Often thought of as a mix between a shearwater and a gull, this thick-necked seabird prefers cold offshore water when not breeding. Rarely seen from land except at colonies. Flies with stiff wingbeats. Seen singly or in small flocks, often loosely associating with other shearwaters when feeding. Light morph birds appear gull-like with a white head and body and silvery-gray wings; note pale patch on upperwing. Dark morph generally sooty gray overall, darker and browner in Pacific. Atlantic and Pacific populations may be separate species.