The carcasses of thousands of small birds called Cassin's auklets have been washing ashore over the last few months from Northern California up to the north coast of Washington.
Scientists along the Pacific Coast have been trying to determine what is causing the large die-off of the birds this winter.
The University of Washington's Coastal Observation and Seabird Survey Team has seen more than 1,200 bodies wash ashore since fall began.
Executive Director Julia Parrish thinks that is only a small fraction of the total number of dead birds. It is probably in the tens of thousands, she said.
Parrish, a professor of marine sciences at the University of Washington, said the die-off was largely a mystery to experts.
The birds have been found mostly starved to death, so the deaths are not a result of an oil spill or a toxic reaction to food, said Lindsay Adrean, a wildlife biologist with the Oregon Department of Fish and Wildlife.
One explanation is that the birds are starving as a consequence of an unusually successful breeding session last year in British Columbia.
Almost every breeding pair laid an egg, and as the young birds fly south for the winter they may not all be finding the small fish and shrimp they normally feed on, Parrish said.
The Pacific has also been a few degrees warmer this winter, which could touch off subtle changes in the food chain that make it harder for the small birds to find sustenance, Adrean said.
But other birds along the coast are not dying at unusual rates, Parrish said.
"If the bottom had fallen out of the ecosystem, you would be seeing everybody dying, but we are not," she said. "There is a little bit of a mystery to it."
It is possible that more bodies are simply being found because the birds are flying closer to the shore to find food, Parrish said. Normally, the birds find food far out at sea.
Typically, spotters see one or two dead birds along a kilometer of beach. Oregon spotters in November saw up to 30 per kilometer and then up to 115 per kilometer in December, Parrish said.
Robert Ollikainen, a volunteer carcass spotter in Cape Meares, Ore., said he walked onto a low sand bar between Tillamook Bay and the Pacific the day after Christmas and came across 126 of the fist-sized birds.
He had never seen so many before. "My God, there were so many of them," Ollikainen said.