Prey Selection and Social Behaviour in Wagtails (Aves: Motacillidae)

Abstract

The feeding behavior of pied [Motacilla alba yarrellii] and yellow [M. flava flavissima] wagtails, small insectivorous birds, was studied on a meadow near Oxford [England]. Prey taken (mainly adult Diptera) was determined by examination of wing remains in the birds'' feces. Flocks of wagtails were studied feeding on insects on the water surface of flooded pools. Changes in their diet over 10 days were related to changes in the absolute abundance of the preferred prey (Chironomidae) and as the numbers of these decreased, more of the alternative prey (Drosophilidae) were incorporated into the diet to maintain the feeding rate. A constant feeling rate, with all individuals foraging in the same way, may have been the mechanism by which flock structure was maintained. Solitary wagtails were observed exploiting insects on dung pats, where there were 5 main prey. The smallest (Staphilinidae and small Sphaeroceridae) were rejected and one (Sepsidae) was distasteful. The largest (Scatophagidae) were patchily distributed and difficult to capture when in swarms and so the next largest prey (large Sphaeroceridae) were also incorporated into the diet. The size range of Scatophagidae eaten corresponded to the optimum in terms of energy intake per unit handling time. The behavior in leaving patches was as predicted by optimal foraging theory. Despite the great differences in social behavior and prey eaten at the 2 feeding sites, wagtails switched rapidly from one to the other. Sometimes these switches resulted in maximization of energy intake but on other occasions they did not, probably because the need to engage in other activities constrained feeding efficiency considered in terms of energy gain.