Where are extra-pair offspring raised: the mother’s, the father’s or the others’ nest?

Abstract

Abstract Extra-pair mating prevails among socially monogamous birds with biparental care. Theoretically, females may lay the resultant eggs in their own, their mating partners or other pairs’ nests, accordingly leading to extra-pair paternity (EPP), quasi-parasitism (QP) and extra-pair conspecific brood parasitism (CBP). Empirical evidence showed EPP to be widespread and QP rare, with extra-pair CBP having never been described. In Tibetan ground tits (Pseudopodoces humilis), we detected EPP, QP and extra-pair CBP in 54%, 47% and 24% of broods, which involved 20%, 16% and 8% of offspring, respectively. Cuckolding and cuckolded individuals in these events were genetically related and moderate inbreeding occurred between extra-pair partners, whereas social mates formed randomly with respect to relatedness. Promiscuity relative to genetic monogamy significantly increased annual reproductive output of both males and females, and extra-pair offspring did not differ from their within-pair siblings in body mass and heterozygosity. While the much high incidence of promiscuity can be ultimately driven by not only males but also females’ pursuit of increasing reproductive success, it may be proximately promoted by frequent kin ties in association with the species’ habit of burrow nesting and roosting, presumably kin-selected benefits may reduce the cost of arising non-descendant offspring. The fact that less than 30 altricial species have been documented with respect to extra-pair maternity (in contrast to over 360 species for EPP) suggests that more attention should be paid towards QP and extra-pair CBP to get a broader perspective on the evolution of avian mating systems.