Helping-at-the-Nest and Group Size in the Arabian Babbler Turdoides squamiceps

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Wright, J. 1998. Helping-at-the-nest and group size in the Arabian Babbler Turdoides squamiceps. - J. Avian Biol. 29: 105-112. The effect of group size on the productivity of communally-breeding bird species has for many years been linked with the various adaptive explanations for helping-at-thenest. This study examines the effect of group size on group provisioning effort and group productivity in the Arabian Babbler Turdoides squamiceps, a species in which helping behaviour is reported to have limited impact on parental reproduction, and has instead been uniquely proposed to operate as a signal within the group to gain social prestige. Contrary to these earlier reports, the present study showed a strong positive and linear effect of group size upon the total number of provisioning visits to the nest and the number of fledglings produced. Larger groups produced larger numbers of young, although individual fledging mass did not decrease with this increase in brood size. Fewer faecal sacs were removed per visit in nests with larger broods, and this was the only variable for which an initial positive effect of group size could be explained by the potentially confounding effect of brood size. In addition, individual provisioning effort was reduced in larger groups, both in terms of number of nest visits and the size of prey items delivered per visit. Although these data were collected over only three breeding seasons, the group sizes and breeding conditions involved are broadly representative of this population during the previous 25 years of records. These results therefore suggest that, under the majority of circumstances, helping-at-the-nest in Arabian Babblers has a positive and linear effect on the reproductive success of parents, as well as substantially reducing the provisioning effort required of other group members. Groups were nearly always made up of closely related individuals, and so these data are consistent with kin selection. Helpers gain indirect fitness benefits from the increased reproductive success of nests, as well as potentially reducing the survival costs of chick feeding for adult relatives. The possibility of direct fitness benefits from helping, especially regarding pseudoreciprocity and adaptive group augmentation, is also discussed with reference to the evolution of helping behaviour and group-living in the Arabian Babbler.