Evolution under the city’s shadow: reproductive divergence of urban and rural great tits revealed by a common garden experiment

Urbanisation profoundly alters natural habitats and reshapes many traits in wild species, including their reproduction. In birds, urban populations typically breed earlier and lay smaller clutches than their rural conspecifics, patterns usually attributed to warmer temperatures and lower food availability in cities. Yet, whether these differences reflect phenotypic plasticity or genetic change remains unclear. To disentangle environmental from genetic contributions, we conducted a common garden experiment with great tits (Parus major) originating from unincubated eggs collected in urban and forest habitats. Nestlings were hand-reared under identical conditions and, the following spring, paired by origin in breeding aviaries. We recorded laying date, clutch size, egg volume, and fertilisation success, and compared them with wild phenotypic distributions. Contrary to patterns in the wild, laying dates did not differ between urban- and forest-origin females, revealing a plastic adjustment of the onset of breeding to the environment. Egg size remained consistently smaller in urban-origin females across both conditions, pointing to a genetic divergence. No difference in fertilisation rate was found between individuals of urban and forest origin in the common garden. Finally, clutch size differences showed striking opposite patterns between experimental and wild birds: in the common garden urban-origin females laid larger, and not smaller, clutches than forest-origin females, suggesting a counter-gradient variation where genetic and environmental effects act in opposing directions on the phenotype. Overall, our study showcases how integrating experimental designs with field monitoring can uncover the hidden dynamics of urban adaptation, including cryptic evolution masked by environmental constraints in cities.