Broad scale environmental conditions, not urbanisation, explain geographic variation in great tit feather microstructure

Urbanisation imposes novel environmental conditions that can shape phenotypic traits including plumage, yet, its effects on characteristics beyond colouration remain poorly understood. Feather microstructure plays an important role in thermoregulation, protection from solar radiation and waterproofing, and its expression is sensitive to environmental conditions. Here, we quantified feather barbule density, a key component of feather microstructure, in great tits (Parus major) from five urban-rural paired populations across Europe to assess its association with urbanisation, temperature and elevation. We found no significant effects of the urban environment or temperature. However, feather microstructure significantly varied between the studied areas and was positively associated with elevation and ultraviolet B (UV-B) radiation, suggesting a potential adaptive response to variation in solar radiation.