Facing rising temperatures in urban environments: the role of phenological plasticity in an urban-dwelling passerine, <i>Parus major</i>

The capacity for birds to adjust their breeding time to variation in spring temperature via plasticity is crucial for insectivorous passerines in temperate areas, particularly in a context of climate disruptions. Recent studies suggested that phenological plasticity varies in response to environmental change via urbanization. We investigated the effects of urbanization on laying date, its phenotypic plasticity in response to spring temperature, and the between-individual variation in laying date, using data from five long-term studies of European great tits, Parus major, in forest and urban areas. First, we compared laying phenology and its plasticity in response to spring temperature between urban and forest populations. We confirmed that birds lay eggs earlier in urban environments in four populations and revealed reduced phenological plasticity in more urbanized environments in two populations. Additionally, we demonstrated greater between-individual variation in laying date in two urban areas. Second, we focused on urban populations only, and showed that the proportion of impervious surface area had little effect on the laying date. Overall, urbanization was associated with earlier breeding and less plasticity, although the strength of these associations varied among cities, likely owing to variation in the intensity of urbanization, landscape connectivity and habitat composition.