The city and forest bird flock together in a common garden: genetic and environmental effects drive urban phenotypic divergence

Abstract

AbstractUrban phenotypic divergences are documented across diverse taxa and are commonly assumed to result from microevolution, but the underlying genetic and environmental drivers behind these phenotypic changes are unknown in most wild urban systems. We censused urban common garden studies in the literature (N= 77) across a range of taxa. Collectively, these studies showed that both genetic and plastic responses can contribute to urban phenotypic divergences, while revealing a lack of studies with vertebrates. We conducted our own common garden experiment using great tit (Parus major) eggs collected along an urbanization gradient in Montpellier, France to: 1) determine whether documented morphological, physiological, and behavioural shifts in wild urban great tits are maintained in birds from urban and forest origins reared in a common garden (N= 73) and 2) evaluate how different sources of genetic, early maternal investment, and later environmental variation contributed to trait variation in the experiment. In line with the phenotypic divergence in the wild, common garden birds from urban origins had faster breath rates (i.e., higher stress response) and were smaller than birds from forest origins, suggesting genetic differentiation has driven these trait differences. Conversely, wild differences in aggression and exploration were not maintained in the common garden, indicating that plasticity to urban conditions likely drives the documented behavioural shifts. Differences between individuals (genetic and environmentally induced) explained the most trait variation in the experiment, while variation among foster nests and captive social groups was limited. Among-individual variation in size and stress response was similar between common garden and wild contexts, whereas among-individual behavioural variation was lower in the common garden than the wild. Our results provide trait-specific evidence of evolution in an urban species where genetic change likely underlies urban shifts in morphology and stress physiology, but that urban behavioural divergences are driven by plasticity.