Variable mating strategies and incest avoidance in cooperatively breeding grey-crowned babblers

Understanding cooperative breeding requires an appreciation of the direct and indirect reproductive benefits available to group members, alongside the costs of group living such as increased potential for inbreeding. We use molecular techniques to determine sex, kinship and parentage to explore the benefits of helping and the mechanisms for incest avoidance in cooperatively breeding grey-crowned babblers, Pomatostomus temporalis. Generally, helpers gained only indirect fitness benefits through helping to raise kin. Breeding vacancies were usually filled by the oldest unrelated helper or by an immigrant if all helpers were related to the surviving breeder. A disproportionate number of helpers were unrelated to the dominant of the opposite sex. This suggests that related helpers disperse when their same-sex dominant dies rather than inherit the breeding position or help an unrelated immigrant to breed. Although monogamy was typical, extrapair paternity, joint-nesting and intraspecific brood parasitism were also revealed. Extrapair paternity was predicted by the relatedness of the dominant pair and appears to be an inbreeding avoidance mechanism, although some cases also suggest within-group reproductive conflict. This study is one of few to show a direct link between the rate of extrapair copulation and incest avoidance between the dominant breeding pair in a cooperatively breeding bird.