Avian functional diversity across an urbanized landscape quantified from citizen science data
Habitat loss due to urbanization is a leading threat to bird populations. In order to conserve avian species diversity, we must understand which species are attracted to urban areas and the traits that allow species to adapt to an anthropogenic environment. Many generalizations regarding urban effects on birds are based on studies of breeding communities, and few investigate urban effects on the functional diversity of species traits. Due to the seasonally dynamic nature of bird communities, it is important to examine variation across seasons. In this study, we used year-round citizen science data from eBird to answer the question: How does habitat variation across an urbanized landscape affect the functional diversity of seasonal songbird bird communities? To answer this question, we paired species occurrence data from several eBird sites in Northern New Jersey and the New York Metropolitan area with species trait data from The Birds of North America Online. We quantified functional diversity as the total branch length of a functional dendrogram for the avian community at each eBird site. Relationships between functional traits and site variation were analyzed using canonical correspondence analysis.
The degree of urbanization (urban, suburban, exurban) did not have a significant effect on functional diversity scores when calculated from a functional dendrogram. However, results of the multivariate analysis did show significant variation in traits among urban, suburban and exurban sites (p<0.05). Bird communities in some large urban parks were actually more similar to non-urban forests than to other urban sites when all seasons were taken into account. Breeding and migratory birds were more closely associated with exurban sites than urban sites. Wintering and year-round residents were more strongly associated with urban sites. These results show that the degree of urbanization can affect the functional diversity of songbirds and that functional diversity is partly dependent on seasonal variation. Specific traits, such as generalist strategies, were more strongly associated with urban habitats. In addition, non-native species were more closely associated with urban sites. These associations match those previously described in the literature using field data. Consequently, this analysis shows the utility of broad scale citizen science data for quantifying the effects of urbanization on bird community structure.