A cross-continental look at the patterns of avian species diversity and composition across an urbanisation gradient

Context As humans become increasingly urban, the need for conservation of nature in cities increases and requires an understanding of the patterns and processes of urban ecosystems. In particular, because humans are the most dominant species in urban areas, understanding the role humans play in these ecosystems (direct and indirect) will be of primary importance. Aims We examine the diversity and composition of bird species across an urbanisation gradient in two cities (Berlin, Germany, and Seattle, Washington, USA). We determine the degrees of species urban tolerance and examine how certain biological traits of species, namely, diet, whether or not species use bird feeders, nest sites and innovation rate, characterise species urban tolerance. Finally, we determine whether human provisioning (bird feeders and nest boxes) influences what types of species persist across the urbanisation gradient. Methods We surveyed bird abundance and species richness using point counts and surveyed human provisioning by conducting door-to-door interviews of residents across an urbanisation gradient in Berlin and Seattle. Key results We found that patterns of species richness were similar in both cities, but that species composition in Berlin changed less across the urbanisation gradient than it did in Seattle. The majority of birds in Berlin were urban tolerant, whereas in Seattle, they were moderately urban tolerant and intolerant. A cluster analysis revealed that, in general, in Berlin, omnivorous, open-nesting birds that use bird feeders and have relatively high innovation rates tended to be urban tolerant. In Seattle, birds that were mostly omnivorous, nested in open cups, and used bird feeders tended to be moderately urban tolerant and they were influenced by provisioning of food by humans. Conclusion Urbanisation and human interactions with birds can act as ecological filters, favouring certain bird species that can lead to varying species compositions across an urban gradient. These differences in species composition across the gradient may be more noticeable in younger cities than in older cities where the filtering process has been occurring for longer time. Implications By providing a variety of habitats and supplementing natural foods and nesting places, urban planners and residents can help conserve bird diversity in urban areas.