To work out how many puffins had actually died, Timothy Jones of the University of Washington used the locations of the known bodies and data on local wind patterns to simulate where the dead birds were coming from. He estimated that between 3,150 and 8,800 tufted puffins perished in the final months of 2016.

What killed these birds? Most of them were intact, with no signs of either predator attacks or disease. Some of them had saxitoxin—a potent poison made by algae—in their stomach, but at levels almost 100 times lower than what would be considered safe for humans to eat. Instead, the most likely cause of death was starvation. The birds were extremely thin, with weak flight muscles and very little body fat. “They literally didn’t have enough to eat and became weak to the point of death,” says Julia Parrish of the University of Washington, who led the study.

Tufted puffins look like fancier versions of the more widely known Atlantic puffins, with elaborate yellow eyebrows that sweep backwards down their neck. On land, they have a clownish, goofy disposition. But in the sea, they become grace personified, using their streamlined body and sickle-shaped wings to fly underwater in pursuit of small fish.

But what if there are no fish to pursue?

The climate of the Bering Sea is changing rapidly. In recent years, the sea ice that would have extended southward in the winter has become unprecedentedly thin and sparse. And that has affected everything from tiny plankton to titanic walruses.

Read: There is no escape for corals

The ice sheets create a layer of super-cold water called the “cold pool,” which sits at the bottom of the Bering Sea. Pollock, cod, and other fish like to congregate in large numbers at the edges of this pool, providing excellent hunting grounds for puffins and other sea birds. When the cold pool doesn’t form, as has been in the case in recent years, the fish spread out over larger distances, and are harder to catch.

And when they are caught, they’re worth less. In the warmer waters, the plankton have shifted toward smaller and less energy-rich species, and the fish that eat those plankton have become similarly thinner in calories. Fish-eating birds, such as puffins, “are going from Clif Bars to rice cakes,” Parrish says. “They have to work much harder to get the same energy content. And this is happening over thousands of square kilometers of ocean, so it’s not like they can say, ‘Oh, there’s no Clif Bars here, so I’ll go to the next grocery store.’”

These changes hit the birds at the worst possible time—when they change their coat of feathers. Puffins use their body feathers as a wetsuit that keeps water away from their skin and helps them retain heat. To keep that suit secure, they regularly replace all their old plumes in a dramatic synchronous molt. Over the next few weeks, they need a lot of energy to grow new feathers, but they can barely fly or dive. For that reason, they typically molt from August to October, when food ought to be plentiful. If it isn’t, the birds don’t make it.