The 162 species of ducks, geese and swans are found throughout the world in all manner of wetland habitats from small ditches to rugged coasts (though they generally eschew the open ocean). They range in size from the small teals (weighing 250g-300g) to the swans, which may weigh 15kg or more. All species are aquatic, with webbed feet and a broad, flattened bill. Nearly all nest on, or beside the water, though a few, such as the Goldeneye nest in holes in trees. Ducks and geese produce large clutches of eggs (up to 10-15), though swans tend to lay fewer.
Swans are amongst the largest flying birds in the world with long necks and a wingspan of over six foot; most of the 7 species have an all-white plumage. The Mute Swan is found in parks and lakes throughout Britain, the other two species are winter visitors from their arctic breeding grounds.
Geese are intermediate in size between ducks and swans and most breed in the far north. The Greylag is the ancestor of all modern farmyard geese, though some wild birds do still breed in the northwest of Britain.
Ducks that occur in Europe can be split into three main groups: the dabbling ducks, such as the Teal, which up end feeding on weed and small insects on or just below the water's surface; the diving ducks, like the Tufted Duck which feed on the bottom for weed and small molluscs or insects, and so have feet set further back; and the seaducks, typified by the Eider, primarily marine species who dive for animal prey, with the mergansers actively pursuing fish and the like. The Mallard is the progenitor of the farmyard duck. Because of their colourful plumage, and ease of keeping, ducks are popular in bird collections and many escape each year - a further problem is that many species will mate with a bird of another species if the opportunity arises, creating confusing (but usually infertile) hybrids.