Colonisation of urban environments is associated with reduced migratory behaviour, facilitating divergence from ancestral populations

How individuals colonising novel environments overcome the diverse suite of new selection pressures is a fundamental question in ecology and evolution. Urban environments differ markedly from the rural ones that they replace and successful colonisation of urban areas may therefore require local adaptation and phenotypic/genetic divergence from ancestral populations. Such a process would be facilitated by limited dispersal to and from the novel habitat. Here we assess divergence in migratory behaviour between seven pairs of urban and rural European blackbird Turdus merula populations along a 2800 km transect across Europe. This former forest specialist is now amongst the most abundant urban birds across most of its range. We use a stable isotope approach due to the lack of sufficient ringing data from multiple urban populations, and compare hydrogen isotopic ratios of tissues grown in the breeding (feathers) and wintering areas (claws) to derive an index of long distance migratory behaviour. We find a tendency for urban blackbirds to be more sedentary than rural ones at all sites and this divergence is particularly strong at the north‐eastern limit of our transect, i.e. in Estonia and Latvia. These urban populations are those that have been established most recently (from the late 1930s to 1950s) implying that urbanisation can promote rapid ecological divergence. The increased sedentary behaviour of urban birds could promote further ecological divergence between rural and urban populations, such as the earlier breeding of urban blackbirds, and in some cases may contribute to their previously documented genetic divergence.