Effects of urbanization on bird phenology: a continental study of paired urban and rural populations

Urban habitats differ from adjacent natural habitats in terms of disturbance regimes, light, temperature, rainfall, habitat distribution and resource abundance. Meteorological differences advance and prolong the growing season in urban habitats compared to nearby rural areas. In turn, urban bird populations may potentially start singing earlier, and reproduce earlier and more frequently than rural populations. However, this prediction has previously only been tested with data from single species using single spatial replicates from rural and urban sites. Here we provide the first general (paired urban and rural populations of 54 bird species) and large-scale (a 3800 km long latitudinal gradient across Europe) empirical evidence for longer and earlier singing periods in urban compared to rural habitats. Effects of urbanization on start and duration of the singing period (as a proxy for the breeding season) were positively related to size of cities and ecological characteristics of species. Bird species that have been urbanized for a long time started to sing earlier and had a more extended singing period in urban compared to rural habitats. We also found that the singing period started later and was shorter at higher latitudes. Geographical variation in phenology was related to temperature and rainfall, although differences between urban and rural habitats were not. Differences in duration of singing periods between paired urban and rural sites were as large as latitudinal differences between southern and northern Europe (5, 6 and 28 d for 3 common species, as compared to a mean latitudinal variation of 17.1 d). This suggests local adjustment to urban environments, either due to evolution or to plasticity of phenological behaviour.