North American bird populations have declined due to numerous factors, including habitat loss (e.g., deforestation due to logging and crop cultivation), changes in land use practices (e.g., increases in urban/exurban development), and invasive species and pathogens (e.g., the recent introduction of West Nile virus, which affects native bird species to differing degrees). We examined bird community diversity in the Chicago metropolitan area, the third largest urbanized area in the United States, which also lies along the Mississippi flyway, a major migration route for North American birds. For this study, data from ten Chicago-area sites located within the flyway for the years 1965-2005 were analyzed using online data from the National Audubon Society’s annual Christmas Bird Count (CBC). The purposes of this study were to: 1) extend the usefulness of the CBC data by developing methodologies to utilize the CBC data for revealing changes in community structure that may be captured by the long-term monitoring of the CBC circles, and 2) examine issues in the statistical use of CBC data, including the influence of sampling effort, a potential source of bias. We calculated several diversity indices: Species Richness (S), the Shannon Evenness Index (E), the Shannon-Wiener Index (H'), and the Simpson Index of diversity (D) using the CBC records. The contributions of sample year, count circle location, and sample effort (“party hours”) to these indices were incorporated into ANCOVAs predicting these diversity components. Significance tests of these independent variables, both singly and in combination, were conducted using F tests of multiple-partial correlation coefficients on ranked diversity indices.
Results/Conclusions
Our analyses revealed changes in bird diversity in the region over time, with differing trends among the ten locations. Overall, the number of bird species increased in the region, but formerly wide differences in diversity across the ten locations have converged to a narrower range. Count circles in more rapidly-growing areas in the region have shown declines in diversity, but established urban areas with more parklands have maintained or increased in diversity. Further analyses revealed that these changes were not due to differences in sampling effort or representation by non-native species. We believe the approaches we used in this study can enhance the usefulness of the CBC data by allowing study of temporal changes in avian community structure and comparisons across regions.