In the storm debris littering a Washington State shoreline, Bonnie Wood saw something grisly: the mangled bodies of dozens of scraggly young seabirds.

Walking half a mile along the beach at Twin Harbors State Park on Wednesday, Wood spotted more than 130 carcasses of juvenile Cassin's auklets—the blue-footed, palm-size victims of what is becoming one of the largest mass die-offs of seabirds ever recorded.

"It was so distressing," recalled Wood, a volunteer who patrols Pacific Northwest beaches looking for dead or stranded birds. "They were just everywhere. Every ten yards we'd find another ten bodies of these sweet little things."

Cassin's auklets are tiny diving seabirds that look like puffballs. They feed on animal plankton and build their nests by burrowing in the dirt on offshore islands. Their total population, from the Baja Peninsula to Alaska's Aleutian Islands, is estimated at somewhere between 1 million and 3.5 million.

Last year, beginning about Halloween, thousands of juvenile auklets started washing ashore dead from California's Farallon Islands to Haida Gwaii (also known as the Queen Charlotte Islands) off central British Columbia. Since then the deaths haven't stopped. Researchers are wondering if the die-off might spread to other birds or even fish.

"This is just massive, massive, unprecedented," said Julia Parrish, a University of Washington seabird ecologist who oversees the Coastal Observation and Seabird Survey Team (COASST), a program that has tracked West Coast seabird deaths for almost 20 years. "We may be talking about 50,000 to 100,000 deaths. So far."

View Images Volunteers have been counting and tagging emaciated juvenile Cassin's auklets like these, which washed ashore near Pacific City, Oregon. Photograph by J. Forsythe, COASST

One Die-Off Among Many

The gruesome auklet deaths come just as scientists around the globe are seeing a significant uptick in mass-mortality events in the marine world, from sea urchins to fish and birds. Although there doesn't appear to be a link to the virus that killed tens of millions of sea stars along the same shores from California to Alaska over the past 18 months, some scientists suspect a factor in both cases may be uncharacteristically warm waters.

The U.S. Geological Survey and others have performed animal autopsies, called necropsies, on several of the emaciated Cassin's auklets. They've found no evidence of disease or trauma—no viruses or bacteria, no feathers coated with spilled oil. The birds appear simply to have starved to death.

"There's very little evidence of food in their GI [gastrointestinal] tracts or stomachs," said Anne Ballmann, with USGS's National Wildlife Health Center.

At first scientists weren't too surprised by the carcasses washing ashore. When young auklets fledge in late summer, they all enter the water at the same time and start competing for food—shrimp-like krill and tiny crustaceans called copepods. For various reasons, last summer's birth class of Cassin's auklets was gigantic. Researchers expected a higher death toll, too.

But they now are perplexed by the sheer numbers of dead birds and the spreading geographic extent of the die-off.

"Death at this level and over this much real estate has to be from more than just that," Parrish said.

By comparison, not one of the five largest U.S. bird mortality events tracked by USGS since 1980 is estimated to have topped 11,000 deaths. In Europe, according to the U.K.-based Royal Society for the Protection of Birds, the worst die-off on record occurred in 1983, when 57,000 guillemots, razorbills, puffins, and other seabirds perished in the North Sea and washed up on the British coast.

"You get some of this with seabirds every year," said David Nuzum, with the Oregon Department of Fish and Wildlife. "You get so many juveniles out there, and they've got this steep learning curve for feeding after being separated from their parents, so you always get a die-off in winter. But I've never seen anything like this, ever, and I've been here since 1985."

View Images Cassin's auklets like this one, swimming near the Santa Cruz Municipal Wharf in November, number between 1 million and 3.5 million along the West Coast of North America. Photograph by Peter LaTourrette

On some beaches the Cassin's auklet death toll was a hundred times greater than any bird die-off ever tallied there, and six times worse per kilometer than the body count recorded after the 1989 Exxon Valdez oil spill in Prince William Sound, Alaska. On a single stretch of beach on Christmas Eve in Oregon, U.S. Fish and Wildlife Service biologist Mike Szumski collected 250 carcasses—and left nearly as many behind.

"You'd find them piled up in clusters on the wrack line, where the tide leaves sea grasses and debris," Szumski said. "Most were in these states of decay, but every now and then we'd see tracks coming out of the water and find a bird that was just barely clinging to life. They were just skin and bones."

The Culprit: Warm Water?

Bill Sydeman, a senior scientist at California's Farallon Institute, said he believes the most likely scenario is that the deaths are related to a massive blob of warm water that heated the North Pacific last year and contributed to California's drought and to 2014 being the hottest year on record.

That water was hotter and stayed warm longer than at any time since record-keeping began. It stretched across the Gulf of Alaska, where a high-pressure system blocked storms, preventing the water from churning to the surface and mixing with air. More warm water eventually moved inward along the coast as far south as California, altering how favorable the environment was for the zooplankton that many fish and birds, including Cassin's auklets, feed on.

That all happened in late summer—about the same time the young auklets began to fledge.

View Images These two dead auklets were among 179 that volunteers counted on December 23 near Neskowin, Oregon. Photograph by P. Shurts/COASST

Research in the waters off Oregon already has shown that some of the tiny crustaceans at the bottom of the marine food chain were replaced by smaller species that provide less nutrition for larger animals. It's still not clear how—or whether—a changing climate contributed to any of these shifts.

But if the base of the food chain has changed, even temporarily, why is the die-off still limited to just one species of seabird?

"That's the thing that's so puzzling to us—we're just not seeing this with common murres or anything else," Parrish said.

Sydeman predicts that this spring or summer the dying might spread to the salmon and forage fish that eat those same plankton species and then perhaps to the murres or other birds that, in turn, eat those fish.

"I think there's a strong possibility of it escalating to affect other species in the near future," he said.