De verspreidingsgeschiedenis van zwanen, ganzen en eenden Deel 1: Zwanen als wegwijzers in de zoögeografie

The swans have been chosen as an introduction to a series of papers on the worldwide zoogeography of the Anseriformes. The basic ideas of this were developed in 1973-1974 during a lecture course for biology students at the Free University at Amsterdam. Recent data on taxonomy, ecology and breeding habits have led to a synthesis of historical zoogeography of swans. In this theory the southern hemisphere species take a crucial position: the Black Swan Cygnus atratus from Australia, the Black-necked Swan C. melanocorypha from southern South America and the Coscoroba Coscoroba coscoroba also from the southern tip of South America. The Coscoroba is considered a first, age-old, gooselike branch of the swan-tree (LIVEZEY 1996 and others), rather than a gigantic form of the whistling duck Dendrocygna tribe (DELACOUR & MAYR 1945). The origin of the swans is sought in Gondwanaland, the praetertiary southern landmass, shortly after it had split into the present southern continents. There must have been a time when the continental fragments had drifted not so far as to prevent early Anseriformes from crossing the still narrow sea barriers. These first swans have left descendants in southern South America, New Zealand (Cygnus sumnerensis, extinct in the 17th century) and Australia. None of these southern species has a wholly white plumage. In view of the photoperiodicity of the beginning of egg-laying in the parks of the Wildfowl Trust at Slimbridge, UK, Kear & Murton (1976:85) already considered a southern hemisphere origin of the swans in a ‘mid-high latitude temperate zone’ the most likely option. At present immaculately white swans live exclusively north of the tropics in North America, Europe and Asia. If these birds are also of a southern hemisphere origin, they must have reached the northern land masses through the long, often interrupted continental American highway at an unknown date. The Mute Swan C. olor is in structure and behaviour closest to the southern species. Although it has at present a characteristically palaearctic distribution, a possible relation, C. paloregonus, is known from a pleistocene fossil from Oregon, USA (HILDEGARDE HOWARD 1964). In view of the theory developed above, this kind of fossil could have been expected. The species of ‘singing/honking swans’ can be arranged in two pairs of superspecies: Whooper Swan and Trumpeter Swan C. cygnus and C. buccinator and Bewick’s Swan and Whistling Swan C. bewickii and C. columbianus. On account of their complicated and ingeniously convoluted trachea, which allow for an impressive resonant honking, these species can be considered to form the recent peak of swan development. Opinions diverge on the question of whether the Old World arctic Bewick’s Swan and the New World Whistling Swan represent one biological species (Tundra Swan), or whether they should be treated as two parapatric species. Mixed breeding pairs with cygnets, and individual swans with intermediate bill coloration, have been reported from northeastern Siberia and elsewhere. One thing seems clear, Bewick’s Swan is the most daring colonist-adventurer by nesting in such inhospitable surroundings as the marshy coast-tundra of northernmost Russia and Siberia. The unpredictable summer weather, already of notably short duration, the exhausting energy needs of the lenght of the migration route, and the unreliable food resource in the wintering areas hardly offer Bewick’s Swan a secure base for a continued existence as a species. Of all swans Bewick’s seems to run the greatest risk of becoming extinct in the geologically near future. The southern hemisphere origin of the diversity of all Anseriformes will be the main theme of the following chapters.