Het voorkomen van de Zwarte Zeeëend Melanitta nigra langs de Nederlandse kust: een evaluatie

This paper summarizes the occurrence of Common Scoters Melanitta nigra along the Dutch North Sea coast, based on seawatching results. Most birds are seen migrating along the coasts of Noord-Holland and the Wadden Isles (figure 1), where hourly averages peak in March/April. While on the mainland coast the only other significant movements occur in late winter (birds moving S), the seasonal pattern on the Wadden Isles show important soulhwestward movements too in late summer (end of June till August) and in autumn (September, October). Yearly fluctuations in the Noord-Holland spring peak turn out to be considerable (figure 2), but do not seem to be connected with southward movements in previous autumns or winters, thus suggesting that the fluctuations might sooner be caused by some external agent than by fluctuations in wintering numbers. Wind direction and force show clear influences on the migrating behaviour of the Common Scoters, supporting the hypothesis that numbers visible from coastal points are higher during onshore winds than during offshore winds and again higher during tail winds than during head winds (figures 3, 4). Indeed, it is shown that in springs with a high percentage of onshore winds during the peak migration period the numbers of birds passing by is higher (figure 5). Extrapolating the hourly means, it is concluded that some 150,000-200,000 Scoters move northward each spring along the Noord-Holland coast, which is about 34000-89000 more than is currently assumed to be wintering south of the Netherlands. These birds should probably be looked for in NW-African coastal waters.