The exploration of the effects of urbanization on bird demography has attracted much attention, and several studies found lower reproductive success in towns, which suggested strong environmental constraints. Here, we conducted a 3-year study to explore the consequences of urbanization on the breeding success of two species that originated in forests, the Blue Tit Cyanistes caeruleus and the Great Tit Parus major. In two replicates of urban and forest habitats, we studied the components of reproductive success. In one replicate of each habitat, we quantified nestling growth over the three breeding seasons, and we collected data on egg quality during one breeding season. The general picture that emerges from our finding is that in urban sites breeding success was lower with smaller clutch sizes, higher clutch, higher brood failure rates and lower survival rates. Our results also showed reduced growth in urban habitats, at the embryonic and nestling stages, with potential adverse consequences on fitness. Crucial ecological factors could explain the observed contrasts between the habitats, and food limitation is among the most likely. Overall, we demonstrated the negative effects of urbanization on the reproductive success of forest birds, and our results were consistent between species and geographic areas for these negative effects. Our results suggest a mismatch between urban environments and the habitat exploitation abilities that birds have evolved in their native forest ecosystems.