Breeding bird diversity in urban forest remnants: key determinants in a metropolitan landscape

As urbanisation is advancing rapidly worldwide, the protection of biodiversity in urban areas is becoming increasingly important. Especially urban forest remnants with valuable habitat structures can be hotspots of biodiversity. However, isolation and fragmentation of such habitats might undermine the effectiveness of biodiversity conservation efforts. Birds are suitable bioindicators for assessing the influence of urbanisation on forest ecosystems. So far, there is a set of studies on forest bird communities in rural woodland fragments, while studies in strongly urbanised landscapes are scarce. Here, we carried out a territory mapping in 19 urban forest remnants in a densely populated German metropolitan region to determine the environmental factors affecting breeding bird diversity. We used multivariate models to identify the relevant local habitat factors and landscape parameters influencing overall number of species, abundance and Shannon diversity and to assess whether there are guild specific responses or with regard to conservation status. We revealed that urban forest remnants can harbour species of conservation concern and often contain high overall bird densities. The most important predictor for the species number was patch area. Further, isolation played a significantly negative role for forest specialists. The availability of favourable habitat structures such as old trees and standing deadwood was the dominant predictor for abundance. Tree/shrub breeders and generalists were negatively influenced by anthropogenic factors like built-up areas. We suggest that maintaining the size of existing forest patches, promoting valuable structural elements and improving connectivity of the adjacent landscape can optimise the ecological potential of urban forest remnants. • urban forest remnants often harbour endangered species and host high bird densities • forest specialists in particular react sensitively to patch size and isolation • bird abundance is positively affected by high densities of old trees and deadwood • built-up areas around forests negatively influence abundance of tree/shrub breeders • differences in bird community composition are mainly driven by nestedness