Aantalsontwikkeling en reproductie van Drentse Bosuilen Strix aluco

The first breeding case of Tawny Owl in the province of Drenthe (2681 km2, situated in the northern Netherlands) was recorded in 1961. Since then, the population slowly increased to 3-5 pairs in 1970-80, c. 10 pairs in the early 1980s and 80 pairs in the mid-1990s. The majority of the pairs was clustered in the southwestern and northern parts of the province (areas with estates and relatively high proportion of deciduous woodland), and few pairs in between (mostly coniferous woodland, planted in 1910-40). After 1995, the population declined to 50-60 pairs in the late 1990s (Fig. 2). Most breeding cases were repotted from nestboxes (probably biased, as other sites were not checked systematically). Mean onset of laying was 6 March (SD=T 1.8, n=41), ranging from 25 January to 27 March (Appendix 2). Early laying was reported in years with high numbers of Wood Mice Apodemus sylvaticus and Common Voles Microtus arvalis; the reverse was found in years of low abundance of voles and mice, as 1997 (Figs. 1 and 3). Clutch and brood size also depended on food supply (Table 1), with highest values in years with an abundant supply of voles and mice (1996 and 1999). Mean clutch size was 4.0, mean brood size 3.4 and mean number of fledglings per successful pair 3.1 (all pairs: 2.3) (cf. Appendix 2). Mortality after leaving the nest site is high. A well-studied pair bred each year in 1993- 2000, producing 31 eggs, of which 24 hatched. From these 24 hatchlings, 16 fledged. At least 7of the 16 owlets disappeared within several weeks after fledging, mainly because of avian predators (Goshawk Accipiter gentilis and Common Buzzard Buteo buteo) ). This high mortality is also visible in the number of Goshawk and Common Buzzard pluckings found in 4466 ha of mainly woodland in western Drenthe (Table 3): 7 Tawny Owls and 38 Longeared Owls over 1990-2000. Tawny Owls were most often depredated in May-July, involving fledged young. Density of Long-eared Owls in Drenthe is 4-12x higher than of Tawny Owls, but Long-eared Owls were only 5x more numerous as prey of avian predators. It therefore seems that Tawny Owls run a high risk of predation, especially between leaving the nest site and reaching independence. The decline in the Tawny Owl population in Drenthe since the mid-1990s is mainly caused by poor recruitment resulting from high mortality (increase in predation pressure from Goshawk and Common Buzzard in 1990s) in the fledging period.