Jaar van de Nachtzwaluw Caprimulgus europaeus in Drenthe 2007

. In spring and summer 2007 most breeding sites in the province of Drenthe suitable for Nightjars, i.e. larger forests and heaths (Fig.l), were checked for the presence of Nightjar. Areas were visited in May-June and again in July-August, and since weather was favourable in the first period and tolerable in the second, it is assumed that few territories were overlooked. A total of 129 territories was counted, and the population in 2007 was estimated at 130-140 pairs. Territories were evenly distributed across suitable habitat, except for some areas in the north where the species could not be detected (Fig. 2). A reconstruction of past fortunes indicates that Nightjar numbers in Drenthe must have declined in 1950-70; many breeding sites were abandoned during this period. In the 1970s, when large dear-fellings were created following two storms in 1972 and 1973, Nightjar numbers stabilised at a level of about 70 pairs through the early 1980s, when finally young plantations on dear-fellings had matured to such an extent that breeding had become impossible (Fig. 3). Interestingly, in the 1980s Nightjars refrained from switching to breeding on heaths, the very habitat occupied nowadays by Nightjars. The decline reached its nadir in the early 1990s, when only few sites in eastern and south-eastern Drenthe held some pairs. From then on, a recovery became evident, first in eastern and central Drenthe, later on also in the western regions. Presently, the spedes has increased more than five-fold compared with the early 1990s. The annual change in number (but only when a 3-year running mean is used) correlates well with the index of summer weather in The Netherlands (Fig. 4). A similar correlation was found with the rainfall anomaly of the Sahel, where presumably part of the population winters (Fig. 5). These correlations do not explain why the species declined in the first place, nor why presendy habitats like heaths are used as breeding site (and rarely so before the 1990s). Does it imply a change in habitat choice, and hence tapping new resources?