The English word swan is derived from the Indo-European root *swen ('to sound, to sing'), as are the German Schwan, Dutch zwaan, and Swedish svan. Young swans are known as cygnets or swanlings. An adult male is referred to as a cob, which comes from the Middle English word cobbe, which means "leader of a group." An adult female is referred to as a pen.
Swans belong to the Anatidae family, which includes the genus Cygnus. Geese and ducks are the closest cousins of swans. Swans belong to the Anserinae subfamily, where they compose the tribe Cygnini alongside closely related geese. They are sometimes classified as a separate subfamily, Cygninae. Swans are divided into six surviving and several extinct species, as well as a species known as the coscoroba swan, which is no longer regarded a real swan. Swans typically pair for life, however "divorce" does occur sometimes, especially after a failed nesting attempt, and if one spouse dies, the surviving swan will mate with another. Each clutch contains anything from three to eight eggs.
Swans eat both on land and in the water. Although they consume modest quantities of aquatic creatures, they are essentially totally herbivorous. Food is acquired by upending or dipping in the water, and their diet consists of aquatic and submerged plant roots, tubers, stems, and leaves.
Swans are known for mating for life and bonding even before sexual maturity. Trumpeter swans, for example, may live up to 24 years and only begin reproducing when they are 4–7 years old, yet they create monogamous pair bonds as early as 20 months. "Divorce" does happen, albeit it is uncommon; one study of mute swans found a 3% incidence of successful breeding couples and a 9% rate of unsuccessful breeding pairs. Even in gregarious and migratory species like the tundra swan, which assemble in vast groups in the wintering grounds, pair ties are maintained year-round. Swans' nests are roughly a meter wide and on the ground near water. Unlike many other ducks and geese, the male assists in the building of the nest and will take turns incubating the eggs. Swans are the only anatids that will do this, apart from whistling ducks. The average egg size (for the mute swan) is 11374 mm long, 340 g in weight, with a clutch size of 4–7 eggs and a 34–45 day incubation period. The nests of swans are fiercely guarded. They will attack anything, even people, that they regard as a danger to their chicks. One individual was believed to have drowned as a result of the incident. Intraspecific aggressive behavior is more common in swans than interspecific hostile behavior for food and refuge. Bewick's Swans are more aggressive towards other species.
