Feeding habits of Great Grey Shrike (Lanius excubitor) wintering in north-eastern Poland: does prey abundance affect selection of prey size?

Abstract

Diet composition and prey selection of wintering Great Grey Shrike (Lanius excubitor) were studied at two sites in north-eastern Poland: in the Mazurian Lakeland and at Bialowieza Glade. The feeding habits of shrikes were recorded on the basis of 1582 prey items found in 1220 pellets, collected in years 1998-2003. Live-trapping of small mammals showed that in both habitats, Microtus voles predominated in rodent communities, with higher densities of the Common Vole (M. arvalis) in the Mazurian Lakeland, and the Root Vole W. oeconomus) in Bialowieza. At both sites, the two main shrike prey groups were small mammals and insects, with a lesser contribution of other prey (passerines, lizards and frogs). Voles comprised between 69 and 86% of consumed food biomass, while other small mammals comprised only 5-13% of prey biomass. Shrikes selectively hunted the Common Vole, especially in the habitat with a lower abundance of this species. On the other hand, in the habitat with a higher abundance of Common Voles, shrikes selected smaller individuals from this population, a feature that was not recorded in the habitat where the Root Vole was predominant. Therefore, intra- and inter-specific prey selection appears to be interrelated and dependent on prey abundance. The prey stored (here, prey impaled on thorns and spikes) by shrikes differed from the prey consumed. The Root Vole was not stored but occurred in the shrike diet. Frogs, newts and lizards were more often stored than consumed. Analysis of the proportion of skulls, pelvises and thigh-bones in pellets, suggests that at the end of winter and in early spring when voles numbers decline, shrikes consumed more whole vole bodies compared to the autumn.