Parid foraging choices in urban habitat and their consequences for fitness

Urban environments are habitat mosaics, often with an abundance of exotic flora, and represent complex problems for foraging arboreal birds. In this study, we used compositional analysis to test how Blue Tits Cyanistes caeruleus and Great Tits Parus major used heterogeneous urban habitat, with the aim of establishing whether breeding birds were selective in the habitat they used when foraging and particularly how they responded to non-native trees and shrubs. We also tested whether they showed foraging preferences for certain plant taxa, such as oak Quercus, which are important to their breeding performance in native woodland. Additionally, we used mixed models to test the impact these different habitat types had on breeding success (expressed as mean nestling mass). Blue Tits foraged significantly more in native than non-native deciduous trees during incubation and when feeding fledglings, and significantly more in deciduous than in evergreen plants throughout the breeding season. Great Tits used deciduous trees more than expected by chance when feeding nestlings, and a positive relationship was found between availability of deciduous trees and mean nestling mass. Overall, the breeding performance of both species was poor and highly variable. Positive relationships were found between mean nestling mass and the abundance of Quercus for Great Tits, but not for Blue Tits. Our study shows the importance of native vegetation in the complex habitat matrix found in urban environments. The capacity of some, but not all, species to locate and benefit from isolated patches of native trees suggests that species vary in their response to urbanisation and this has implications for urban ecosystem function.