PLATTEEUW M (1980) The moult migration of the Shelduck Tadorna tadorna along the Dutch North Sea coast. LIMOSA 53 (4): 121-128.

During the years 1975-1978 regular seawatching has been carried out in summer from several observation points along the North Sea coast of the Dutch mainland (figure I). The quite impressive northerly moult migration of the Shelduck Tadorna tadorna was one of the most remarkable features of these observations. In each of the years the largest number of migrants passed between 5 and 15 July (figure 2). At about this time the British Shelducks leave their breeding areas, so it may well be assumed that tile migration to the German moulting areas via the Dutch coast takes place in a remarkably short time. In 1976, the year with the highest numbers recorded, a second migratil)n peak occurred at the end of July (figure 2). An extrapolation over the years 1976-1978 suggests that nearly 16% of the British population may pass along the Hondsbossche Zeewering (Noord-Hoiland). The proportion of second year birds, which have not yet bred, decreases rapidly from the beginning of June to the end of July (40-5%). Presumably this phenomenon is caused by earlier departures of non-breeding birds from the vicinities of the breeding areas.
      Along the Hondsbossche Zeewering more migrating Shelducks are seen than from the observation points in Zuid-Holland (figure 3). Probably this is the result of concentration caused by the influence of the Dutch coast. It has been shown that the best visible migration of Shelducks takes place during head winds (figure 5). It therefore seems likely that effects of concentrations mainly occur under this condition. During following winds Shelducks would tend. to fly high and go over land to the Heligoland Bight moulting areas, ignoring the coastline. This route is definitely shorter and flying thus would be more profitable energetically. The fact that, unlike the situation above the English mainland, a high cloud coverage and a moderate visibility (ef figures 6, 7) do not obviously suppress migration tentatively confirms this hypothesis. Common Shelduck Tadorna tadorna

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