Voedselkeus van Havik Accipiter gentilis, Sperwer A. nisus en Buizerd Buteo buteo in de Flevopolders

The breeding birds of woodland in the Noordoostpolder and Oostelijk and Zuidelijk Flevoland (totalling 6178 ha) were censused during 1989, 1990 and 1994 (Fig. 1). These forests were planted between 1947-82 in the newly reclaimed polders of the former Lake üsselmeer (mostly on sandy and clayish soils). The raptor population is not yet stabilised here (de Roder, Nap & Bijlsma 1994). During the census, all raptor nests were mapped and checked, breeding success was monitored and food items and pluckings were collected on a regular basis between March and July. Choice of prey of Goshawk (Table I), Sparrowhawk (Table 2) and Common Buzzard (Table 3) did not differ much from the findings elsewhere in The Netherlands. Locally abundant and relevant prey species were captured most often, especially doves, pigeons, thrushes, Jays and Starlings by Goshawks, thrushes and fmches by Sparrowhawks and thrushes, moles and voles by Common Buzzards. The importance of thrushes as prey, as compared to other study areas in The Netherlands, sterns from the high densities of Blackbirds and Song Thrushes in the polder forests (average densities of resp. 81-146 and 64-102 pairs/100 ha in the various forests), On the other hand, hole-nesting birds (especially tits) did not figure prominently in the diet of Sparrowhawks (Appendix I). The forests are still too immature to contain a multitude of natural holes (and nestboxes are scarce) and are therefore not yet suitable for large numbers of hole-nesting birds. The majority of captured prey was juvenile, viz. 60.6% of 287 Goshawk pluckings, 67.1% of 505 Sparrowhawk pluckings and 96.7% of 89 prey remains on Common Buzzard nests (Appendix 1).