Genetic variation in cue sensitivity involved in avian timing of reproduction

1. Annual variation in the timing of avian reproduction is associated with predictive cues related to ambient temperature. Understanding how these cues affect timing, and estimating the genetic variation in sensitivity to these cues, is essential to predict the micro-evolutionary changes in timing which are needed to adapt to climate change. 2. We carried out a 2-year experiment with great tits Parus major of known genetic background, which were kept in pairs in climate-controlled aviaries with simulated natural photoperiod and exposed to a seasonal change in temperature, where the two treatments differed by 4 °C. We recorded the dates of laying the first and last eggs and timing of moult, as well as physiological proxies associated with reproduction: plasma luteinizing hormone (LH), prolactin, and gonadal size at four-weekly intervals. 3. The temperature treatments did not affect first-egg dates, nor gonadal growth or plasma LH and prolactin concentrations. However, birds terminated egg laying, regressed their testes and started their moult earlier at higher temperatures. 4. There were marked family differences in both the start of egg laying, with sisters from early laying maternal families laying early, and in the termination of laying, indicating that there is heritable variation in sensitivity to cues involved in timing. 5. Our experiment, the first to use genetically related individuals in an experimental design with a natural change in photoperiod and biologically realistic temperature differences, thus shows that genetic adaptation in cue sensitivity is possible, essential for species to be able to adapt to a warming world.