Helping enhances multiple components of reproductive success in the cooperatively breeding apostlebird

1. We examined the influence of helpers on reproductive success in the apostlebird using data gathered over three breeding seasons (116 nests, 34 breeding groups). Initially, annual productivity and individual components of reproductive success were examined in relation to group size (two to 17 birds). The effect of helping per se was then examined in a reduced data set by comparing (a) the predictive power of group size and actual helper numbers, and/or (b) the productivity of groups that changed in size between nests or seasons. 2. Group size was positively and linearly related to annual fledgling production and the number of offspring that survived to the following season. This was attributed to the presence of helpers, as the number of birds that fed nestlings, but not group size, was significantly related to annual fledging success. Neither group size nor helper number influenced fledgling survival over the subsequent winter. 3. The probability of re-nesting after a successful first brood increased with group size. Among double-brooded groups, the interval between nests was inversely related to group size. 4 Clutch size increased linearly with group size among groups with two to five birds, but was not related to group size among larger groups. A matched comparison of groups that changed size between years further suggests that clutch size was facultatively adjusted to group size, a phenomenon rarely reported in species with non-breeding helpers. 5. Hatching success was not related to group size or the number of birds participating in incubation. 6. The number of birds provisioning the brood was a better predictor of feeding rate per chick than group size, and was significantly and inversely related to the incidence of nestling starvation. However, there was no relationship between group size and fledgling body mass. 7. Fledging success per brood increased linearly among groups with three to five members. The only case of pair-breeding was unsuccessful, suggesting that this species is one of only a few birds that may be obligately cooperative. 8. We found no evidence that helpers influenced the likelihood of nest predation, or the probability of group members surviving to the following season.