Thousands of dead Cassin’s auklets, a small seabird, have been washing ashore along the Pacific Coast from British Columbia, Canada to San Luis Obispo, California, and scientists have yet to figure out exactly why, The Statesman Journal reported.

One Tillamook, Oregon resident found 132 dead birds on a beach; 126 of them were Cassin’s auklets. On the next two days, another Oregonian found dozens of dead birds on two different beaches.

“I estimate there were probably 30 to 50 per mile,” Dave Miller, who found the birds, told the SJ. “I’ve never seen that many before.”

While it is normal for some seabirds to die over the winter, especially during big storms, said wildlife veterinarian for the Oregon Department of Fish and Wildlife Julia Burco. But mass die-offs like this, which is known as a “wreck,” aren’t common.

Preliminary reports from tests on the birds run by Oregon State University show that the birds are starving to death.

“It doesn’t look like a toxin,” said Burco. “It’s more likely due to weather and food supply.”

Although experts aren’t sure exactly what’s going on, some speculate that the dead birds could be the result of a successful breeding season.

Julia Parrish, Executive Director of the University of Washington’s Coastal Observation and Seabird Survey team, explained to The LA Times that almost every breeding pair laid an egg, and food might be scarce as the birds migrate south for the winter.

“If the bottom had fallen out of the ecosystem, you would be seeing everybody dying, but we are not,” said Parrish. “There is a little bit of mystery to it.”

Other experts cite climate change as the cause. Particularly violent storms could be affecting the birds by forcing them into areas they’re not used to or are preventing them from foraging for food, the SJ reported. Climate change, which is responsible for a warmer, and therefore more acidic ocean, which affects the “tiny zooplankton, such as krill, the birds feed upon.”

“The suggestion is that the ocean, for some combination of reasons, is less abundant for their food sources,” said Phillip Johnson, executive director of the Oregon Shores Conservation Center.

Johnson also noted that the US Geological Survey’s National Wildlife Health Center in Wisconsin is also studying the dead Cassin’s auklets to figure out exactly what is going on.

“To be this lengthy and geographically widespread, I think, is kind of unprecedented,” he said. “It’s an interesting and somewhat mysterious event.”