Neergang van Blauwe Kiekendief en opkomst van Bruine Kiekendief op Schiermonnikoog

From 1978 till 1985 more Hen Harriers than Marsh Harriers hatched on Schiermonnikoog; from 1991 onwards the reverse was true. In the intervening years (1986 – 1990) there is a sharp increase in the number of hatching pairs of the Marsh Harrier, be it with large fluctuations (Figure 1). This is probably related to a massive departure of Marsh Harriers from the Lauwersmeer, after this area had been colonized by the Red Fox from 1984 onwards. In the decline of the Hen Harrier on Schiermonnikoog competition for breeding places with the Marsh Harrier undoubtedly played a part. This competition should however not be overrated. For if this competition is the determining factor, years with few pairs of Hen Harriers would always be years with many Marsh Harriers and vice versa. This appears not to be the case; competition alone, therefore cannot have been the sole determining factor (Figure 2). On the other hand the number of successful hatching pairs of the Marsh Harrier appears to be related to the number of cold days in the month of April. From 1991 onwards, this also applies to the Hen Harrier (Figures 3 and 4). In the period in which the hatchlings of the Hen Harrier were growing up they suffered from rain. The more rainy days in this period, the fewer young in a nest survived (Figure 5). For the Marsh Harrier such a relation was not found. Also the number of Rabbits, an important food for the harriers, appears to be affected by rain in the aforementioned period (Figure 6).