Croci, Solène, Butet, A., and Clergeau, Philippe. 2008. Does urbanization filter birds on the basis of their biological traits? The Condor 110 (2): 223-240.

Urban sprawl spreads further every year, at the cost of natural, rural, and agricultural space. Despite the seemingly inhospitable environment urban landscapes represent for wild animals, some remain and even thrive in towns and cities. Others… not so much. Croci et al. suggest this is because the urban environment acts as an ecological filter, blocking species with some biological traits while allowing others to pass. They select 12 medium-sized cities in France and Switzerland and identify 110 bird species that are present in the region and therefore could be present in the cities. Species that were confirmed to breed inside the cities were classified as “urban adapters” (45 species), while species that didn’t breed in the cities were “urban avoiders” (65 species). None of these species were raptors, unless you count shrikes (which you shouldn’t). The authors then assessed whether certain traits were generally associated with avoiders or adapters, and found that the answer was yes. Traits such as forest, mountain, or cliff habitat-adaption, omnivory, and nesting off the ground (along with others) were generally found amongst adapters, but not avoiders. They concluded that urbanization does indeed act as a filter on species based on their traits.