The effects of prey size on diet differentiation of seven passerine species at two spring stopover sites

Prey size was evaluated for seven passerine trans-Saharan migrant species at two spring stopover sites in Sardinia, Italy. The species considered were Pied Flycatcher Ficedula hypoleuca, Spotted Flycatcher Muscicapa striata, Redstart Phoenicurus phoenicurus, Garden Warbler Sylvia borin, Whitethroat Sylvia communis, Willow Warbler Phylloscopus trochilus and Wood Warbler Phylloscopus sibilatrix. The analysis was made for three prominent prey types: beetles (Coleoptera), ants (Hymenoptera: Formicidae) and “flying prey” (wasps and bees Hymenoptera, excluding ants] and flies, Diptera, pooled), The prey size distribution in the diets of some species was very similar to that caught in our insect traps, showing that our estimates of availability are, at least partially, relevant. On the whole, diets deviated from food available in a species consistent way. The size distributions of “flying prey” differed between the two sites but were almost constant in the birds' diets. In contrast, size distributions of available beetles and ants at the sites were similar but were different in the birds' diets. Different feeding behaviour of the birds, in terms of physiological constraints during migratory stopovers, is discussed. Paired species comparisons show that the diets of most species differ significantly in the distribution of the size classes of at least some prey types. The conclusions drawn from the comparisons of the size distributions of all prey items collected from a bird species in one season are very similar to the conclusions drawn from comparisons based on the presence/absence of a size class per faecal sample. We compared the similarities of diet with and without using information on prey size. With the exception of the Pied Flycatcher and the Redstart at one of the study sites, size information did not add to diet segregation. Certain size classes within prey types tended to be common in the diet of these migrant passerines. However, specialization on certain size classes within broad taxonomic categories was not evident.