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

Abstract

Urban environments present novel ecological challenges to wild species. Understanding whether species responses to urban living are adaptive or maladaptive is critical to predicting the impacts of urbanisation on biodiversity. In birds, urban populations generally exhibit reduced reproductive investment (clutch size) compared to forest populations. However, whether smaller clutches are adaptive, or a result of environmental constraints is unclear. Here, to investigate these two hypotheses, we quantified the ability of urban and forest blue tits (Cyanistes caeruleus) to lay new eggs upon egg removal. Consistent with the constraint hypothesis, our results suggest that urban females do not lay new eggs, at least to the same extent as forest birds. Meanwhile, forest birds laid approximately two additional eggs. As urban blue tits did not lay replacement eggs, our experiment resulted in a brood reduction and nestlings from urban experimental nests had higher survival than those from urban control nests, suggesting that females may be misjudging urban habitat quality and produce a clutch too large to be sustained. Taken together, our results suggest that urban females may experience constraints that limit egg formation and/or exacerbate the trade-off between female survival and egg production. This has important implications for urban green space management.