Ardea
Official journal of the Netherlands Ornithologists' Union

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Boere G.C. (1976) The significance of the Dutch Waddenzee in the annual life cycle of arctic, subarctic and boreal waders. Part 1. The function as a moulting area. ARDEA 64 (3-4): 210-291
The subject of this article is the significance of the Dutch Waddenzee as a moulting area for waders. It is part of a general study on the importance of the Waddenzee for hundreds of thousands of arctic, subarctic and boreal waders. The data in the present study come from 6,375 adults and 1996 first year waders, belonging to 21 species. They were caught on the. Isle of Vlieland in the period of December 1971 to December 1975. Emphasis was put on the post-nuptial and pre-nuptial moult, and special attention was paid to wing and tail moult. Body moult was quantified by a simple method and presented in percentages per decade. It appeared that the weight of moulting birds of all species stays at a relatively low level during this period, although it coincides with optimal availability of food in the area. The total moult period of all species ranges from the end of June (Redshank) up to the beginning of December (Oystercatcher), showing differentiations in timing and duration of the post-nuptial moult: e.g. Dunlin 90 days, Bar-tailed Godwit 104 days, Oystercatcher 110 days. Timing and duration of primary moult in the Waddenzee are compared with those of the same species moulting at other localities, especially Morocco and Mauritania. Differences in timing of moult and moult duration were found between second year birds and older and were particularly clear in the Oystercatcher, Turnstone, Grey Plover and Bar-tailed Godwit. Differences in moult duration were found between sexes of Bar-tailed Godwit and Dunlin. It appeared from retraps that the start of the moult period can vary from year to year in individual birds and the species as a whole. This may be due to the variable meteorologic conditions and food situations in the arctic breeding areas, which can cause delay in breeding and subsequently also in moult. Variations in environmental conditions in the breeding areas might be responsible too for annual fluctuations of numbers of birds arriving in the Waddenzee in active or arrested wing moult, as occurred in the Bar-tailed Godwit. No moult was observed in the primaries and secondaries of first year birds and only some first year Turnstones and Redshanks showed some moult of the tail feathers in the autumn. First year birds have a partial body moult in autumn, which occurs a few weeks later than in the adults. Not all individuals of a particular species were caught in moult; they were apparently migrating to other moulting areas. Among these species were Turnstone, Bar-tailed Godwit Grey Plover, Sanderling and Redshank. Only of the latter species it was possible to distinguish these non-moulting birds on sub-specific level, namely Tringa t. totanus. Of these Continental Redshanks, as well as of other common passage migrants like Greenshank, Curlew Sandpiper and Ringed Plover, small numbers actually start their moult in the Waddenzee, but arrest it to complete it elsewhere. Many Grey Plovers, wintering in the Waddenzee, arrest their outer two or three primaries and some secondaries to be moulted in spring. This is probably because of their relatively late onset of the primary moult which coincides with decreasing biological production of the area. Just before moult completion weight of all species increased because of fat accumulation for subsequent migration and/or wintering. Large numbers leave the Waddenzee after moult completion to winter elsewhere, especially the British Isles. Controls of ringed birds showed that individual birds can moult one particular year in the Waddenzee and another year in the British Isles. This shows that both areas are equal as moulting sites, but the carrying capacity differs, at least in the period July until October and during the winter. Considerable numbers of waders use the Waddenzee during spring migration to moult into the breeding plumage. Some species, e.g. Bar-tailed Godwit, are then more numerous than in autumn; others are almost absent, such as the Curlew Sandpiper. In the general discussion, comments are given on several facts which make the Waddenzee so important as a moulting area, such as: a. extensiveness of the area, b. high biological production, c. low predation rate, d. short distance between feeding and resting places, and e. ecological differentiation in prey choice and feeding areas, avoiding interspecific competition; the avoidance being favoured by different moult periods of species moulting in the area. The significance of the Dutch Waddenzee as a moulting area in world perspective is discussed.


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