Laying date in a Mediterranean population of pied flycatchers (Ficedula hypoleuca)

The origin of natural selection is found in the environmental heterogeneity that determines a variation in relative fitness among phenotypes. However, individuals in wild populations are exposed not a single but to a plethora of biotic and abiotic environmental factors. Surprisingly, the relative influence of multiple environmental conditions on 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 response. By using individual-based information from a short-lived migratory passerine, 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 over 29 breeding seasons. Selection penalised late breeders using number of recruits as proxy of fitness. 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 colder years. In addition, non-linear selection on laying date was shaped by minimum temperatures in May, with selection on laying date changing from null to negative as the breeding season advances. 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 overtime, 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 the influence of local climatic variables on selection in addition to global climatic trends, may constrain phenotypic change.