Adults are 33–36 cm (13–14 in) long, with long, pink legs, and a long, rather thin black bill. The birds are generally black above and white below, with a white head and neck (with a varying amount of black, species-dependent). Males have a black back, often with a greenish gloss or sheen. Females' backs have a brownish hue, contrasting with the black remiges. In populations where the top of the head is normally white (at least in winter), females tend to have less black on the head and neck the entire year-round, while males often have much more black, particularly in summer. This difference is not clear-cut, however, and males usually grow all-white head feathers in winter.
