Common on lakes, reservoirs, and slow-moving watercourses, in fresh and brackish marshes, and occasionally on saltwater. Breeding pairs aggressively territorial but nonbreeding flocks can number in hundreds. Often out on open water and dives readily; also feeds by upending or grazing on grassy shore. Adult distinctive: slaty black overall with white bill and forehead shield. Immature has duller bill, like adult by midwinter. When walking on shore, note big feet with lobed toes.