Het voorkomen van Nachtzwaluwen Caprimulgus europaeus als broedvogel op de Zuidoost-Veluwe

The occurence of the Nightjar Caprimulgus europaeus as a breeding bird on the Southeast-Veluwe, the Netherlands In the period 1974-1987 the Nightjars on the Southeast-Veluwe (the Netherlands) were censused. This area comprises mostly wood, some large heatherfields and a small area with sanddrifts. Along the borders some agricultural fields are situated (Figures 1,4, table 1). The soils are sandy, dry and originate from the Saale glacial period. In the medieval times most of the forest disappeared and heather took over. Later on also the heather disappeared because of overexploitation by men. In these times large sanddrifts developed (Figure 4). The podsolic soiltypes are the most common (Figure 3). In the former sanddrifts hardly any soil development has taken place since. The census in 1974 was quite extensive (Figure 7). We estimate 30-50 pairs for that period. From 1979 onwards nearly every year the whole area was investigated with regards to Nightjars. Since 1974 the numbers reduced to 15-20 pairs in 1984-1985 (Table 2). In 1986 and 1987 an increase was noticed. The most stable population occurred in a former sanddrift (Table 3), which is nowadays partly heather (mainly Calluna vulgaris), partly primary pine forest (Pinus sylvestris) with bare sandy patches. The population of Nightjars fluctuated more on large dear-cuts and in old open pine forests (Figure 10, 11) (Table 3). Most Nightjars were found in the centre of the census area. Only a few one kilometer squares comprised more than 5 upto 7 pairs as a maximum (Figure 9). In most of the kilometer squares they were not found every year (Figure 8). The distribution of Nightjars as shown in the Figures 8 and 9 is quite similar to that of the former sanddrifts (Figure 3), as far as heatherfields occur (Figure 4). Most pairs were located at former sanddrifts, then secondly on sandy heatherfields, less on large clear-cuts and in old open pine forests (Table 4, Figure 9). The density was the highest at the former sanddrifts and the lowest in the old open pine forests (Table 5). In some small areas the density could rise up to more than 6 pairs/100 ha. In the period 1930-1950 the Nightjar was quite common in the census area, they even breeded near the city of Arnhem (Figure 5). We estimate for this period at least 100 pairs in the census area. In 1959 the population was estimated to be at least 70 pairs (Figure 6), Since World War 11, probably earlier, the population decreased. The main factors were the changes in the environment of the heatherfields and forests. Since the afforestation of sanddrifts and heatherfields the environment got more and more the character of forest instead of heather. During the last few decades the acid rain caused an increase in the productivity of the ecosystems. This caused a decrease in the number of plant species, fungi and mosses, and probably the number of insects which are characteristic for this type of community. This may have had an adverse effect on the Nightjar, Recreation was a second reason for the decline. Since 1974 the northeastern and southern parts were not occupied anymore because of increased recreational activities (cf. Figures 6,7, 8). As a third reason human activities like towndevelopment (cf. Figures 5,8) and habitat destruction caused a decline. The increase in number of territories since 1986 is hopeful. We wonder if this will continue.