Climate change causes an evolutionary change in seasonal timing of a long-distance migrant bird

Abstract Climate change alters ecological interactions, and, because observed phenotypic responses are often insufficient 1,2 , evolutionary responses are required 3 . However, these have rarely been reported, and species seem to either adapt through plasticity 4 or suffer population declines 5 . Earlier work suggested that especially long-distance migrants cannot adjust their complex annual cycle by phenotypic plasticity, because they lack predictable cues at their distant wintering grounds 6,7 . Here we show that the ongoing advance in spring arrival and laying dates in a pied flycatcher population results from cross-generational processes. Using a 51-year time-series we show that during the 25-year period with a strong increase in spring temperature, the population advance in laying dates could only be partly explained by phenotypic plasticity, and primarily resulted from an evolutionary change. Spring arrival from the African wintering grounds advanced simultaneously, and this was unrelated to conditions at the breeding grounds. Individual plasticity of laying date to temperature resulted from shortening the interval between arrival date and laying date, whereas the cross-generational effect was caused by advancing the arrival date. This observed evolution of migration timing can only proceed with ongoing climate change if conditions at the wintering grounds or during migration will not constrain earlier arrival in spring.