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– Puffins are in trouble.

The birds have been in decline, especially since the 2000s. The potential culprits are many: fickle prey, overfishing, pollution. Scientists say that climate change is diminishing food supplies and the fact that puffins are tasty — and thus hunted — hardly helps.

Researcher Annette Fayet is trying try to solve the mystery of the dwindling Atlantic puffins. In Iceland, where the largest population of Atlantic puffins is found, their numbers have dropped from roughly 7 million to about 5.4 million. Since 2015, the birds have been listed as “vulnerable” by the International Union for Conservation of Nature, meaning they face a high risk of extinction in the wild.

The birds are cherished by Icelanders as part of their history, culture and tourist trade — and, for some, their cuisine. “The puffin is the most common bird in Iceland,” said Erpur Snaer Hansen, acting director of the South Iceland Nature Research Center. “It’s also the most hunted one.”

The puffins also have suffered because of the decline of their favorite food, silvery sand eels. That collapse correlates to a rise in sea surface temperatures. The temperature of waters around the country is governed by long-term cycles of what is known as the Atlantic Multidecadal Oscillation, with periods of colder water alternating with warmer. Between the 1965-1995 cold cycle and the current warm cycle, Hansen said, winter temperature records show about 1 degree Celsius of additional warming — disastrous for the sand eels.

Without as many sand eels in the water, the birds have to fly farther to find food for themselves and their chicks. The colonies’ decline suggests these birds are working too hard for their supper. “Flying, for puffins, is very demanding,” said Fayet, who is monitoring four puffin colonies. “It is a big energy cost for them.”

Hansen said 40 percent of the population of Icelandic puffin chicks is losing body mass over time, another bad sign. When the adults can’t catch enough to feed themselves and the chicks, they make an instinctive Malthusian choice; the chicks starve.

“These birds are long lived, so you don’t just see them plummeting down,” Hansen said. In the long run, he warned, “It’s not sustainable.”