Onderzoek naar Sperwers Accipiter nisus in westelijk Noord-Brabant in 1993-97

Preliminary data are presented concerning a study of Sparrowhawks in two adjacent areas in western Noord-Brabant: a 1100 ha large woodland is compared with open farmland (22.900 ha) in which woodlots make up c. 450 ha. The latter area started to become occupied by breeding Sparrowhawks from 1992 onwards. Both areas differed only slightly in proportion of egg-laying pairs (96% in both areas), onset of laying (30 April and 3 May respectively in woodland and farmland), clutch size (4.5 and 4.6 resp.), brood size (3.8 and 4.0), breeding success (2.6 and 2.9 fledglings per nest) and proportion of first-year birds among breeding males (17.9 and 24.5%) and females (25.5 and 27.0%)(Table 1), as well as diet (Table 3). Causes of failure were mainly desertion (including disappearance) in woodland, and predation and weather in farmland (Table 2), the latter caused by the fact that many pairs breed in deciduous trees (less coverage, vulnerable to strong winds). Nestlings ringed in woodland had a higher recovery rate than those in farmland (Table 4), mostly because a higher proportion of woodland nestlings remained in the vicinity of the natal site. Over 1993-97, 18% of 33 females in woodland bred more than once, i.e. 2x 2, 2x 3, 1x 4 and 1x 5 years; this group of females produced 48% of all fledglings. In farmland, 10% of 57 females bred more than once, i.e. 4x 2, lx 3 and 1x 4 years, producing 35% of all fledglings. The data so far suggest only small differences in breeding performance between both habitats, but a higher turnover and more competition in farmland compared to woodland.