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

Urban environments are expanding globally, presenting novel ecological challenges to which species might not be well adapted. Understanding whether species responses to urban living are adaptive or maladaptive is critical to predicting the future impacts of urbanisation on biodiversity. Urban breeding birds exhibit reduced reproductive investment (clutch size) compared to neighbouring non-urban populations. However, whether this reduction is an adaptive response or a result of physiological constraints is unclear. Here, we investigated the ability of urban and forest blue tits (Cyanistes caeruleus) to lay new eggs following an egg removal manipulation. Consistent with the expectation that constraints limit clutch size, egg removal did not induce urban females to lay replacement eggs. Meanwhile, forest birds laid approximately two replacement eggs after egg removal. Additionally, we found that the size of replacement eggs from forest females declined over the lay sequence, a pattern not observed in urban clutches. Hatchlings from experimental nests were lighter in both habitats, with lighter hatchlings having reduced survival. Furthermore, as urban blue tits did not lay replacement eggs, egg removal resulted in a brood reduction in the city and nestlings from urban experimental nests had higher survival than those from urban control nests. Overall, our results suggest cities place constraints on egg production in urban birds. Urban females may experience energetic or nutrient limitations that restricts egg formation and/or exacerbates the trade-off between survival and egg production. Additionally, females may be misjudging urban habitat quality, due to time constraints when laying, and produce a clutch too large to be sustained in the city. ### Competing Interest Statement The authors have declared no competing interest.