Diet composition and prey choice by the Great Grey Shrike Lanius excubitor during the non-breeding period: comparing two methods of diet analysis

Abstract

ABSTRACT Capsule The diet of the Great Grey Shrike Lanius excubitor during the non-breeding period was assessed using analysis of pellets and larders. The two methods gave different results but together showed a preference for voles, crickets and dung beetles. Aims To describe the diet composition of the Great Grey Shrike based on analyses of the contents of pellets and larders; to compare the two methods, and to compare prey taken in relation to its local availability. Methods The research was carried out during the non-breeding period of the Great Grey Shrike in an agricultural landscape of east-central Poland in 2014–2018. The diet was assessed through the analysis of the contents of larders and regurgitated pellets. The abundance of potential prey was assessed with pitfall traps for invertebrates and live traps for small mammals. Results In larders, the ratio of vertebrates to invertebrates was 34.3 : 65.7 (n = 528 prey items), with Orthoptera, Coleoptera and Rodentia predominant. In pellets, the ratio was 64.6 : 35.4 (n = 237 prey items in 155 pellets), with Rodentia and Coleoptera being most numerous. The proportions of the most frequently found prey taxa differed significantly between the two methods of diet analysis. In relation to potential prey availability, Great Grey Shrikes showed a preference for Common Voles Microtus arvalis but seemed to avoid mice Apodemus spp. and Diptera. The greatest discrepancies between the methods were for Coleoptera, Lepidoptera and Orthoptera. Conclusion The results of the pellet and larder analyses differed, mainly because of the large number of Orthoptera found in the larders; shrikes can use them to mark their territories and so they may remain uneaten. Both analytical methods should be used simultaneously to determine the diet composition.