We tested the hypothesis that segregation in wintering areas promotes population differentiation in a sentinel North Pacific seabird, the rhinoceros auklet (Cerorhinca monocerata). We collected tissue samples for genetic analyses on five breeding colonies in the western Pacific Ocean (Japan) and 13 in the eastern Pacific Ocean (California to Alaska), and deployed light-level geologgers on 12 eastern Pacific colonies to delineate wintering areas. Loggers were deployed previously on one colony in Japan. There was strong genetic differentiation between populations in the eastern vs. western Pacific. Deep-ocean habitat along the northern continental shelf appears to act as a barrier to dispersal; abundant in the western and eastern Pacific Ocean, the rhinoceros auklet is virtually absent as a breeder in the Aleutian Islands and Bering Sea, and no loggered birds crossed the North Pacific in the non-breeding season. Late Pleistocene glaciation over the North Pacific also might have forced a southward range shift that isolated the western and eastern populations. While genetic differentiation was strongest between the eastern vs. western Pacific, there was also extensive differentiation within both regional groups. In pairwise comparisons among eastern Pacific colonies, the standardized measure of genetic differentiation (F’_ST) was negatively correlated with the extent of spatial overlap in wintering areas. That result supports the hypothesis that segregation in the non-breeding season promotes genetic structuring. Strong natal philopatry and a neritic foraging habit probably also play roles. Widely distributed, vulnerable to anthropogenic stressors, and exhibiting extensive genetic structure, the rhinoceros auklet encompasses the scope of the conservation challenges posed by seabirds.

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