Do temporal trends in Christmas Bird Counts reflect the spatial trends of urbanization in southwest Ohio?

Abstract

We compared the temporal and spatial effects of urbanization on birds by examining the greater Cincinnati region Christmas Bird Count (CBC) data from 1960–1998, and contrasting it to the current winter distribution and abundance of birds across an urban gradient in Butler County, OH. As urban zones expand, the changes in the regional bird community through time should reflect changes similar to those seen along a gradient of urban land-use. Along the gradient, the pre-development bird species (assumed to be those found at the most undisturbed site) drop out gradually as the land-use becomes more urban. This local extinction is accompanied by the invasion of other species that exploit the changes brought about by urbanization. Consequently, we expected the CBC data to reflect a shift, over time, from predominantly native species to invasive species, as urbanization spread in the region. However, results from the CBC data did not reflect the patterns observed along the spatial urban gradient. CBC circles, because of their size (approx. 384 km2) may contain enough fragmented habitat to support small populations of some native species, even near rapidly expanding urban centers such as the greater Cincinnati region. The nature of CBCs encourages coverage of as many habitat types as possible in order to observe the greatest number of species. Together, these factors produce data that do not accurately represent spatial population trends. Thus single CBC circles may be at a scale inappropriate for assessing the impacts of expanding human populations on native bird communities.