Circadian (i.e. daily) regulation of behaviors is thought to provide fitness benefits to organisms by enabling them to anticipate diel changes in the environment, such as sunrise. A common behavior among socially monogamous songbirds that usually takes place in the early mornings is copulating with partners outside of the social pair bond (i.e., extra-pair mating). Thus, variation in when individuals begin their daily activity may influence their reproductive success; early risers may be better able to gain copulations and be able to guard their partners and minimize their risk of being cuckolded compared to late risers. Sexual selection may thus play an important role in shaping circadian behaviors, but this assumption has yet to be tested in free-living animals. Here we experimentally weakened endogenous circadian rhythmicity and thus anticipation of dawn in male great tits (Parus major) in the wild through the subcutaneous administration of an implants filled with melatonin shortly before egg-laying began in this population. Selection, particularly sexual selection, may shape circadian phenotypes of wild vertebrates which enable anticipation of important and predictive diel changes in an individuals biotic and abiotic environment.