Nesthabitat en broedresultaten van Drentse Boomvalken Falco subbuteo in 1990-98

The breeding status of Hobbies in the province of Drenthe in 1990-98 is summarised and put into perspective. Clutch size was 2x 1, 1x 2, 12x 3 and 2x 4 (mean 2.9/nest), brood size at ringing 4x 1,5x 2,10x3and 1x 4 (mean2.8). Onsetof laying varied between 30 May and 13 June (mean 6 June, N=21, cf. Appendix 1). Of 32 nests in which eggs were laid, 20 fledged one or more young. Breeding in pine woods (in disused nests of Carrion Crows Corvus corone in Scots pine Pinus sylvestris) almost ceased completely during this period. These traditional breeding sites (altogether some 2500 ha with 80 pairs) became progressively depleted during the 1990s, probably because of the high predation rate (mainly by Goshawks Accipiter gentilis, but also by Common Buzzards Buteo buteo, both common predators in this region). It is unknown whether predation as such has been responsible for the disappearance of Hobbies from traditional pinewood-territories, or the risk of predation, or both. Research in farmland, however, showed a widespread occurrence of Hobbies at a much lower density (1 pair/2000-3000 ha). It is argued that -given the large area of farmland (some 160,000 ha)- the supposed population decline may in fact have been a redistribution of territories over a much larger area of farmland, a switch hitherto undetected because very little systematic bird census work is carried out in farmland by competent birdwatchers.