Wat is het predatierisico voor Wespendieven Pernis apivonis in de Nederlandse bossen bij een afnemend voedselaanbod voor Haviken Accipiter gentilis

From the 1980s onwards, and especially since the 1990s, Northern Goshawks breeding in Dutch forests planted on sandy soils in the northern and central Netherlands face serious food-stress, caused by precipitous declines of their main prey species, such as pigeons (including racing pigeons), thrushes, corvids, starlings and rabbits. In a study plot on the Veluwe (110 km², 63% forested), this decline amounted to >75% in biomass (spring census) between the mid-1970s and 2000. In 1970-74, when Goshawk numbers were still small (but increasing), none of 20 Honey-buzzard nests was affected by predation. During 1975-89, when the increasing population of Goshawks levelled off at 30-36 pairs in 1981-90, predation on Honey- buzzards affected between 7.7 and 12.3% of Honey-buzzard nests (summed in 5-year periods). This proportion steeply increased to 17% in 1990-99 and 33% in 2000-04 (Fig. 3), i.e. while the Goshawk population was declining (Table 3). In a study plot in western Drenthe (45 km², 64% forested), a similarly food-stressed and declining population of Goshawks depredated between 25.0 and 30.8% of all Honey-buzzard nests in the three five-year periods between 1990 and 2004 (Fig. 3). Predation involved nestlings, fledglings and/or adults (Fig. 2). Although reproduction of Dutch Honey-buzzards has been consistently poor over the last 15 years (on average 0.45 young/pair, range 0.00- 1.17/year; Table 2), and depredations on adult Honey- buzzards have increased, the Honey-buzzard population is not (yet) in serious decline. A typical example of predation is described for western Drenthe (in 2003). A pair of Honey-buzzards took over a Common Buzzard Buteo buteo nest that had failed in the early chick stage on 13 May 2003. The nest was built in the canopy of a Scots pine Pinus sylvestris, well hidden in dense mixed deciduous-coniferous forest with a visibility at nest level of 30 m at most. The single egg hatched on 10 July, and the fledgling followed the normal growth program until day 32 (for body mass, cf. Fig. 1), with a provisioning rate of 1-2 wasp combs per day (Table 1) and an unknown – but presumably small – number of frogs. From then on, several two-day periods of undernourishment (each time resulting in temporary drops in body mass of 125-240 g) were followed by recovery after realimentation. The female had attended the nestling till day 31, but was not seen at a later date (presumed to have left the breeding area). Flence, the chick was fed by the male only, and was left unattended on the nest until day 41, when it was grabbed by a juvenile female Goshawk (Photos 1 and 2). The Goshawk did not start plucking, but ripped open the ventral side (entrails partly taken out) and commenced eating the muscles from pelvic limb and sternum without killing the Floney-buzzard. Upon disturbance, it tried but failed to lift the Floney-buzzard from the ground, and was forced to leave its prey behind. The Floney- buzzard then died within 5 minutes.