Helping-at-the-nest in Arabian babblers: signalling social status or sensible investment in chicks?

Abstract

Helping-at-the-nest in Arabian babblers,Turdoides squamicepshas been uniquely explained as an altruistic ‘signal’ used to ‘show-off’ and gain ‘social prestige’ within the group. Evidence from other cooperatively breeding bird species, however, instead suggests that helpers invest in nestlings in order to gain kin-selected benefits. This study investigates patterns of provisioning behaviour among parents and helpers for 27 broods over three breeding seasons, and assesses the potential for chick-feeding to operate as a signal in Arabian babblers. Provisioning rate accounted for much of the variation in fledging brood mass, suggesting that observed provisioning effort had potential fitness consequences for the brood. All group members provisioned at similar rates and made similar adjustments in feeding effort, irrespective of sex and/or dominance rank within the group. Chick age had a positive linear effect, and group size a negative linear effect, upon provisioning rates of all group members. There was no obvious evidence that nestling provisioning operated as a signal within groups in any way. Contrary to earlier reports, individuals did not appear to ‘interfere’ with the provisioning efforts of subordinates; individuals in larger groups did spend longer in the nest tree before reaching the nest, but this was unaffected by sex, dominance rank and presence of other (more dominant) birds. These results suggest that provisioning by both helpers and parents in Arabian babblers functions not as a signal, but as sensible investment in chicks. In this species, high relatedness within groups may provide helpers with kin-selected benefits via such investment, although possible advantages from augmentation of group size cannot be ruled out.