Sometimes hybridizes with U. lomvia, perhaps more frequently than hitherto believed#R. Race hyperborea poorly differentiated; possibly better synonymized with nominate. Birds of Faeroes sometimes separated subspecifically as spiloptera and populations of small islands in S Baltic as intermedia, but claimed differences probably reflect individual variation; both are synonymized with nominate. Form ibericus (from W Iberia) is included in albionis. Five subspecies recognized.
Winters mainly in boreal waters, S in Atlantic to Georges Bank (sea off Outer Gulf of Maine) and W Europe, and in Pacific to S Japan and S California.
Food and feeding
Status and conservation
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