Extrapair fertilizations (EPFs) have been found in most socially monogamous passerine species. EPFs are generally beneficial for males because they increase the number of sired offspring. The benefit accrued by females, however, is less obvious. Maternal benefits may involve fertility insurance, increased genetic variability of the offspring, and improved offspring quality via compatible genes or 'good genes'. In a Hungarian population of the collared flycatcher, we investigated whether the occurrence of extrapair young (EPY) in a brood could be predicted by the traits of the females or their social mates, and whether EPY were superior to their half sibs in terms of growth and. edging condition. We found that 55.7% of the broods contained EPY. The females' participation in extrapair copulations (EPCs) was not related to any of the characteristics of their mates (body size, condition, wing and forehead patch size). The EPY did not differ from their half sibs in any measures of offspring quality. The half sibs had similar embryonic and postembryonic growth and. edged with similar body condition. Female body size was related to extrapair paternity: larger females were less likely to produce mixed-paternity broods. This suggests that the role of female traits in EPCs deserves more attention. We also found that male nestlings grew faster than females, although females could catch up by the time of. edging, so we argue that sex-dependent development should be taken into account in studies using nestling growth as a measure of nestling quality. (C) 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.