Common mergansers have a long, slender, serrated, red or orange bill with a wide base. Males have a green head, a mostly white body, and a black and gray back. Female has a gray body with a rusty head that is sharply defined against the white neck. Voice is a hoarse croaking car-r-r-r or rapid cackling notes. Mergansers are divers, and the legs are far back on the body; on land, the posture is upright.
Similar species both have larger crests: Red-breasted merganser males have gray sides, a dark chest and back, and a ragged crest; females have a pale rusty head with an indistinct border between rusty head and gray-white breast. Hooded merganser males have black and white head, crest, and back, with chestnut flanks; females are brown with rusty crest.