The effect of urbanisation on avian morphology and latitudinal gradients in body size

Abstract

Urban areas occupy a large and growing proportion of the earth. Such sites exhibit distinctive characteristics relative to adjacent rural habitats, and many species have colonised and now successfully exploit urban habitats. The change in selection pressures as a result of urbanisation has led to trait divergence in some urban populations relative to their rural counterparts, but studies have generally been local in scale and the generality of differentiation thus remains unknown. The European blackbird Turdus merula is one of the commonest urban bird species in the Western Palearctic, but populations vary substantially in the length of time they have been urbanised. Here we investigate patterns of morphological variation in European blackbirds occupying 11 paired urban and rural habitats across much of the urbanised range of this species and spanning 25° of latitude. First, we assessed the extent to which urban and rural blackbirds are differentiated morphologically and the consistency of any differentiation across the range. Paired urban and rural Blackbird populations frequently exhibited significant morphological differences, but the magnitude and direction of differentiation was site dependent. We then investigated whether the nature of latitudinal gradients in body‐size differed between urban and rural populations, as predicted by differences in the climatic regimes of urban and rural areas. Blackbird body‐size exhibited strong latitudinal gradients, but their form did not differ significantly between urban and rural habitats. The latitudinal gradient in body size may be a consequence of Seebohm's rule, that more migratory populations occurring at high latitudes have longer wings. We conclude that while there can be substantial morphological variation between adjacent urban and rural bird populations, such differentiation may not apply across a species’ range. Locality specific differentiation of urban and rural blackbirds may arise if the selection pressures acting on blackbird morphology vary in an inconsistent manner between urban and rural habitats. Alternatively, phenotypic divergence could arise in a stochastic manner depending on the morphological traits of colonists, through founder effects.