Diet, condition and site fidelity of wintering Great Grey Shrikes Lanius excubitor in the Netherlands: Is there a relationship with grazing?

In this paper the results of an ongoing study since 2002 on wintering Great Grey Shrikes in two areas in the Netherlands are presented. We assessed diet (based on 7, 632 pellets, of which 90% was found under roosting trees), site fidelity (based on resightings of 48 colour-ringed individuals) and condition (body mass at ringing, corrected for wing length) in relation to the type of grazing management in the shrikes' winter territories. Body mass of ringed Great Grey Shrikes increased with wing length, but we found no relation between condition and age of the bird, date or time of ringing or weather during the two preceding weeks. Average condition of shrikes in winter territories grazed by cattle was significantly higher (by 3%) than in ungrazed territories (Fig. 1), but this depended on one extreme individual in a grazed site. The diet of wintering Great Grey Shrikes differed strongly between study sites (Fig. 2). In wet heathlands 45% of the prey items were voles and mice (mainly Micromys minutus), in dry heathlands only 29% (mainly Microtus arvalis and Clethrionomys glareolus). Based on prey weights, voles and mice formed 64% and 77% of the diet in wet and dry heathlands, respectively (Fig. 3). Dung beetles (mainly Typhaeus typhoeus) were frequently caught particularly in dry heathlands (42% of prey items), but were not important in terms of weight (<5%). Diets differed only marginally between grazed and ungrazed sites (Fig. 4). Great Grey Shrikes exhibited a high fidelity to their winter territories: only one of the 17 birds resighted during a following winter was found more than 2 km from the ringing locality. Also within winters only 4 out of 40 individuals were resighted further than 2 km away from the ringing site.