Nest defence and offspring provisioning in a cooperative bird: individual subordinates vary in total contribution, but no division of tasks among breeders and subordinates

Research aimed at understanding the evolution of costly, seemingly altruistic helping behaviour in cooperative birds has strongly focused on a single aspect of helping—offspring provisioning. However, various other activities are also important for successful breeding, most notably nest defence, and emphasis on a single aspect of helping might hamper a full understanding of the evolutionary drivers of cooperation and seemingly altruistic helping behaviour. Nonetheless, the extent to which social classes (i.e. dominant breeders and subordinate helpers), as well as different individuals within social classes, divide tasks in cooperative groups remains largely unknown. Here we tested whether individual purple-crowned fairy-wrens, Malurus coronatus, show task division in the two forms of costly offspring care important for reproductive success: nestling provisioning and nest defence (to predator models presented at nests). Subordinates and breeders generally contributed to both tasks. Breeders defended marginally more than subordinates, and provisioned at a consistently high level, irrespective of their individual contribution to defence, whereas subordinates’ investment in both types of care was positively correlated. Thus, we found no evidence for division of tasks between or within social classes, highlighting it may be absent in facultatively cooperative birds even when activities are costly and important for reproduction. This suggests that in such species, cooperative improvement of overall investment may be more effective than enhanced individual efficiency at different tasks. In many social species, cooperation between individuals to achieve communal goals is important for successful reproduction. When multiple individuals are working together, division of tasks can occur, with tasks unevenly distributed among group members. Research on cooperative breeding, where subordinate helpers seemingly altruistically assist others in raising their young, has however mainly focused on offspring provisioning, while subordinates can often contribute to several breeding tasks. Thus, if task divisioning occurs, this would call into question conclusions about adaptive benefits of helping based on provisioning effort alone. Here we use nestling feeding watches and predator model presentations at nests to quantify nestling provisioning and nest defence effort by individuals in a classical cooperative breeder, the purple-crowned fairy-wren. We show a lack of task divisioning by breeders and subordinates, which is reassuring for our current understanding of the evolution of cooperative breeding.