An avian urban morphotype: how the city environment shapes great tit morphology at different life stages

Urbanization is a worldwide phenomenon associated with tremendous modifications of natural habitats. Understanding how city dwelling species are affected by those changes is becoming a pressing issue. We presently lack fine scale spatio-temporal studies investigating the impact of urbanization across different life stages and along urbanization gradients. Based on 8 years of monitoring of urban and forest great tits (Parus major), we investigated how city life shapes morphological characteristics at different life stages in the city versus the forest, and within the urban habitat (along naturalness and pedestrian frequency gradients). We found that urban nestlings were significantly smaller than forest ones, but not in lower body condition. Urban breeders showed reduced tarsus, wing and tail lengths compared to forest birds. Within the city, variation in nestling tarsus length and body condition along the naturalness gradient highly depended on the year, with no consistent pattern. For breeders, tarsus length and body condition were positively correlated to the naturalness gradient, although only in 2019 for tarsus, and only in older individuals for body condition. Finally, we found that males had smaller wing lengths in more urbanized parts of the city. These results suggest that urbanization affects morphology early on in development, influencing many morphological attributes. While the mechanisms underlying the urban morphotype remain to be determined, we discuss the potential origins for the documented differences between forest and urban morphotypes, and argue that they most probably result from urban environmental constraints linked to food availability.