RIJNSDORP AD (1984) Distribution, seasonal numbers, and population trend of wintering Wigeons Anas penelope in the Netherlands. LIMOSA 57 (1): 1-6.

Based on the results of a questionnaire sent to birdwatchers all over the country, the distribution of Wigeon in the Netherlands is described (fig. 1). Important haunts occur in the coastal Waddenzee and Delta area, and in inland pasture areas in Friesland and Noord-Holland. The seasonal distribution in the Netherlands (fig. 2) and NW. Europe (fig. 3) is affected both by the geographical position and by the type of feeding habitat.
      In autumn, Wigeon visit predominantly coastal sites and saltmarshes; in winter and spring they concentrate in inland pasture areas. In the Waddenzee, numbers peak in October-November, in the bordering inland pasture areas in Friesland in February-March (fig. 2). Averaged over the period September-March, Wigeon spend about 50% of their time in coastal areas and 50% in inland pasture areas (fig. 4). In Lake Grevelingen (Delta area) where Wigeon feed on eelgrass Zostera marina, numbers peak in October, whereas they visit the surrounding saltmarshes mainly during the winter (table 1). In 1974-78 an average of nearly 400 000 Wigeon wintered in the Netherlands spending here a total of 64.5 million Wigeon days per winter.
      To study the development of the population size of wintering Wigeon in the Netherlands a January index was calculated for the period 1957-81 (fig. 5). From 1967 onwards the index compares the total number of Wigeon counted on a number of sites in year i with that in the same sites in the reference year 1973. Because not all sites were counted each year, between 224 and 340 sites could be compared. For the period 1957-66 the index is based on the ITBON counts which were much less extensive (9-33 sites). With a geometric mean regression of the ITBON index on the midwinter index, January index values for the earlier years were estimated. In 1957-73, numbers of Wigeon fluctuated around a constant level (fig. 5). In 1974 the number started to increase. In 1978 the January index reached a peak value of more than four times the index value in the reference year. In the severe winter of 1978/79 numbers dropped to the 1973 level. In 1980-81 the index again reached the high values of 1977-78.
      Two hypotheses can explain the increase in the January index: (1) a northward shift of the centre of the winter distribution, (2) an increase in the size of the NW European flyway population. A comparison of the seasonal distribution of Wigeon before and during the increase of the January index (fig. 6) indicates that the increase can be explained partially with hypothesis 1. However, the increase in the January index in the Waddenzee area (fig. 7) together with a lack of change in the seasonal distribution in that area (fig. 6) indicates also an increase in the actual population size. Eurasian Wigeon Anas penelope

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