High frequency of extra-pair fertilisations in the moustached warbler, a songbird with a variable breeding system

We examined the genetic mating system of the moustached warbler Acrocephalus melanopogon, a multi-brooded songbird that sometimes breeds in trios (one female and two males). The second male, which appears relatively late in the breeding cycle, participates in all forms of parental care but provides less care than the pair male. DNA fingerprinting revealed a high frequency of extra-pair paternity: 39% (7/18) of the broods contained extra-pair young, comprising 27% (12/44) of the chicks. An additional 7 broods contained at least 9 chicks (out of 20) that were not sired by the resident male. We were able to assign paternity for 11 (52%) of the extra-pair young. Of these, 7 (64%) were sired by secondary males and 4 (36%) by close neighbours (nesting within 50 m). Consistently, pairs with extra-pair young had more close neighbours, and they also tended to have a secondary male at the nest. Caring for nestlings that they might have sired through extra-pair copulations may be a way for secondary males to salvage their reproductive effort. They may also provide parental care because of the possibility of pairing with the female in future breeding attempts. Our results suggest that female moustached warblers obtain direct benefits (i.e., help with brood care) from extra-pair copulations. However, a skewed fertilisation pattern implies that genetic compatibility between mates and copulation partners may also influence female extra-pair behaviour in this species.