Duck is the common name for a number of Water Bird species. The ducks are divided between several subfamilies. Swans and Geese are not considered ducks. Ducks are mostly aquatic birds, mostly smaller than the swans and geese and may be found in both fresh water and sea water. Ducks are sometimes confused with several types of unrelated water birds with similar forms, such as Loons, Grebes, Gallinules and Coots. The overall body plan of ducks is elongated and broad. The ducks are relatively long-necked, although not as long-necked as the Swans and Geese. The body shape of diving ducks varies somewhat from this in being more rounded. The bill is usually broad and contains serrated lamellae which are particularly well defined in the filter-feeding species. In the case of some fishing species the bill is long and strongly serrated. The scaled legs are strong and well developed, and generally set far back on the body, more so in the highly aquatic species. The wings are very strong and are generally short and pointed, and the flight of ducks requires fast continuous strokes, requiring in turn strong wing muscles. Many species of duck are temporarily flightless while Moulting; they seek out protected habitat with good food supplies during this period. This moult typically precedes migration. Most duck species breed once a year, choosing to do so in favourable conditions. Ducks also tend to make a nest before breeding.