Voedselecologie van overwinterende Klapekster Lanius excubitor op het Groote Zand

From 1988-2002 a Great Grey Shrike wintered on the Groote Zand, a heather field of 100 ha in Northem-Drenthe. Scattered over the undulating field are standing solitary pines, oaks and birches. The vegetation is diverse with Calluna vulgaris, Erica tetralix, Empetrum nigrum and Molinia caerulea as most occurring species. Until 1995 nearly every month the whole area was searched for pellets and prey remains. After 1995 mainly pellets were collected beneath the sleeping tree. The two data sets are incomparable, because the share of vertebrates in the diet will be overestimated when the sample is biased towards pellets. Great Grey Shrikes prey upon nearly all animals with a weight less than 50 g (Table 1). In the course of the winter the proportion of birds in the diet decreases (Fig, 1), Lizards are fed upon mainly in March. Mice, voles and shrews reach the highest proportion in December. Their occurrence in the diet correlates with day lenght (Fig 3.). Probably a short period of daylight forces the shrikes to concentrate on heavy prey. The proportion of Short-tailed Voles as a proportion of all mammals tends to decrease in the course of the study period (Fig. 2). Since Short-tailed Voles are the most common heavy prey, their abundance on heather fields could be an important clue for the quality of the winter territory. During the breeding season vole abundance also proved to be a key factor in reproduction of Great Grey Shrikes in the central part of the Netherlands (Bijlsma 1995).