Bird traits in urban-rural gradients: how many functional groups are there?

Recent analyses of communities have examined the variation of species traits along environmental gradients. These papers highlight a combination of several traits, instead of variation of individual traits, to better explain the effect of urbanization on bird communities. Exploratory factor analysis (EFA) allows the identification of an underlying structure of a broad set of data. EFA can be a useful tool for generating functional groups from highly correlated biological traits in bird communities and determining its variation along gradients of urbanization. Birds were counted along an urban-rural gradient during spring 2009-summer 2010. Species were classified using 15 biological traits related to the use of space. The EFA was calculated from a matrix where rows were sampling units (n = 75), and columns represented counts of individuals with each trait (n = 15). Four functional groups were obtained. Functional group 1 comprised resident species feeding gregariously on the grond, nesting in buildings, having an omnivorous diet, and being most abundant in the more urbanized areas. Functional group 2 was most abundant at intermediate levels of urbanization and represented solitary species that nest in trees, feeding on vegetation and with carnivorous and nectarivorous diets. Migratory behavior, insectivorous and granivorous diets, aerial feeding and ground nesting were representative of two functional groups in rural areas. Responses to urbanization by these functional groups are consistent with the classifications of response guilds (urban exploiters, urban adapters, and urban avoiders). Thus, EFA allows a link between concepts generated from the analysis of species and the analysis based on biological traits.