LENSINK R (1986) Migration of Crossbills Loxia curvirostra over the Netherlands during the invasion in 1983. LIMOSA 59 (3): 105-110.

Common Crossbill Loxia curvirostra In this paper, the invasion of Crossbills in the Netherlands in the summer and autumn of 1983 is discussed, based on systematic observations of migrating birds (fig. I, tab. 1). Firstly, the seasonal and geografical patterns based on simultaneous countings over the country (25 counting sites), mainly during the early morning hours of nine days, are described. Most birds were seen at the end of August, the end of September, and the second half of October (fig. 2), and in the central part of the Netherlands, which is covered with a large area of coniferous woodlands (fig. 3). Secondly, the geografical and seasonal patterns, based on frequent observations over the entire season on seven observations points are described (lJ2 hour before to 2 hours after sunrise, post 12 the entire day). Based on these data, several peaks could be recognized (fig. 4), resulting in two distinct migration periods, viz. 17 June through up to 2 September and from 3 September through 30 November. Non-systematical observations in several parts of the country showed that the invasion started already in the end of May. Median dates of migrations, extrapolated totals per season, and the maximum numbers for each period are given in tab. 2. Again, most intense migration was observed in the central part of the Netherlands. The large numbers in the central part of the country might partly be a result of a relatively low altitude of migration caused by the attractive landscape there. Migration peaks were highly synchronized for all parts of the Netherlands. The median dates of migration, however, gradually shifted in westerly and southeasterly direction, either after fattening up of birds or after depletion of food resources. It may be that birds in SE. Netherlands originate partly from other populations than those seen elsewhere. The peaks were one or two weeks earlier than in Belgium, France, and England. The origin of the invasion must be found in Scandinavia and Russia. The peak in May-June may be a result of overpopulation in Scandinavia, the peaks in July-August and September-November of food shortage in Scandinavia and Russia, respectively. The numbers of birds participating in the invasion were probably larger than during the invasion in 1979 (tab. 3).

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