Unmasking black and yellow in a common garden: genetic and environmental drivers of urban colour divergence

Communication signals of many taxa, such as colouration, differ in urban areas compared to natural habitats. However, the mechanisms shaping these divergences-genetic or environmental-remain unclear. We studied urban and forest great tits (Parus major) from populations where carotenoid-based colouration is reduced in urban habitats compared to forest, and first examined whether they differed in their melanic patches. We then used a common garden experiment to determine if the colour differences were maintained when birds were raised from the unincubated egg stage in a common environment. In the wild, urban males showed smaller black ties than forest males, and along an urbanisation gradient within urban habitat. Common garden birds of urban origin exhibited reduced chromatic yellow breast and smaller ties compared to forest-origin birds. In contrast, common garden birds from both habitats did not differ in yellow brightness or ultraviolet chroma. These results suggest that divergences in carotenoid saturation and melanin patch are primarily driven by genetic changes or early maternal investment in eggs, whereas yellow brightness or ultraviolet chroma are more environmentally determined. These findings provide information on the mechanisms driving phenotypic divergence in urban birds, with potential implications for mate choice, social dynamics, and evolutionary trajectories in anthropogenic landscapes.