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The human population is rapidly urbanizing. While this will undoubtedly present challenges for humans it will also place pressure on birds in these areas. To better manage these spaces in a manner that promotes avian biodiversity, we must first come to understand how human development affects the distribution and abundance of bird species and guilds within cities and if patterns observed previously in large cities can be extended to smaller ones.
Breeding birds were surveyed along a gradient of increasing urbanization in Iowa City, Iowa, during the summers of 2014 and 2015. Study areas included a forested park, recreational park, low density residential area, medium density residential area, high density/mixed-use area, and urban core. Birds were censused a total of four times at each site using variable circular plot counts. Landscape characteristics were measured using a high resolution land cover dataset and tree canopy model. Regression models were developed to investigate relationships between the bird community and land cover characteristics.
Bird species richness, diversity, and evenness all decreased with increasing urbanization, while biomass and the number of individuals peaked in the urban core. The community shifted from non-native, resident, granivorous, multi-brooding building nesters in highly developed areas to native, migrant, invertivorous, single-brooding, tree and tree cavity nesters at the least developed sites. Regression models indicated varied relationships among landscape characteristics and species richness and community prevalence of functional guilds. Native, migratory, invertivorous, tree cavity-nesting, and single-brooding species showed negative relationships to variables measuring the built environment, while non-native, resident, granivorous, building nesting, and multi-brooding species showed positive relationships to these measures.
Overall, the response of avian functional guilds to varying levels of urban intensity in Iowa City were remarkably similar to the results of previous studies. This suggests that much of what has been learned previously concerning avian responses to human development can be extended to planning and implementing conservation strategies in smaller cities.