Onderzoek naar overwinterende Torenvalken Falco tinnunculus en Buizerds Buteo buteo in Midden Zeeuws-Vlaanderen: werkwijze en aantallen

During waterbird surveys in midwinter in the province of Zeeland, most observers also counted raptors and other species along the side. Experiments showed that such surveys, being not specifically focused on raptors, missed 20-45% of Eurasian Kestrels and 15-30% of Common Buzzards present in the area. Therefore, a specific method for counting wintering raptors was developed, primarily based on mapping during at least four complete surveys in December and January (preferably on bike), stops and scans of the environment every 250-500 m and recording age, sex and plumage whenever possible (Appendix 1). These records are entered on maps in the field, and separately analysed on species maps at home, using standardised criteria for enumerating the number of individuals involved (taking note of age, sex and plumage). Although not yet tested, it seems that this method suffices in farmland but fails in areas with a scattering of orchards, woodlots and woods (raptors difficult to detect). The impact of extreme weather conditions on the detectability of wintering raptors is normally small (as noticed during a serious drop in temperature in December 1996), but heavy snowfall may force raptors to occupy more secluded sites (hence difficult to detect; this situation did not occur yet). Wintering raptors were counted in the central section of Zeeuws-Vlaanderen in the southwestern Netherlands. It consists of a conglomerate of 44 polders, varying in size between 13 and 1500 ha, and totalling 83 km² (77% arable, 10% industrial, 5% grassland, 4% open water, 3% woodland and 1% orchard). Most polders are diked (total length of dikes >100 km). Local winter weather is normally mild, with a mean January temperature of 3.8°C over 1971-2000. The wintering number of Eurasian Kestrels varied between 35 and 76/100 km² in 1993-99 (Table 1), those of Common Buzzards between 24 and 64/100 km² (Table 2). Comparing the number of wintering birds to the number of breeding Kestrels, it seems that at least part of the breeding population departs from the breeding area. On the other hand, at least half of the wintering population of Common Buzzards apparently consists of immigrants.