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..., , , ---

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

This dataset contains all the data that was collected and analysed for a project investigating whether urban and forest blue tit populations respond differently to an egg removal experiment.

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Mark D. Pitt, Norah S.S. Alhowiti, Claire J. Branston, Eugenio Carlon, Jelle J. Boonekamp, Davide M. Dominoni, Pablo Capilla-Lasheras. **Environmental constraints can explain clutch size differences between urban and forest blue tits: insights from an egg removal experiment**.

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For any further information, please contact: Mark Pitt, email: [[e-mail]](mailto:[e-mail])

## Description of code:

The statistical software used to analyse our data was R v.4.3.2. Scripts are numbered in order of where they appear in the manuscript. Each script contains a small description to the section of the manuscript to...