Thousands of carcasses of small birds known as Cassin's auklets have been washing up in recent months on the coastline from Northern California to the north coast of Washington and the cause of the species' large-scale deaths is baffling scientists.
Cassin's auklets (Ptychoramphus aleuticus) are small seabirds that nest in small burrows. Although the species is listed as Least Concern and is still numerous, some of its populations have significantly declined.
Since fall, a team of experts from the University of Washington has found over 1,200 bodies of these birds washed ashore but Julia Parrish, executive director of the University of Washington's Coastal Observation and Seabird Survey Team (COASST) thinks that this number represents only a small fraction of the total number of dead birds, which she estimates to be likely tens of thousands.
Although some seabirds normally die during harsh winters and particularly when there are big storms, the massive die-off of Cassin's auklets currently happening is unusual. Parish, who is also a professor of marine sciences, said that the mass die-off is a mystery to experts.
Wildlife biologist Lindsay Adrean of the Oregon Department of Fish and Wildlife said that the dead birds were mostly starved to death, which indicates that they did not die because of toxic reaction to food or an oil spill.
It is possible that the birds are starving as a result of an unusually successful breeding season last year with nearly every breeding pair managing to lay an egg. Parrish said that some of the young birds flying south for the winter may not have found small fish and shrimp to feed on.
Some experts consider climate change as another possible cause. Unusually violent storms, for instance, could be pushing the birds to go to unfamiliar areas or prevent them from foraging. The Pacific has also been notably warmer this winter and experts said that this could cause subtle changes in the food chain, which could make it harder for the birds to find food.
"The suggestion is that the ocean for some combination of reasons is less abundant for their food sources," said Oregon Shores Conservation Coalition executive director Phillip Johnson.
Parrish, however, noted that other bids along the coast do not die at unusual rates, which makes the phenomenon more baffling.
"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."
The cause of the birds' mysterious death could be unveiled later though as the U.S. Geological Survey's National Wildlife Health Center in Wisconsin is already conducting necropsies on the dead Cassin's auklets; and COASST is studying the unusual phenomenon.