The noisy neighbor conundrum: what influences the value of urban sites for forest birds?

Birds are a prominent component of urban biodiversity yet many species, particularly forest-dependent birds, are vulnerable to landscape transformation. We examined the relative influence of three factors that potentially influence forest bird communities in residential areas of Melbourne, Australia: (i) local habitat attributes; (ii) types of land use; and (iii) biotic interactions with an aggressive native species, the noisy miner (Manorina melanocephala). We also investigated characteristics of urban sites that contribute most to landscape-level diversity of forest species. We systematically surveyed forest birds at 300 sites in a range of urban land-uses; with sites clustered in 30 landscapes representing gradients in housing cover and tree cover. Composition and richness of forest bird communities differed among land-use types: sites with greater cover of native canopy trees had a distinct and richer avifauna. However, a pervasive influence on communities was the relative abundance of the noisy miner, a native species that aggressively excludes small insectivores. Sites in native vegetation made the greatest contribution to landscape-level richness of forest birds, but also were negatively influenced by noisy miner occurrence. The dominance of the noisy miner poses a challenge for conservation because actions to improve urban habitat for forest species may also benefit this aggressive species. Conservation of forest birds will depend on larger forest tracts, fringing residential areas, which are less suitable for the noisy miner and serve as potential source populations for forest species. Within residential areas, enhancing understorey complexity with native shrubs and mid-storey will help improve urban habitats for forest birds.