Direct and indirect effects of pollution on the foraging behaviour of forest passerines during the breeding season

Direct and indirect effects of acid deposition on the foraging behaviour of three forest passerine species, the Crested Tit (Parus cristatus), Long-tailed Tit (Aegithalos caudatus), and Coal Tit (P. ater), during the breeding season were studied. In two areas, one affected by pollution from a nearby coal-fired power station and the other unaffected by pollution, we measured needle density and arthropod availability on tree branches; both factors are recognised as affecting the foraging behaviour of forest birds. Tree-site use and movement patterns of birds searching for prey were also determined in both areas. Branch needle density was lower in the polluted area, as were densities of the main arthropod groups found on needled twigs (Homoptera and Aranae); however, the total abundance of arthropods did not differ between the areas, and some groups even seemed to increase slightly in abundance in the polluted area. Bird behaviour differed between zones in both tree-site use and foraging activities. The Coal Tit showed the greatest number of differences between the areas. The three species showed lower pecking and higher vigilance rates in the polluted area. Higher vigilance rates are interpreted as a response to a higher predation risk due to thinned canopies in polluted forests. The results are discussed in relation to species' breeding performance and ecology. It is concluded that, through increasing levels of stress, the indirect effects of pollution have a negative impact on bird foraging behaviour during the breeding season.