The world is seasonal and organisms’ adjustment of their seasonal timing to this environmental variation is crucial for their fitness. Climate change is strongly impacting seasonal timing which makes a better understanding of the potential for micro-evolution of timing in natural populations essential. As any phenotypic change ultimately involves changes in the physiological mechanism underlying timing, we need to unravel the genetics of these mechanisms. I will carry out a highly integrated eco-evo-devo project on the causes and consequences of genetic variation in timing of reproduction in great tits (Parus major), an ecological model species for that we recently developed state-of-the-art genomic tools. I will develop a powerful instrument to study this timing mechanism by creating selection lines of early and late reproducing birds using genome-wide, rather than phenotypic, selection. The phenotypic response of selection lines birds will be assessed both in controlled environment aviaries and in birds introduced to the wild. To unravel how selection has altered the birds’ physiology I will measure key components of the physiological mechanism at the central, peripheral and egg production levels. As a unique next step I will then introduce selection line birds into a wild population to assess the fitness of these extreme phenotypes. This will enable me, for the first time, to estimate the selection on timing without confounds, which I will compare with traditional estimates using observational data. Finally, I will integrate genetics, physiology and ecology to hindcast the rate of genetic change in our wild population and validate this rate using DNA sampled over a 20-year period. This innovative project integrates state of the art developments in ecology, genetics and physiology (eco-evo-devo) will set new standards for future studies in other wild species and will be of key importance for our predictions of evolutionary responses to a warming world.