Analyses on prey composition of overwintering Great Grey Shrikes Lanius excubitor in southern Finland

Abstract

Diet composition of Great Grey Shrikes Lanius excubitor was studied during nine winter seasons (October-March) in southern Finland. In pellets, the main preys were Microtus-voles (35.4% of vertebrates by number), the harvest mouse Micromys minutus (27.5%), the common shrew Sorex araneus (14.8%), the house mouse Mus musculus (7.0%) and birds (7.0%). Invertebrates constituted about 0.1-1.7% of the total prey biomass. The shrikes hunted birds in an opportunistic manner, no preference for larger species within the suitable size-range was evident but tits dominated in the diet (61.2%). Compared to other species this was not due to favourable availability or prey selection but rather a result of higher vulnerability. The relationship between diet diversity and prey availability was studied using snow cover as an indicator for reduced availability of small mammals. Accordingly, the use of voles decreased in midwinter and during periods with snow, whereas birds and especially mice showed a corresponding increase. However, the overall vertebrate prey diversity was only slightly increased. A comparison between studies in northern and central Europe revealed a higher vertebrate prey breadth in the north. These observations are discussed in the light of optimal foraging theories predicting increased prey diversity in conditions with lower availability.