Ardea
Official journal of the Netherlands Ornithologists' Union

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Piersma T. (1988) The annual moult cycle of Great Crested Grebes. ARDEA 76 (1): 82-95
This study analyses the moult patterns of all feather tracts of Great Crested Grebes throughout the non-breeding season. It is based on an examination of about 1000 carcasses of grebes which had accidentally drowned in fishing nets in Lake IJsselmeer, The Netherlands from August 1978 to December 1985. No differences were detected in the body moult schedules of adult males and females. From August through December, but not from January to March, first winter birds showed patterns which were slightly different from the adults. The moult of all feather tracts was very intensive in August-October. Most body feather tracts showed little moult in midwinter, but the flanks never stopped moulting. In late winter and early spring, only the head and neck showed a (second) peak in moult intensity (i.e. the acquirement of a summer plumage). A peculiar 'autumnal head plumage' is described: a head plumage which looks like a pale adult breeding plumage with shorter crest and tippets, which consists of a different feather generation. The simultaneous moult of the flight feathers and, concurrently, the moult of the assembly of wing coverts, takes place in the period August-first half October, with much individual variation but no sexual differences in the timing of this moult. It was calculated that it takes an individual grebe about 17 days to complete the wing moult, which is less than was hitherto assumed. The upper wing coverts are moulted slightly earlier than the under wing coverts. In the discussion, the possible functional correlates of the moulting patterns and the biology of the grebes are explored. It is suggested that the maintenance of moulting flank feathers throughout the winter, is to provide the birds with a continuing resource of ingestible feathers, which may be an important digestive aid. A feather deposition rate (g feather mass1day) was calculated, to examine the annual changes in total moult intensity. Within a year, feather deposition rates vary with a factor 8.5. The highest values for the feather deposition rate (c. 0.85 g/ day) are much like those reported for other species if body mass differences are taken into account. Grebes do not moult most heavily when thermal conditions are optimal, but a little later. This may relate to the extended breeding season of this species, or to a late built up of profitable fish stocks.


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