Geographic differences in the phenology of gonadal development and moult, but not of egg laying, are genetically based in a small songbird.

To forecast how fast populations can adapt to climate change, it is essential to determine the evolutionary potential of different life-cycle stages under selection. In birds, timing of gonadal development and moult are primarily regulated by photoperiod, while laying date is highly phenotypically plastic to temperature. We tested whether geographic variation in phenology of these life-cycle events between populations of great tits () has a genetic basis, indicating that contemporary genetic adaptation is possible. We carried out a common garden experiment in which we bred first- and second-generation pairs in captivity originating from eggs from Gotland (Sweden) and Hoge Veluwe (The Netherlands), two populations that showed different temperature sensitivity of laying date in a recent meta-analysis. We recorded the phenology of egg-laying, moult and gonadal size in early spring. We found no significant differences in laying date between the populations, but they did differ in moult timing and testis size. This implies that under climate change the timing of gonadal development and moult, which are mainly regulated by photoperiod, will not respond to increased temperature but can respond by genetic adaptation in response to selection, while the opposite holds for laying date, perhaps indicating that plasticity is constraining genetic adaptation.