An article in Oecologia published by Danish ornithologist Anders Pape Møller compared difference in bird population densities between urban and rural habitats and the year of urbanization. The study found:

Population density was on average 30 % higher in urban than in rural habitats, although density reached as much as 100-fold higher in urban habitats in some species. Invasive urban bird species that colonized urban environments over a long period achieved the largest increases in population density compared to their ancestral rural habitats. 

This is a departure from the fact that most animals occur at highest densities when in their native "natural" habitats. According to the paper, "these results suggest that successful invasion of urban habitats was associated with gradual adaptation to these habitats as shown by a significant increase in population density in urban habitats over time."

Via http://www.springerlink.com/content/c027244101682703/

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