Dissecting factors behind temporal trends in the timing of breeding in two songbird species – evolutionary change or phenotypic plasticity?

Warming climate has led to significant phenological advances in many plant and animal populations. Whether these advances represent evolutionary responses or phenotypic plasticity remain typically unknown. Using 53 years long time-series of individually marked Great (Parus major) and Willow Tits (Poecile montanus), we investigated whether the significant breeding time advances in these species could be explained to result from evolutionary responses, phenotypic plasticity, or both. In the case of both species, we did not find any evidence for changes in breeding values for timing of breeding, suggesting that the observed changes do not have a genetic, and hence, evolutionary basis. In contrast, we found that annually fluctuating environmental effects explained most of the variation in first egg lay dates, suggesting that advances in breeding time were attributable to phenotypic plasticity. We further inferred that phenotypic plasticity in response to spring temperatures can fully explain the observed advancement of Great Tit phenology over time, whereas Willow Tits have advanced their phenology much beyond what would be expected from phenotypic plasticity in response to spring temperatures. The latter observation suggests that some other yet unidentified environmental factor, uncorrelated with spring temperatures, likely explains about half of the advancement in their breeding time.