Nachtzwaluwen Caprimulgus europaeus op de Zuidwest-Veluwe

During 1974-1987 Nightjar distribution and breeding biology was studied on the Southwest-Veluwe, an area of 11.443 ha in the central part of The Netherlands (figure 1a). The study area mainly consists of woodland (62%), heaths (15%), sand dunes (1%) and farmland (18%) (figure 1b). The soil is characterized as noncalcareous high podzol with very poor, not-loamy sand. The average yearly temperature is 8.8°C. Annual rainfall in 1974-87 averaged 772 mm, of which 47% dropped during April-September. Of 215 territories mapped in 1974-87,61% was located in sand dunes, 27% in sandy depressions amidst Scots Pines and birches where the woodland had died as a result of flooding in 1968, 9% on sandy heatherland, 2% on dear-fellings and 1% in open Scots Pine forest. Local densities could be very high, viz. 17 pairs on 25 ha of sand dunes and dead forest in 1974. Average nearestneighbour distances in the latter cluster of pairs was 80 m (variation: 30-120 m). Simultaneously occupied nests were found 45, 55 and 60 m apart, probably typical where clusters of pairs occur. However, solitary breeding took place in 26% of the pairs (at least 500 m from the nearest neighbour) (see also figure 3). Several cases of nest-site fidelity were recorded. One nest-site was occupied during 1975-78 (eggs laid at exactly the same spot in each year); in 1979-80 a new nest-site was in use some 15 m from the old site; the roosting site of the male did not change during 1975-80. Another nest-site, also with eggs laid at the same spot, was in use in 1979, 1983 and 1984. Based on various sources, a reconstruction of the qualitative distribution in the fifties (figure 1c) and sixties (figure 1d) was made. Whereas Nightjars were still common throughout the study area in the fifties, a decrease of 50% was found in the sixties. Despite systematic coverage of the study area since 1974, the species showed a further contraction in range in the seventies (-60%, figure 1e) and the eighties (-25%, figure 1f). The present situation seems to be relatively stable after having reached its nadir in 1981 (figure 2). The average number of territories per occupied square of 100 ha decreased from 3.5 in 1974-79 till 1.7 in 1980-87. There is ample evidence that an average density of 3,5 territories/occupied 100 hasquare was applicable for the fifties. The present population amounts therefore to 6% of the number of pairs in the fifties, and this is an optimistic estimate considering the poor coverage of the area in earlier decades! Males arrived earlier on the breeding grounds than females, first dates being respectively 14 April and 16 May. It is thought that the majority of territories becomes occupied in the course of May and early June. Research on the registration efficiency of Nightjars shows that territories within loose colonies are probably occupied earlier than isolated territories (figure 5). The date of departure is poor known. The only observation of a migrating Nightjar was made in broad daylight on 25 August 1984. The last observation of a Nightjar in its breeding habitat was on 3 September 1982. Start of singing was closely related to the period around civil twilight, viz. 10-70 minutes after sunset and before sunrise (figure 4). The actual start of singing was influenced by the absence or presence of clouds. Song and display normally lasted 15-45 minutes. During warm, dry summers (1976!) singing continued all night, except for a lull of several hours around midnight. Of 43 nests found, 9 failed during the egg stage and 7 during the nestling period. Partial losses (3x one addled egg, 2x one young disappeared) were rare. In sum, 49 fledglings were produced, viz. 1.1 young/pair and 1.8 young/succesful pair. Start of breeding was influenced by temperature, with on average a later start of breeding in years in which the warmth sum of May and June did not exceed 850° C (figure 6). Breeding success was also much lower in those years (50%), as compared to warm, dry breeding seasons (62%) (table 1). Double brooding was apparently rare: at least twice in 1976, once in 1982 and possibly once in 1975 and 1986. Indications for double brooding were solely obtained during warm summers, It is thought that weather conditions in The Netherlands are normally unfavourable to permit the production of second broods.