Stocky, medium-sized seabird like a mix between a shearwater and a gull. Variable plumage. Light morph birds are white below and gray above like a gull. Dark morphs are generally sooty-gray overall, darker and browner in the Pacific. All but the darkest birds have a pale patch on the upperwing. Also note thick pale bill with orange tip. Flies with stiff wingbeats, often wheeling and arcing like a shearwater. Can be seen singly or in flocks, often loosely associating with other seabirds when feeding. Breeds on cliffs at high latitudes across the Northern Hemisphere; otherwise found offshore in cold water. Rarely seen from land except at colonies. Atlantic and Pacific populations may be separate species.