Voorjaarstrek van Dwergmeeuwen Larus minutus langs de Noordzeekust
Sula , Volume 4 - Issue 3 p. 90- 98
Of the three distinct breeding populations of Little Gulls, only the most westerly is likely to pass regidarly through W. Europe on the way to or from important wintering areas in the Western Mediterranean, along the coast of Brittany (France) or at the IJsselmeer (Netherlands). Some, scanty, information on population size is available only for the Baltic and Finland (3000-4000 pairs) and there is no information at all on yearly fluctuations in numbers. Since the yearly spring seawatches, carried out along the Dutch North Sea coast (1974-1987) invariably show important movements of this species, concentrated strongly within a short period of time (April/May, cf. figures 1, 2), it was considered worthwhile to make a tentative analysis of the amount of migration and the yearly fluctuations. The number of Little Gulls passing by proved to be considerable, extrapolated numbers fluctuating in recent years between 5000 and almost 30,000 birds at the Noord-Holland coast (figure 3). The numbers passing by in the ’best’ years (1981, 1984 and 1985) are by far larger than the population estimates mentioned before, so it is suggested that the rest of NW. Russia still holds a substantial but unknown number of breeding birds. Analyzing the yearly fluctuations, it was found that in springs with predominating S- and SW-winds relatively few Little Gulls were seen (figure 4). It is suggested that in these conditions the predominance of tail winds may favour the shorter but more hazardous overland route described in previous studies, following the larger rivers up from the spring staging area at the Camargue (France). Confrontedd with head winds, however, birds might prefer to follow the coastline, which permits feeding “en route”. Yearly fluctuations did not correlate positively with the percentage of immature birds (compare figures 3 and 5), suggesting that population dynamics did not influence the migrating numbers. It seems even more likely that during the ’best’ springs a higher proportion of adult birds passed by. Adult birds seem to have a choice between two alternative routes: the overland route and the coastal route. We suggest tentatively that a predominance of head winds in April/May favours the choice for the latter.
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