The Anatidae are for the most part of similar habits, and frequent seas, lakes, rivers, and watery spots generally, being found to a great extent in winter on the shore, especially where mudflats are exposed by the ebbing tide, and beds of such food-plants as _Zostera_ (grass-wrack) are uncovered. Large flocks, which include many migrants, are formed at that season, and in spring the ganders and drakes commonly collect into parties while the female is incubating, which she does during twenty-one to forty-two days. Later in summer the majority of the Family shed their quills simultaneously, and conceal themselves until again capable of flight, the males then becoming dull in colour for several weeks, and resembling the other sex.[112] _Merganetta_ is found only on the torrents of the Andes; _Hymenolaemus_ and _Salvadorina_ being also residents on mountain streams. The noisy flight is extremely powerful, and much swifter than it appears, the wedge-shaped formation which Geese affect being especially noticeable; some forms, however, are practically flightless, such as _Nesonetta_ and the adult Steamer-Duck (_Tachyeres_). All the Anatidae swim exceptionally well, diving being carried to its perfection in the marine Fuligulinae; while the partially-submerged {114}position with erect tail when feeding is known to every one. The various Swans have a whooping, trumpet-like, or hissing note; that of Geese is a harsh cackle, a gaggling sound, a clang or a "honk." Ducks do not always quack, but have whistling or grating cries in addition. The usual food is vegetable, consisting of grass, _Chara_, _Zostera_, _Ulva_, and other plants; but Mergansers live chiefly on fish, and the bill of fare is varied by grain, pulse, berries, frogs, insect-larvae, worms, molluscs, and crustaceans. The nest is placed on the ground in thick herbage, or sometimes almost in the water; holes in banks, hollow trees, or even branches at a slight elevation being chosen in certain cases: it is composed of heather, grass, moss, leaves, or rarely seaweed and twigs, and is lined with down, added gradually from the parent's breast during incubation. The eggs, which vary in number from two (_Biziura_) to about a dozen, are smooth and hard-shelled, with a plain white, creamy, or green coloration, and are commonly covered when left. The young return for a time to the nest at night, and are carefully tended by the female, who is occasionally assisted by the male, especially in Swans. It is not certain how tree-building Ducks convey the nestlings to the water, though it has been stated that they are carried in the bill; but it is no uncommon sight to see ducklings and cygnets climb upon their mother's back and hide beneath her wings when danger threatens.