Populations of the Atlantic puffin, a colorful species of seabird in the auk family, are declining—and nowhere is the trend more evident than in northern Norway.

Nesting birds in that country have decreased by up to 2 million since 1980, though estimates vary. Scientists offer differing explanations, but most point to reduced feeding stocks as a major concern.

A warming ocean has opened up northern feeding grounds for mackerel, which consume...