Selection of foraging habitat by grassland birds: effect of prey abundance or availability?

Vegetation structure and food availability can significantly modify the composition of farmland avifauna. In the 2006 breeding season we tested the effect of food resources (density of epigeic invertebrates) in two local habitats on foraging of farmland birds. We have been exploring how intensively the foraging birds utilise meadow and pasture habitats in an extensively used farmland area of Central Poland. Two plots were selected in adjacent meadow and pasture each of 0.18 ha where bird and invertebrate sampling was conducted in May 2006. We set five Barber traps active for two weeks at each plot to survey for the epigeic invertebrates which form the main part of farmland birds' diet. In total, we trapped over two thousands invertebrate individuals (mainly Aranea, Coleoptera and Hymenoptera). Visual observations of foraging birds were performed from an elevated observation point located at a 25 in distance from the edge of the plots. We recorded 191 observations of foraging by 12 bird species (mainly Sturnus vulgaris L., Upupa epops L., Cuculus canorus L., Motacilla alba L., Corvus corone L., C. monedula L., Pica pica L.). We found that overall invertebrate density at the meadow was 2.5-fold higher than at the pasture. This was also the case with the invertebrate groups that were preyed upon by the birds. We recorded however a 10-fold higher frequency of foraging of birds at the pasture, as compared to the meadow. Preference of a bird species for foraging at the pasture was inversely correlated with its body mass. We discuss the importance of pastures for the preservation of farmland avifauna.