Ducks of remote islands 5 The endemic ducks of remote islands DAVID LACK Introduction with the names and measurements of the resident forms of Anas. On large islands This paper is concerned with all those and on islands near continents many ducks which have formed distinctive sub­ species of ducks coexist. For instance, 13 species restricted to one particular small species, 7 in the genus Anas, breed regu­ and remote island or archipelago. Only larly in Britain (Parslow 1967), 16 species, dabbling ducks in the genus Anas are 6 in the genus Anas, in Iceland (F. involved. Many species in this genus have Gudmundsson pers, com.), 8, 5 in the an extremely wide range, but the average genus Anas, in the Falkland Islands number of subspecies per species is as (Cawkell and Hamilton 1961) and 7, with low as 1.9 for the 38 species recognised 2 more in the nineteenth century, 4 in the by Delacour (1954-64). Hence the fact genus Anas, in New Zealand (Williams that nearly all the ducks on remote small 1964). Some of the Falklands and New islands are separate subspecies means that, Zealand forms are endemic, not so those for ducks, they belong to relatively of British or Iceland. Although the isolated populations. In contrast, on some various species of ducks look superficially of the archipelagos concerned, notably alike, especially when seen together in a the Hawaiian Islands and Galapagos, collection of living waterfowl, they have various of the land birds are distinctive in fact evolved an adaptive radiation species or even genera. Ducks evidently within the limits imposed by their aquatic wander widely and frequently, being life, and even the species of Anas helped by their powerful flight and their evidently differ sufficiently in feeding ability to settle on the sea. habits for up to six to coexist in the same area (Olney 1965). The islands and species concerned In contrast, on the remote small islands set out in Table I, only a single species of The islands concerned, all either tropical duck breeds, on Hawaii, Laysan, Wash­ or subantarctic, are set out in Table I, ington (in the Line archipelago), the Table I. Ducks in the genus Anas on remote islands. male male female weight of egg wing length cidmen body weight in proportion in mm. in mm. in gm. clutch to body weight TROPICAL ISLANDS A. platyrhynchos MALLARD A. p. laysanensis LAYSAN 201 39.5 450 5 10.9% A. p. wyvilliana HAWAII 220 46 500 8 8.5% nominate northern 265 53 1000 11 5.4% A. streperà GADWALL A. p. couesi WASHINGTON (Line Is.) 199 37 ——— nominate northern 271 41.5 —— — A. bahamensis BAHAMA PINTAIL A. b. galapagensis GALAPAGOS 203 42.5 ——— nominate S. America 214 43 ——— A. gibberifrons GREY TEAL A. g. remissa RENNELL (Solomon Is.) 187 33 —— — A. g. gracilis Australasia 202 39 ——— SUBANTARCTIC ISLANDS A. acuta (NORTHERN) PINTAIL A. a. eatoni KERGUELEN & CROZETS 218 32 450 4-5 — nominate northern 271 54 700 8 — A. georgica CHILEAN PINTAIL A. g. georgica SOUTH GEORGIA 217 34 465 5 8.0% A. g. spinicauda S. America 245 42 706 8 6.0% A. castanea CHESTNUT TEAL A. c. aucklandica AUCKLANDS & CAMPBELLS (136) 40 400 3-4 c.18.0% A. c. chlorotis New Zealand 199 44 590 6 10.5% A. c. castanea Australia 218 41.5 500 9 8.0% Notes: A. s. couesi is now extinct. Delacour (1954-64) admitted a separate subspecies near to A. a. eatoni on the Crozets and another near to A. c. aucklandica on Campbell Island, but these cannot be sustained. He also treated A. aucklandica (including the form chlorotis in New Zealand) as a separate species from A. castanea, but I have followed Frith (1967) in regarding them as conspecific. These are the only breeding ducks on the islands concerned except for the Grey Duck A. superciliosa, of which the New Guinea-Polynesian form A. s. pelewensis (male wing 245 mm., culmen 20.5 mm. and female weight 670 gm.) breeds on Rennell, and the New Zealand form A. s. superciliosa (male wing 259 mm., culmen 51.5 mm. and female weight 1000 gm.) breeds on the Aucklands. The measurements of male wing length and male culmen are the middle points between the extremes given by Delacour (1954-64) and the weights are from Lack (1968), assembled by J. Kear. Ducks of remote islands 7 Galapagos, Kerguelen, the Crozets and rather than from one of the continental South Georgia, and only two on Rennell forms independently. and the Aucklands. The second species Delacour (1954-64) followed other on the two latter islands is the Grey Duck authors in treating the Auckland Island A. superciliosa, which is not endemic on Teal as a separate species from the either, and which ranges widely over Australian Chestnut Teal A. castanea, but Polynesia, New Guinea and Australasia, this is a purely taxonomic decision, and and has recently colonised the only im­ it is generally agreed that these two forms portant subantarctic island, Macquarie, are closely related; following Frith (1967) which does not have an endemic duck they are here regarded as conspecific, (Carrick 1957). A. superciliosa is together with the intermediate New Zea­ decidedly larger than the other species land Brown Teal, which is presumably with it on Rennell and the Aucklands the immediate ancestor of the Aucklands (see note to Table I). form. Each of these island ducks belongs At least in Hawaii, Laysan, the Line to a species resident on the nearest main­ islands and the Galapagos, the existence land, except for the unlikely occurrence of only one resident species of duck is of the Northern Pintail A. acuta on Ker­ not due to the failure of other species to guelen and the Crozets, but this island get there. Twelve other species, 8 in the form at times shows the male plumage genus Anas, have occurred in the pattern characteristic of A. acuta, so the Hawaiian archipelago, usually in winter, identification can hardly be doubted. The namely Pintail Anas acuta (regular), Galapagos form of the Bahama Pintail American Wigeon A. americana, Green­ A. bahamensis and the Rennell form of winged Teal A. crecca, Shoveler A. cly­ the Grey Teal A. gibberifrons are much peata (regular), Blue-winged Teal A. dis­ less distinctive than the other island ducks cors, European Wigeon A. penelope, discussed in this paper. Northern Mallard A. p. platyrhynchos Perhaps the most striking point brought (the presumed ancestor of the resident out by Table I is that, while there is only endemic), Garganey A. querquedula, one endemic form on each of the archi­ Lesser and/or European Scaup Aythya pelagos concerned, it is in each case affinis and/or A. marila, Canvasback A. derived from a different mainland species valisineria, Bufflehead Bucephaia albeola (except that both Laysan and adjacent and Ruddy Duck Oxyura jamaicensis Hawaii have the Mallard A. platyrhyn­ (Udvardy 1961a, b; Clapp and Sibley chos). This state of affairs has perhaps 1967), while one migrant duck, Anas dis­ arisen through the historical factor of cors, is regular in the Galapagos which species happened to arrive and to (Leveque et al. 1966). That, despite these become established first. periodic visitors, there is only one resi­ dent species of duck in each of these Differences in ecology and feeding archipelagos presumably means that there is an ecological niche for only one and The continental species of Anas, includ­ that, once established, it can exclude the ing those from which the island forms rest. The resident Gadwall A. streperà on were derived, live primarily in fresh Washington Island is now extinct, and it waters and eat primarily vegetable matter, will be interesting to see whether one of and all except one of the island forms do the other six species ,of Anas recently likewise. Most of them, however, appear recorded from the Line archipelago will to have a wider range of feeding habits become resident there. than their respective mainland forms. The mainland species from which each Thus the A. platyrhynchos of Hawaii eats of these island forms evolved are named not only plant foods but many molluscs in Table I. The only uncertain identifi­ (Delacour 1954-64). The A. acuta of Ker­ cation is that of the Laysan duck as a guelen feeds mainly on inland waters and form of the Mallard A. platyrhynchos, lagoons but also on the sea shore, and but the Hawaiian form is intermediate takes a much higher proportion of animal between them in appearance, and some food, chiefly crustacea, than the mainland males of the latter have the characteristic form (Paulian 1951; Milon and Jouanin plumage pattern of the northern race of 1953). The A. georgica of South Georgia the Mallard. The latter point suggests likewise feeds partly on the shore and that the Hawaiian form is derived from takes more animal food than the mainland the Northern Mallard, not the Mexican form (Murphy 1936), and A. castanea of subspecies, as the latter lacks distinctive the Aucklands often feeds on the sea, in male plumage. The Laysan form could the areas of kelp (Oliver 1955). The ex­ well be derived from the Hawaiian form, ceptional form is A. platyrhynchos of 8 Wildfowl Laysan, which often swims, but which form’s habit, already mentioned, of taking feeds almost entirely on land, on lepidop- many small crustacea. Again, the Auck­ terous larvae, adult Diptera and other lands duck has a narrower beak than the insects picked from the vegetation. Hence Australian and New Zealand forms of A. it has a different and more specialised castanea, but whether this is related to its niche than continental A.