No place for ground-dwellers in cities: A meta-analysis on bird functional traits

Urbanization is one of the most severe forms of environmental alteration, in which increasing human settlement leads to an unprecedented loss of natural areas, thereby threatening global biodiversity and associated ecosystem functions. Consequently, the evidence base needs to be strengthened in order to understand how this man-made alteration affects urban biodiversity, and hence develop appropriate conservation measures. According to our expectations, urbanization processes influence the abundance of passerine birds through functional traits, what we studied within the frame-work of a systematic review using phylogenetically controlled meta-analyses. We tested four specific predictions: (i) Migration strategy will in-fluence responses to urbanization, and tropical migrants will be the least numerous in urban areas; (ii) Birds nesting at ground level will be negatively affected by urbanization, while birds nesting at higher levels will suffer less; (iii) In terms of foraging technique, ground probers will be negatively; and (iv) birds with insectivorous diet will be the most disadvantaged in cities. Bird species (N = 53) were studied along urbanization gradients that ranged from highly urbanized areas to adjacent natural areas. Our findings revealed that the impact of urbanization on the abundance of bird species is modulated by certain functional traits. Partial or short distance migrants, ground nesters, ground gleaners and granivores were the group of species most negatively influenced by urbanization. Species sensitive to urbanization were those that are linked in some way to open grassland areas. This indicates that cities need more intact and extensively managed grasslands to sustain bird communities, which provide valuable ecosystem services.