Effects of predation risk on foraging behaviour and group size: adaptations in a social cooperative species

Abstract

There is a divergence between models examining the evolution of group living in species in which groups are largely based on families and those based on fluid aggregations of nonrelatives. In the former, the onus has been on ecological and demographic factors that select for offspring philopatry; in the latter, the importance of factors such as foraging success and predation risk are more typically emphasized. We examined the association between predation risk and both group size and foraging behaviour in the chestnut-crowned babbler, Pomatostomus ruficeps, a family-living cooperatively breeding bird that does not appear to face classic ecological or demographic constraints on dispersal and breeding. Groups were more likely to encounter, and be attacked by, avian predators when dependent young were present. Large groups were also more likely to encounter a predator, but less likely to be attacked by it, consistent with a benefit of group living through early predator detection or confusion effects. In addition, the average risk of predation for a given individual was reduced in large groups compared to small ones, owing to the dilution effect. That predation might partly select for group living in this species is boosted by findings showing reductions in ground foraging and increases in sentinel behaviour when predation risk was higher. We conclude that predation might represent an important force selecting for sociality in chestnut-crowned babblers, and highlight the need for future studies to consider more explicitly inherent benefits to group living in the evolution of vertebrate cooperative breeding systems.