to use slightly more western and northwestern waters from the Aleutians to the Chukchi Sea.

The immediate cause of the bird deaths is starvation.

"They just simply aren't able to find the food that they need to survive," Renner said. Necropsies conducted by the National Wildlife Health Center in Wisconsin found the dead murres were emaciated, with no food in their gastrointestinal systems and no fat on their bodies.

But what's behind the starvation?

Renner said biologists are focusing on three potential culprits that may be working independently or in concert with one another. And a common thread is heat, likely related to the "Blob" of warm water that persisted in 2014 and 2015 in the North Pacific and pushed temperatures as much as 3 degrees Celsius (5.4 degrees Fahrenheit) above normal.

"Warm water is implicated," she said.

Warmer waters might have affected murre food supplies or altered the birds' food needs by changing their metabolism, she said. Many past die-offs have been associated with warm waters, supporting the argument that the Blob is to blame, she said. The investigation is complicated because biologists have unanswered questions about the winter diet of murres, birds famous for their deep dives to forage for fish in summer.

"We know a lot more about what they eat in the summer than what they eat in the winter," Renner said.

Another suspect is a series of strong storms that might have scattered already stressed birds this winter, she said.

A third suspect is harmful algal blooms, which proliferate in warm waters and have been connected to some other marine animals' deaths.

So far, toxins associated with such algal blooms have not been found in dead murres examined by the National Wildlife Health Center. But it is possible that the signs of the toxins would have vanished long before the tests, even if they killed the birds, because the toxins don't linger in body tissue and instead are generally found in food in animals' digestive systems -- something missing from these murres' carcasses.

Renner said she is not yet worried about the die-off threatening Alaska's overall common murre population, roughly estimated at 2.8 million.

Still, troubling signs warrant monitoring in the future.

A breeding colony in the Barren Islands that is usually teeming in late summer with adult murres tending their young was deserted this year, she said. The site, at East Amatuli Island, usually has nesting birds crowded into the cracks of the rock face, but this year, "nobody was home," Renner said. "In more than three decades of monitoring murres in the Barrens, we've never had complete reproduction failure before." Similar failures occurred at some other nesting colonies, though not at all, she said.

Common murres and whales -- which are the subject of a National Oceanic and Atmospheric Administration investigation after several were found dead this summer -- are not the only Gulf of Alaska marine animals to fall victim to ailments believed to be related to warm waters in 2014 and 2015.

Tammy Hoem Neher, a NOAA scientist working with the multiagency program Gulf Watch Alaska, listed a wide range of changes in the marine systems observed during the period of unusual warmth.

Kachemak Bay saw an eight-fold increase in sea otter deaths, with carcasses showing signs of toxins produced by harmful algal blooms, Neher said at the symposium. Sea stars in Kachemak Bay in 2015 were found stricken with a wasting disease similar to that which has killed large numbers of the animals elsewhere on the U.S. West Coast, she said. One hypothesis is that the unusually warm waters exacerbated other stresses on the sea stars, she said.

But at least one Gulf of Alaska marine population thrived in the new conditions, Neher said. Fish-eating resident killer whales have feasted on big runs of salmon, fattening up without having to swim very far, she said.

"They kind of lazed around day to day," she said.