Fluctuating selection driven by global and local climatic conditions leads to stasis in breeding time in a migratory bird.

The origin of natural selection is linked to environmental heterogeneity, which influences variation in relative fitness among phenotypes. However, individuals in wild populations are exposed to a plethora of biotic and abiotic environmental factors. Surprisingly, the relative influence of multiple environmental conditions on the relative fitness of phenotypes has rarely been tested in wild populations. Identifying the main selection agent(s) is crucial when the target phenotype is tightly linked to reproduction and when temporal variation in selection is expected to affect evolutionary responses. By using individual-based data from a 29-year study of a short-lived migratory songbird, the pied flycatcher (Ficedula hypoleuca), we studied the relative influence of 28 temperature- and precipitation-based factors at local and global scales on selection on breeding time (egg laying) at the phenotypic level. Selection, estimated using the number of recruits as a proxy for fitness, penalized late breeders. Minimum temperatures in April and May were the environmental drivers that best explained selection on laying date. In particular, there was negative directional selection on laying date mediated by minimum temperature in April, being strongest in cold years. In addition, nonlinear selection on laying date was influenced by minimum temperatures in May, with selection on laying date changing from null to negative as the breeding season progressed. The intensity of selection on late breeders increased when minimum temperatures in May were highest. Our results illustrate the complex influence of environmental factors on selection on laying date in wild bird populations. Despite minimum temperature in April being the only variable that changed over time, its increase did not induce a shift in laying date in the population. In this songbird population, stabilizing selection has led to a three-decade stasis in breeding time. We suggest that variation in the effects of multiple climatic variables on selection may constrain phenotypic change.