Does urbanization filter birds on the basis of their biological traits?

Urbanization leads to the biotic homogenization of global avifauna. We hypothesized that urbanization acts as a filter on species traits and, therefore, that urban passerines share biological traits explaining their capacity to tolerate urban constraints. We investigated 18 biological traits of passerines related to their general biology, distribution, breeding, and morphometry. In a regional analysis conducted on passerine data from one Swiss and 11 French cities (regional analysis), we identified urban adapters (tolerant species) and urban avoiders (intolerant species), and compared their traits. In a local analysis conducted on passerine data of 13 woodlands located along a short rural-urban gradient, we identified groups of species associated with particular vegetation structures within or particular landscape structures around woodlands. We associated each of these species groups with a tolerance level to urbanization and compared their traits. Regional analysis revealed that urban adapters prefer forest environments, are sedentary, omnivorous, widely distributed, high-nesters with large wingspans. Urban avoiders seem to allocate more energy to reproduction than do urban adapters, to the detriment of adaptation to new environments such as urban areas. Local analysis did not reveal any link between traits and species tolerance levels. At large spatial scales, urbanization seems therefore to act as a filter on species traits. However, the urban constraints that filter species at such large scales do not seem to be the same ones that determine species distribution at local scales. Analyses of traits are powerful tools to understanding regional community composition between urban and rural areas.