Ardea
Official journal of the Netherlands Ornithologists' Union

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Bourne W.R.P. & Warham J. (1966) Geographical variation in the Giant Petrels of the genus Macronectes. ARDEA 54 (1-2): 45-67
1. The Giant Petrels which breed around the antarctic and subantarctic zones of surface water of the Southern Ocean show small differences in appearance, proportions, and behaviour, and have breeding seasons six to eight weeks apart. The two forms breed alongside each other and remain distinct at Macquarie Island, which lies on the convergence between these two zones of surface water, so it appears that they must be treated, at least there, as distinct, sibling species, showing sympatric character divergence. An intermediate population with a breeding behaviour of the southern type occurs in the Falklands. 2. The name Macronectes giganteus (Gmelin) applies to the southern form. This form breeds on the Antarctic Continent, on the islands of the Scotia Arc north to South Georgia and the Falklands, on Heard Island and Macquarie Island, and possibly on some other islands near the Antarctic Convergence. It is polymorphic, with a pale phase that is white with occasional dark feathers throughout life, and a dark phase which is dark brown when immature becoming grey-brown with a pale head or face when adult, and, at Macquarie Island, with an unmarked greenish to yellowish brown bill. It breeds colonially in open situations late in the year, and young birds are highly migratory. 3. The name Macronectes halli Mathews applies to the northern form. This form breeds in the subantarctic zone of surface water at the Chatham, Auckland, Antipodes, Campbell, Stewart, Macquarie, Kerguelen, Crozet, Marion, and Gough Islands, and possibly elsewhere in this zone as at Amsterdam Island. There is no evidence that it is polymorphic. Young birds are also dark brown; old birds resemble the dark phase of the southern form, but are darker and browner, with dark heads, and a white face with a well-defined mottled dark border. At Macquarie Island breeding adults have yellow-brown bills with dark markings inside the nails. This form tends to nest in August throughout its range, and at Macquarie Island tends to nest alone in sheltered places. There is so far no evidence that it migrates. 4. Populations of Macronectes as a whole conform to recognised zoogeographical or ecogeographical rules, becoming smaller, darker and browner with larger bills and legs in lower latitudes. There is a break in the continuity of this trend between the two species, which vary in a parallel way. It seems possible that white birds congregate in the far south, while it is young birds of the dark phase of this population, which have the darkest plumage of all, which are known to migrate north. The differences in the natural history of the two forms of Giant Petrel seem most likely to be general adaptations for different environmental conditions over different zones of surface water. The differences in the colour and marking of the face and bill of the two forms, which appear to be most marked where they breed together on Macquarie Island, may provide interspecific recognition characters which enable them to recognise each other in their characteristic displays.


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