Swans are from the Anatidae family within the Cygnus group. The nearest relatives of the swans are the geese and the ducks.
Swans are grouped with closely related geese in the Anserinae subfamily, where they join the Cygnini tribe.
Often they are known to be a separate subfamily, Cygninae.
- Domain: Eukaryota
- Kingdom: Animalia
- Phylum: Chordata
- Class: Aves
- Order: Anseriformes
- Family: Anatidae
- Subfamily: Anserinae
- Genus: Cygnus
There are six living and several extinct species of the swan; in comparison, there is a species known as the coscoroba swan which is no longer considered to be one of the true swans.
Swans typically mate for life, but divorce often happens, particularly after a nesting loss, and if a mate dies, the remaining swan will take another. The number of eggs in each clutch can be from three to eight.
Swans are the only current members of the Anatidae family of waterfowl and are among the only aerial birds in the world.
The largest birds, including the mute swan, the trumpet swan and the huge swan, can exceed a length of more than 1,5 m (59 in) and weigh more than 15 kg (33 lb).
Their wingspans can be more than 3.1 m (10 ft). Compared to closely related geese, they are somewhat larger and have proportionally larger legs and necks.
Adults often have a layer of unfeathered skin between their eyes and their bills. Both sexes have identical plumage, but the males are usually larger and heavier than the females.
Swan populations in the Northern Hemisphere have pure white plumage, whereas populations in the Southern Hemisphere are mixed black and white.
The Australian black swan (Cygnus atratus) is completely black except for white flying feathers on its wings; the black swan chicks are light grey. The black-necked Swan of South America has a white body with a black neck.
The legs of the Swans are typically dark blackish-grey, except for the black-necked Swan of South America, that possesses pink legs. Their bill colour varies: the four subarctic species have black bills with different levels of yellow, and all the others have red and black patterns.
While birds do not have teeth, swans have beaks with close edges that appear like tiny jagged ‘teeth’ as part of their beaks used to capture and consume aquatic plants and algae, but also molluscs, tiny fish, frogs and worms.
The mute swan and black-necked swans have lumps at the base of their bills on the upper mandible.
Swans are usually present in temperate areas, seldom present in the tropics. In-flight, a party of swans is called a bevvy or a wedge.
There are four (or five) populations in the Northern Hemisphere, one population in Australia, one endangered species in New Zealand and the Chatham Islands, and one species in Southern South America.
They are absent from tropical Central America, northern South America, Asia, and Africa as a whole. One species, the mute swan, has been introduced in North America, Australia and New Zealand.
Several animals are migratory, either in whole or in part. The mute swan is a partial migrant that lives in western Europe but is entirely migrating to Eastern Europe and Asia.
The whooper swan and the tundra swan are completely migrating, and the trumpeter swans are almost completely migrating.
There is some evidence that the black-necked swan is migrating across some of its territories, but extensive studies have not determined if these movements are long-range or short-range migration.
Swans feed on water and land. They are almost exclusively herbivorous, although small numbers of marine species may be consumed.
In the sea, food is obtained by up-and-down or dabbling, and its diet consists of seeds, tubers, stems and leaves of aquatic and submerged plants.
Swans are considered to be a life partner, and usually unite long before they reach sexual maturity.
Trumpeter swans, for example, who can live for as long as 24 years and only start breeding at the age of 4–7, form monogamous pair bonds as early as 20 months.
‘Divorce,’ while uncommon, occurs; one study of mute swans showing a 3 per cent average for pairs that breed successfully and 9 per cent for pairs that do not.
The pair bonds are established across the year, including in gregarious and migratory species such as the tundra swan, which assemble in large flocks in the wintering grounds.
Swan nests are on the ground above water and around a meter long. Unlike many other ducks and geese, the male tends to build the nest and also takes turns incubating the eggs.
Alongside the whistling ducks, swans are the only ones to do this. The average egg size (for the mute swan) is 113×74 mm, with a weight of 340 g, a clutch size of 4 to 7 and an incubation time of 34–45 days.
Swans are very protective of their eggs. They will viciously attack everything they see as a threat to their chicks, even humans. One man was accused of drowning in such an attack.
The intra-specific aggressive activity of Swans has seen to be more common than inter-specific activity in food and shelter. Aggression with other animals is more apparent in Bewick’s Swans.