Until recently, the Moustached Warbler was considered strictly monogamous. However, in our study we observed males other than the pair male incubating, feeding and defending nestlings. This additional care was far less extensive than that provided by the breeding male. Parental care by extra males seems to facilitate their later forming of a pair with the female for a subsequent brood. This is suggested by the following results: intrusions at foreign nests were strongly male-biased; high rates of non- aggressive encounters between intruders and pair females; helping behaviour occurred mostly in the presence of females; in subsequent second broods, two documented cases of mate switching at nests with extra male care in contrast to two cases of mate fidelity at nests where extra males did not help.