Anatidae is a biological family of birds that includes ducks, geese and swans. The family has a cosmopolitan distribution, occurring on all the world’s continents except Antarctica and on most of the world’s islands and island groups. Many anatids are migratory, although tropical and subtropical species remain close to breeding grounds during non-breeding season. All spacies are aquatic, with webbed feet and a broad, flattened bill. They range in size from the small teals (weighing 250 – 300 g) to the swans, which may weigh 15 kg or more. The family contains around 146 species.