Duck is the collective name for waterfowl in the subfamily Ducks of the family Duckidae of the order Geese, or true ducks. Ducks are relatively small, with short necks and larger beaks in some genera. The legs are located behind the body, resulting in a waddling gait. Most true ducks (including several species incorrectly called geese because of their individual size and shape) differ from swans and geese in the following characteristics: males change their plumage twice a year; females lay more eggs per clutch and have smooth egg shells; the legs of both sexes are covered with overlapping scales; and the quacking and plumage show some degree of sex differentiation. All true ducks, with the exception of the Warbling Duck and the Sea Duck, are sexually mature within the first year and pair up only during the breeding season, unlike swans and geese, which mature later and pair up throughout their lives. Based on their characteristic behaviors, ducks can be divided into three main groups: drilling ducks, diving ducks, and perching ducks. The mallard is a typical water-drilling duck and is the ancestor of most domestic ducks. Perching ducks such as the Moscow Duck have long claws and are the most arboreal ducks. Diving ducks include the most marine species. Green-headed ducks fly from the south to the north in the spring to lay their eggs, and then to the south in the fall to overwinter. They lose their migratory flightiness when they are domesticated by humans, and people don't let them stop laying to hold their eggs in order to get more eggs. Over time, domestic ducks have lost the ability to hatch eggs.