Swan - Systematics and Evolution

Systematics and Evolution

All 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 (del Hoyo et al., eds, Handbook of the Birds of the World); 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.

Genus Cygnus:

• Subgenus Cygnus
  • Mute Swan, Cygnus olor, is a Eurasian species that occurs at lower latitudes than 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 species, often semi-domesticated descendants of domestic flocks, are naturalized in the United States and elsewhere.

• Subgenus Chenopis
  • Black Swan, Cygnus atratus of Australia, and introduced in New Zealand
    • †New Zealand Swan, Cygnus atratus sumnerensis, an extinct subspecies of the Black Swan from New Zealand and the Chatham Islands
• Subgenus Sthenelides
  • Black-necked Swan, Cygnus melancoryphus of South America
• Subgenus Olor
  • 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 is a small swan that breeds on the North American tundra, further north than the Trumpeter Swan. It winters in the USA.
    • Bewick's Swan, Cygnus (columbianus) bewickii is the Eurasian form that migrates from Arctic Russia to western Europe and eastern Asia (China, Japan) in winter. It is often considered a subspecies of C. columbianus, creating the species Tundra Swan.

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. Among them was the giant Siculo-Maltese C. falconeri, which was taller (though not heavier) than the contemporary local dwarf elephants (Elephas falconeri).

Fossil swans:

• †Cygnus csakvarensis (Late Miocene of Hungary) - formerly Cygnanser
• †Cygnus mariae (Early Pliocene of Wickieup, USA)
• †Cygnus verae (Early Pliocene of Sofia, Bulgaria)
• †Cygnus liskunae (Middle Pliocene of W Mongolia)
• †Cygnus hibbardi (?Early Pleistocene of Idaho, USA)
• †Cygnus sp. (Early Pleistocene of Dursunlu, Turkey: Louchart et al. 1998)
• †Giant Swan, Cygnus falconeri (Middle Pleistocene of Malta and Sicily, Mediterranean)
• †Cygnus paloregonus (Middle Pleistocene of WC USA) - includes "Anser" condoni and C. matthewi
• †Dwarf Swan Cygnus equitum (Middle - Late Pleistocene of Malta and Sicily, Mediterranean)
• †Cygnus lacustris (Late Pleistocene of Lake Eyre region, Australia) - formerly Archaeocygnus
• †Cygnus sp. (Pleistocene of Australia)

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). Anser atavus is sometimes placed in Cygnus.

The Coscoroba Swan (Coscoroba coscoroba) from South America, the only species of 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.