THE INFLUENCE OF STREAM ACIDIFICATION AND RIPARIAN LAND-USE ON THE FEEDING ECOLOGY OF GRAY WAGTAILS MOTACILLA-CINEREA IN WALES

Studies of the feeding ecology of a riparian species, the Grey Wagtail Motacilla cinerea, were undertaken during the breeding season at 21 sites in upland Wales to assess the potential effects of stream acidification and bankside land use. Grey Wagtails foraged in a variety of habitats in and around the river but showed particularly strong selection for riparian shingle. Although the abundance of aquatic invertebrates was significantly reduced in acidic streams by comparison with circumneutral streams, invertebrate abundance in the riparian zone, 5-10 m from the stream, was not. Instead, invertebrate abundances here reflected bankside land use. Water-trap catches were significantly reduced alongside streams through conifer forest by comparison with broadleaved woodland and moorland. Catches by sweep net in most cases were significantly greater alongside streams through broadleaved woodland than in either moorland or conifer. Although the biomass of water-trap catches in moorland and broadleaved woodland was similar, the Grey Wagtail breeding period correlated most closely with the seasonal patterns of prey biomass and prey size along streams through broadleaved habitat. Adult Diptera were overall the most commonly used prey. However, diet reflected land use, with larval Lepidoptera important for nestlings at broadleaved sites, where these prey were most abundant. Aquatic insects contributed a minority of items in the diet (9-33%), being lowest along acid streams. Brood provisioning was not obviously impaired at acidic sites, nor in any type of land use. Recorded load sizes were close to those predicted from energetic calculations. The low reliance on aquatic prey, and absence of any effect of acidification on invertebrate abundances in the riparian zone, help to explain the lack of any obvious relationship between stream acidification and Grey Wagtails. This contrasts with the situation found among Dippers Cinclus cinclus. Enhanced prey abundances along streams in broadleaved woodland, and the close match between the Grey Wagtail breeding season and phenology of prey availability at broadleaved sites, help to explain why Grey Wagtails prefer wooded streams.