Unalaska, AK – Observers in Unalaska still aren't sure what killed more than 2,400 seabirds that washed up on the island's beaches starting Wednesday night. But they're leaning towards the hypothesis that the birds, called shearwaters, may have run into one or more fishing boats offshore in Unalaska Bay.

Marine biologist Reid Brewer is coordinating a volunteer effort in Unalaska to document the shearwater die-off, in conjunction with the Washington state-based Coastal Observation and Seabird Safety Team and the U.S. Fish and Wildlife Service. He says that while the boat strike may not be the whole story, it seems like a reasonable explanation.

Fish and Wildlife Service biologists are analyzing some of the shearwater carcasses at the agency's lab in Madison, Wisconsin. In the meantime, eight volunteers are still working with Brewer to monitor the beaches around Unalaska for more dead birds. But Brewer says the numbers are decreasing steadily, down to about 1,200 this afternoon.