De verspreidingsgeschiedenis van zwanen, ganzen en eenden Deel 2: wilde eenden: hoe soorten geografisch met elkaar verbonden zijn, of: hoe soorten zich wereldwijd ontplooein

Descriptions are given of the structural and behavioral characters and of the geographical and ecological distribution (together the zoogeographies! characters) of 14 species of dabbling ducks in the subgenus Anas: Mallard A. platyrhynchos, Mexican Duck A. diazi, Mottled Duck A. fulvigula, American Black Duck A. rubripes, Hawaiian Duck A. wyvilliana, Laysan Duck A. laysanensis, Chinese Spotbill A. zonorhyncha, Indian Spotbill A. poecilorhyncha, Philippine Duck A. luzonica, Pacific Black Duck A. superciliosa, Mariana Duck A. oustaleti, Meller’s Duck A. melleri, Yellow billed Duck A. undulata, African Black Duck A. sparsa (Chapter 1). Some authors treat the Mexican Duck and the Mottled Duck as members of either the Mallard or the American Black Duck (Delacour & Mayr 1945). Others consider the Chinese and Indian Spotbills as conspecifics under the name Anas poecilorhyncha (Vaurie 1965, Ripley 1961), or prefer to lump these with the Philippine Duck and the Pacific Black Duck, also under the name of A. poecilorhyncha (Delacour & Mayr 1945, Johnsgard 1978). When evaluating the interrelationship of the species concerned, the following details have been taken into account: the phenetic comparison of DNA-structures by Sibley & Ahlquist (1990) and Sibley & Monroe (1990, 1993), the cladistical analysis of structure and anatomy by Livezey (1991), the complex courtship displays described by Johnsgard (1965, 1978), and above all the zoogeographical characters of each species. Problems with species limits are apparent with respect to the satellites and relatives of the Mallard (Chapters 2, 3, 6) and the Pacific Black Duck (Chapters 4, 5). As a rule the moderate taxonomic positions by Livezey (1991) and Sibley & Monroe (1993) have been followed. The highly isolated endemic ducks of the Hawaii Islands are treated as established colonists of the Mallard: the Laysan Ducks thought to be the oldest and most obviously modified species, and the Hawaiian Duck, which seems to be of a younger, but nonetheless respectable age (Chapter 6). Data on interspecific hybrids in nature and in captivity are given in each species account (Gray 1958, Gillham & Gillham 1996). Some of these are most surprising, such as a hybrid obtained by Bonhote (1907) in England of the combination Indian Spotbill x Mallard x Pacific Black Duck x Meller’s Duck x Northern Pintail A. acuta (Chapter 7, fig. 13). Apparently hybrids offer poor possibilities for estimating interrelationships of these ducks. Of the species mentioned the Mallard with its occurrence in Europe, Asia and North America has by far the widest distribution. Equally ‘by far’ male Mallards have the most brilliant and colourful breeding or definitive alternating plumage (Chapter 2, 9). The remaining species can boast sober unisex, spotted neutral or feminine plumages only. The Mallard is therefore suggested to be the present climax of the evolutionary development of these ducks. By accepting this theory the origin of all species of the subgenus Anas has been shifted to the southern continents, which are the floating remaining fragments of the early tertiary Gondwana landmass. Particularly the species inhabiting at present New-Zealand, Australia, India and Madagascar must be the direct descendants of the oldest representatives of the group (Chapters 5, 7, 9). Most probably the Bronze-winged Duck Anas specularis of southern South America is the modified descendant of the oldest of these duck species in this part of the broken-up Gondwana continent (Chapter 9). Of the African species mentioned here, one, the unisex-clad Yellow-billed Duck shares the ecological flexibility of the Mallard. The other, the African Black Duck, is specialized for life in clear runnig waters, including wild-water mountain torrents (Chapter 8). The Black Duck is thought to represent the oldest branch from the stem of Gondwa- na-ducks treated here. It has, however, not yet reached the high degree of specialization of the three other known Southern Hemisphere wildwater ducks (Chapter 8). Thus, the ultimate origin of the Mallard and its satellites, relatives and specialists has been suggested to be the primordial southern landmass of Gondwana, as was the case in the swans (Voous 2000). The swan Coscoroba and the Bronze-winged Duck are therefore of comparable great age. Probably it is the Chinese Spottbill that is pointing towards the origin of the Northern Mallard complex (chapter 9).