THE EFFECTS OF PREY SIZE ON DIET DIFFERENTIATION OF 7 PASSERINE SPECIES AT 2 SPRING STOPOVER SITES

Prey size was evaluated for seven passerine trans-Saharan migrant spec ies at two spring stopover sites in Sardinia, Italy. The species consi dered were Pied Flycatcher Ficedula hypoleuca, Spotted Flycatcher Musc icapa striata, Redstart Phoenicurus phoenicurus, Garden Warbler Sylvia borin, Whitethroat Sylvia communis, Willow Warbler Phylloscopus troch ilus and Wood Warbler Phylloscopus sibilatrix. The analysis was made f or three prominent prey types: beetles (Coleoptera), ants (Hymenoptera : Formicidae) and ''flying prey'' (wasps and bees [Hymenoptera, exclud ing ants] and flies, Diptera, pooled), The prey size distribution in t he diets of some species was very similar to that caught in our insect traps, showing that our estimates of availability are, at least parti ally, relevant, On the whole, diets deviated from food available in a species consistent way, The size distributions of ''flying prey'' diff ered between the two sites but were almost constant in the birds' diet s, In contrast, size distributions of available beetles and ants at th e 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 distri bution of the size classes of at least some prey types, The conclusion s drawn from the comparisons of the size distributions of all prey ite ms 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 wi th and without using information on prey size, With the exception of t he Pied Flycatcher and the Redstart at one of the study sites, size in formation did not add to diet segregation. Certain size classes within prey types tended to be common in the diet of these migrant passerine s. However, specialization on certain size classes within broad taxono mic categories was not evident.