Data from: Environmental constraints can explain clutch size differences between urban and forest blue tits: Insights from an egg removal experiment

Urban environments present novel ecological challenges to wild species. In
birds, urban populations generally exhibit reduced clutch sizes compared
to forest populations. However, whether smaller urban clutches are
adaptive or a result of environmental constraints is unclear. To
investigate these two hypotheses, we quantified the ability of urban and
non-urban blue tits (Cyanistes caeruleus) to lay new eggs after an
experimental manipulation aimed to increase egg production. We removed the
first four eggs laid by urban and forest birds to test their ability to
produce new eggs. If the urban environment imposes constraints on egg
production, we predicted that urban birds would not lay new eggs. If the
small clutches of urban birds are an adaptive response, we predicted they
would lay new eggs to reach the optimal clutch size for the
environment. Consistent with the environmental constraint
hypothesis, our results suggest that urban females do not lay new eggs to
the same extent as forest birds following egg removal. Forest birds laid
approximately two new eggs after our experimental manipulation, while
urban birds laid approximately 0.36 new eggs following egg
removal. Our manipulation resulted in a brood reduction in the
urban habitat, and nestlings from urban experimental nests had higher
survival than those from urban control nests. This suggests that females
might be misjudging urban habitat quality and produce a clutch with more
eggs than nestlings they can rear. Overall, our results suggest
that environmental constraints could limit the number of eggs that urban
females lay, generating urban versus non-urban differences in this trait.