Extra-pair copulations (EPC) are a rule rather than an exception in socially monogamous birds. Despite widespread occurrences, the benefits of female infidelity remain illusive. Most attention has been paid to the possibility that females gain genetic benefits from EPC. Fitness comparison between maternal half-siblings is considered to be a defining test of this hypothesis. Recently it was shown that these comparisons may be confounded by within-brood maternal effects. One such effect may be the distribution of half-siblings in the laying order. This possibility is difficult to study since it would be necessary to detect from which egg the chick hatched. Here we used a new approach for egg-chick assignment. We cross-fostered eggs on an individual basis among a set of nests of the collared flycatcher Ficedula albicollis. After hatching, chicks were ascribed to mothers and therefore to individual eggs by molecular-genetic methods. Extra-pair young predominated early in the laying order. Under natural conditions, this should give them a competitive advantage over their half-siblings mediated by hatching asynchrony. However, we experimentally synchronized hatching. After this treatment extra-pair young did not outperform within-pair young in any studied trait including survival up to recruitment and several indicators of reproductive success and attractiveness. We obtained only modest sample size for the last two traits and did not test for extra-pair success of male offspring. Thus we cannot exclude the possibility of slight advantages of EPY during their adult phase of life. However, our data tentatively suggest that more likely reason for females’ EPCs is the insurance against the infertility of a social mate.