Functional diversity of wintering urban and rural passerines in the northeastern United States quantified from Christmas Bird Count data

Background/Question/Methods

Bird populations have been heavily affected by urban expansion due to habitat loss. 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. Factors such as temperature and resource availability may make urban habitats more attractive to a more functionally diverse community in the winter months. In this study, we focused on over-wintering birds and aimed to answer the question: How does urbanization affect the species richness and functional diversity of wintering birds? We combined and analyzed species occurrence data from the Audubon Society’s Christmas Bird Count (CBC) and species trait data from The Birds of North America online. We examined passerine species richness and functional diversity (FD) in and around three major cities in the Northeastern United States. We quantified functional diversity (FD) in R as the total branch length of a functional dendrogram for the avian community at each CBC circle. 

Results/Conclusions

Functional diversity was higher overall for urban areas (FD = 16.2) than surrounding rural areas (FD = 14.7; p =.005). In addition to higher functional diversity, urban areas had higher species richness than rural circles. The mean species richness for urban and rural circles was 63.6 and 51.9 respectively. Other studies have shown that bird species richness is higher in moderately disturbed sites than in both rural areas and the urban core. Because our city data include suburban areas, our results may reflect this pattern. However, this pattern has not been well documented for functional traits, especially in wintering birds. Urban environments are characterized by high habitat heterogeneity and can offer abundant food resources, which may support a wider range of functional traits within bird communities. This analysis shows the utility of broad scale citizen science data for quantifying the effects of urbanization on bird communities.