Extra-pair paternity is common among socially monogamous birds, but whether females benefit from having extra-pair copulations remains unresolved. In this study I staged single extra-pair copulations in captive pairs of socially monogamous Japanese quail (Coturnix japonica), and used extra-pair males of different colour to identify extra-pair fertilisations. This eliminated among-female variation in extra-pair copulations as a source of variation in extra-pair fertilisations, and tested whether a single extra-pair copulation would insure against infertility of the social male. Overall, the probability of extra-pair fertilisation was 0.46 for the first egg that was fertilised after extra-pair copulation, but this rapidly declined over consecutive eggs in the laying sequence. However, a single extra-pair copulation was effective in ensuring fertilisation of the majority of a typical clutch in the few cases were the social male was completely infertile. Hence, my results show that variation in extra-pair paternity can be independent from variation in extra-pair copulation behaviour, and that a single, strategically timed, extra-pair copulation can largely insure against social male infertility. Among-female variation in extra-pair fertilisations, and their declining probability over the laying sequence, as typical for many bird species, can thus, in principle, be parsimoniously explained by a uniform female fertility insurance strategy.