Introduction. Extra-pair paternity (EPP) in the Pied Flycatcher is a common phenomenon in the Moscow region and other parts of species' range. The comparison of quality of cuckolded and non-cuckolded males is one of the central places in investigation of this phenomenon. Our main task, in particular, was to reveal if the cuckolded males demonstrate worse parental behaviour in comparison with non-cuckolded ones? N.Tinbergen and K.Blagosklonov hypothesize courtship feeding to be a hint to good parental features (ссылка?!!). We propose good courtship feeding to be correlate to good female guarding. If yes, non-cuckolded males could be better chick feeders in comparison to cuckolded males. Taking into account the presence of helpers and occasional feeders among strange nest-visitors in PF population (Ilyina 2012), the former could be proposed to be biological fathers of EPP chicks and our second task was to analyze composition of visitors and to check their possibility to be relative to visited chicks. Behaviour of strange birds Materials and Techniques. We investigated the relation between EPP and parental care in Pied Flycatcher (Ficedula hypoleuca) at the Zvenigorod biological station in 2011-2015. Using visual observations and/or video recording of Pied Flycatchers (PF), which reared chicks at the age of 9-13 days, we evaluated their feeding rates (FR) and number and size of preys in each delivered food portion. Sizes of broods varied from 4 to 8 chicks. Broods with EPP chick was not significantly different in average pull number, age of nestlings and terms of breeding in comparison to broods without EPP chicks in investigated nest-boxes. Length of prey was estimated in comparison to the length of beak of parent bird. Each brood was observed during 3 hours. Behaviour of strange visitors were analyzed also. There were observed and genotyped 27 broods of socially monogamous pairs and 6 of them included EPP nestlings. For detail information concerning technique of genetical analisys see poster Kerimov et.al. Behavior of social parents I. We did not found parental behaviour of cuckolded males to be noticeably worse than the same of non-cuckolded ones. Revealed asymmetry in parental investment of mates with EPP broods depends on the enlarged ones of females. We suppose females coming in extra-pair copulations to be more experience. III. We believe an important role in extra-pair copulations to be dependent in the large degree on female's readiness to copulate with extra-pair males. If females try to choose males which will be good feeders for her offsprings is still unclear. Acknowledgment. Financial support was provided by RFBR (grant № 13-04-01309-а).