Maria Moiron, Celine Teplitsky, Birgen Haest, Anne Charmantier, Sandra Bouwhuis

Understanding the mechanisms by which populations can adapt to changing environmental conditions is crucial for predicting their viability. In the context of rapid climate change, phenological advance is a key adaptation for which evidence is accumulating across taxa. Among vertebrates, phenotypic plasticity is known to underlie most of this phenological change, while evidence for micro-evolution is very limited and challenging to infer. In this study, we quantified phenotypic and genetic trends in timing of spring migration using 8032 dates of arrival at the breeding grounds obtained from observations on 1715 individual common terns (Sterna hirundo) monitored across 27 years, and tested whether these trends were consistent with predictions of a micro-evolutionary response to selection. We observed a strong phenotypic advance in arrival date, with birds arriving on average 9.34 days earlier over the study period. This phenotypic trend translated into an advance in breeding values, which accounted for c. 7.4 % of the observed change in the population. The Breeder’s equation and Robertson’s Secondary Theorem of Selection predicted qualitatively similar evolutionary responses to selection, although the estimate from the latter was uncertain, and those theoretical predictions were largely consistent with observed genetic patterns. Overall, our study therefore provides rare evidence for micro-evolution to underlie (part of) an adaptive response to climate change in the wild, and illustrates how a combination of adaptive micro-evolution and phenotypic plasticity facilitated a shift towards earlier migration in this natural population of common terns.