Habitat urbanization and its effects on birds

By transforming landscapes, human activity creates new types of habitats with altered environmental characteristics that never existed before. As the process of habitat urbanization bears impact on more and more natural habitats, it is essential for us to understand the changes we bring forth in the ecological forces shaping urban animal communities. Birds are perhaps the most frequently studied model organisms by urban ecologists. It is a well known general pattern that urban avian communities have typically reduced species richness, while the density of a few successful species is often higher in cities than in adjacent more natural habitats. But it is less understood which mechanisms generate and uphold these community-level changes. In this review we discuss the most important components of the urban environment influencing birds' physiology, behaviour or morphology, and compile several recent studies to illustrate their effects. To understand urban food webs we also review the results of bottom-up and top-down approach which suggest that altered food availability and predation may play key roles in forming recent urban bird communities. We encourage future research to focus more on experimental, manipulative studies, that would help us not just to realize general patterns but shed more light on the mechanisms, the underlying processes prompting changes in urban bird communities.