Swan

Etymology and terminology

The English word 'swan', akin to the German Schwan, Dutch zwaan and Swedish svan, is derived from Indo-European root *swen (to sound, to sing). Young swans are known as swanlings or as cygnets; the latter derives via Old French cigne or cisne (diminutive suffix -et "little") from the Latin word cygnus, a variant form of cycnus "swan", itself from the Greek κύκνος kýknos, a word of the same meaning. An adult male is a cob, from Middle English cobbe (leader of a group); an adult female is a pen.

Description

Swans are the largest extant members of the waterfowl family Anatidae, and are among the largest flying birds. The largest species, including the mute swan, trumpeter swan, and whooper swan, can reach a length of over 1.5 m (59 in) and weigh over 15 kg (33 lb). Their wingspans can be over 3.1 m (10 ft). Compared to the closely related geese, they are much larger and have proportionally larger feet and necks. Quite unusual for birds, swans have "teeth" - jagged parts of their bill that are used for catching and eating fish. Adults also have a patch of unfeathered skin between the eyes and bill. The sexes are alike in plumage, but males are generally bigger and heavier than females.

The Northern Hemisphere species of swan have pure white plumage, but the Southern Hemisphere species are mixed black and white. The Australian black swan (Cygnus atratus) is completely black except for the white flight feathers on its wings; the chicks of black swans are light grey. The South American black-necked swan has a white body with a black neck.

Swans' legs are normally a dark blackish grey colour, except for the South American black-necked swan, which has pink legs. Bill colour varies: the four subarctic species have black bills with varying amounts of yellow, and all the others are patterned red and black. Although birds do not have teeth, swans have beaks with serrated edges that look like small jagged 'teeth' as part of their beaks used for catching and eating aquatic plants and algae, but also molluscs, small fish, frogs, and worms. The mute swan and black-necked swan have lumps at the base of their bills on the upper mandible.

Distribution and movements

Swans are generally found in temperate environments, rarely occurring in the tropics. A group of swans is called a bevy or a wedge in flight. Four (or five) species occur in the Northern Hemisphere, one species is found in Australia, one extinct species was found in New Zealand and the Chatham Islands, and one species is distributed in southern South America. They are absent from tropical Asia, Central America, northern South America and the entirety of Africa. One species, the mute swan, has been introduced to North America, Australia and New Zealand.

Several species are migratory, either wholly or partly so. The mute swan is a partial migrant, being resident over areas of Western Europe but wholly migratory in Eastern Europe and Asia. The whooper swan and tundra swan are wholly migratory, and the trumpeter swans are almost entirely migratory. There is some evidence that the black-necked swan is migratory over part of its range, but detailed studies have not established whether these movements are long or short range migration.

Behaviour

Swans feed in water and on land. They are almost entirely herbivorous, although they may eat small amounts of aquatic animals. In the water, food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants.

Swans famously mate for life, and typically bond even before they reach sexual maturity. Trumpeter swans, for example, who can live 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", though rare, does occur; one study of mute swans showing a 3% rate for pairs that breed successfully and 9% for pairs that do not. The pair bonds are maintained year-round, even in gregarious and migratory species like the tundra swan, which congregate in large flocks in the wintering grounds. Swan's nests are on the ground near water and about a metre across. Unlike many other ducks and geese, the male helps with the nest construction, and also take turns incubating the eggs, and alongside the whistling ducks are the only anatids that will do this. Average egg size (for the mute swan) is 113×74 mm, weighing 340 g, in a clutch size of 4 to 7, and an incubation period of 34–45 days. Swans are highly protective of their nests. They will viciously attack anything that they perceive as a threat to their chicks, including humans. One man was suspected to have drowned in such an attack.

Systematics and evolution

Evidence suggests that the genus Cygnus evolved in Europe or western Eurasia during the Miocene, spreading all over the Northern Hemisphere until the Pliocene. When the southern species branched off is not known. The mute swan apparently is closest to the Southern Hemisphere Cygnus; its habits of carrying the neck curved (not straight) and the wings fluffed (not flush) as well as its bill color and knob indicate that its closest living relative is the black swan. Given the biogeography and appearance of the subgenus Olor it seems likely that these are of a more recent origin, as evidence shows by their modern ranges (which were mostly uninhabitable during the last ice age) and great similarity between the taxa.

Phylogeny

Species

Genus Cygnus

• Subgenus Olor
  • Mute swan, Cygnus olor, is a Eurasian species that occurs at lower latitudes than the whooper swan and Bewick's swan across Europe into southern Russia, China and the Russian Maritimes. Recent fossil records, according to the British Ornithological Union, show Cygnus olor is among the oldest bird species still extant and it has been upgraded to "native" status in several European countries, since this bird has been found in fossil and bog specimens dating back thousands of years. Common temperate Eurasian birds, often semi-domesticated descendants of domestic flocks, are naturalized in the United States and elsewhere.
• Subgenus Chenopis
  • Black swan, Cygnus atratus of Australia, introduced into New Zealand and the Chatham Islands
• Subgenus Sthenelides
  • Black-necked swan, Cygnus melancoryphus of South America
• Subgenus Cygnus
  • Whooper swan, Cygnus cygnus breeds in Iceland and subarctic Europe and Asia, migrating to temperate Europe and Asia in winter
  • Trumpeter swan, Cygnus buccinator is the largest North American swan. Very similar to the whooper swan (and sometimes treated as a subspecies of it), it was hunted almost to extinction, but has since recovered.
  • Tundra swan, Cygnus columbianus breeds on the Arctic tundra and winters in more temperate regions of Eurasia and North America. It consists of two forms, generally considered to be subspecies.
    • Bewick's swan, Cygnus (columbianus) bewickii is the Eurasian form that migrates from Arctic Russia to western Europe and eastern Asia (China, Korea, Japan) in winter.
    • Whistling swan, Cygnus (columbianus) columbianus is the North American form.

The coscoroba swan (Coscoroba coscoroba) from South America, the only species in its genus, is apparently not a true swan. Its phylogenetic position is not fully resolved; it is in some aspects more similar to geese and shelducks.

Fossil record

The fossil record of the genus Cygnus is quite impressive, although allocation to the subgenera is often tentative; as indicated above, at least the early forms probably belong to the C. olor – Southern Hemisphere lineage, whereas the Pleistocene taxa from North America would be placed in Olor. A number of prehistoric species have been described, mostly from the Northern Hemisphere. In the Mediterranean, the leg bones of the giant swan (C. falconeri) were found on the islands of Malta and Sicily; it may have been over 2 meters from tail to bill, which was taller (though not heavier) than the contemporary local dwarf elephants (Palaeoloxodon falconeri).

• Subgenus Chenopis
  • †New Zealand swan, Cygnus sumnerensis, an extinct species related to the black swan of Australia

• Other subgenera (see above):
  • †Cygnus csakvarensis Lambrecht 1933 (Late Miocene of Hungary)
  • †Cygnus mariae Bickart 1990 (Early Pliocene of Wickieup, U.S.)
  • †Cygnus verae Boev 2000 (Early Pliocene of Sofia, Bulgaria)
  • †Cygnus liskunae (Kuročkin 1976) (Middle Pliocene of western Mongolia)
  • †Cygnus hibbardi Brodkorb 1958 (?Early Pleistocene of Idaho, U.S.)
  • †Cygnus sp. Louchart et al. 1998 (Early Pleistocene of Dursunlu, Turkey)
  • †Giant swan (Cygnus falconeri) Parker 1865 sensu Livezey 1997a (Middle Pleistocene of Malta and Sicily, Mediterranean)
  • †Cygnus paloregonus Cope 1878 (Middle Pleistocene of west-central U.S.)
  • †Dwarf swan (Cygnus equitum) Bate 1916 sensu Livezey 1997 (Middle – Late Pleistocene of Malta and Sicily, Mediterranean)
  • †Cygnus lacustris (De Vis 1905) (Late Pleistocene of the Lake Eyre region, Australia)
  • †Cygnus sp. (Pleistocene of Australia)
  • †Cygnus atavus (Fraas 1870) Mlíkovský 1992

The supposed fossil swans "Cygnus" bilinicus and "Cygnus" herrenthalsi were, respectively, a stork and some large bird of unknown affinity (due to the bad state of preservation of the referred material).