Snake-directed mobbing in a cooperative breeder: anti-predator behaviour or self-advertisement for the formation of dispersal coalitions?

Abstract.

The Arabian babbler (Turdoides squamiceps), a cooperatively breeding bird, mobs a wide range of its natural predators, mainly snakes and raptors. The response of babblers to a life-like, gypsum model of the horned viper (Cerastes gasperettii) was studied in the Arava valley, Israel. I considered three alternative hypotheses to account for mobbing behaviour: investment in other group members, predator-prey interaction and self-advertisement for the formation of dispersal coalitions. Mobbing response varied with group structure: family groups mobbed more than complex groups in which subordinates were potential breeders. Subordinate group members – potential dispersers – mobbed more than dominant breeders, and females mobbed more than males. Babblers did not increase their mobbing response when vulnerable fledglings were present. The results suggest that babblers may increase their investment in anti-predator behaviour when surrounded by close kin, and that immatures may learn about their potential predators during mobbing. However, snake mobbing by Arabian babblers may also serve functions other than anti-predator defence. It is possible that participation in this risky activity may be an honest signal by which subordinate group members advertise their quality as potential members of dispersal coalitions. However, present data provide only indirect evidence, and more work is needed to assess fitness consequences of advertisement for individual group members.