Lange voedselvluchten bij Buizerds Buteo buteo in het broedseizoen: een gevolg van voedselschaarste?

During observations from tree tops in western Drenthe in late June and early July 2009, three adult Buzzards belonging to two nests were seen transporting a frog to their nest located respectively 1600 and 1000 m from the site of capture (a woodland fen). In the preceding years (1990-2008), maximum foraging flights, as visually observed from tree tops and on the ground (not using radio tags or other devices), ranged between 500 and 800 metres from the nest. The year 2009 was characterised by very poor numbers of voles and mice (Microtus arvalis, Clethrionomys glareolus, Apodemus sylvaticus) and rabbits Oryctolagus cuniculus, and this was reflected in a mammal-deficient diet during the breeding season, a high proportion of nonbreeding Buzzard pairs, poor breeding success and high mortality among nestlings. Food, as assessed from prey remains found during 48 visits to 5 nests, consisted of birds (N=28), frogs and toads (N=B) and grass snakes Matrix natrix (N=3). Never before since 1990, the diet of breeding Buzzards in this region had shown such low diversity, and a complete lack of mammals. However, a tendency of lower prey diversity in Buzzard diets had been apparent already since the early 2000 s, and a decline in the frequency with which prey remains were found on nests had been noted since 1998. The latter coincided with a decline in the number of prey remains found per nest visit (in 1998-2009 only twice above the long-term average for 1990- 2009, the latter based on 1660 visits to 118 nests). These data suggest a long-term negative change in the fortunes of Buzzards in western Drenthe, largely food-driven. This is thought to have been caused by declining biodiversity in farmland and largescale conversion of farmland into set-aside, the latter rapidly being overgrown with dense carpets of rough herbage (preventing access to small mammals). Where the enriched top soil has been removed, prey numbers (moles, voles, birds, earthworms) have dropped considerably, with concomitant problems for raptors, even in a versatile species like the Buzzard. Long-distance foraging flights may therefore punctuate food scarcity.