The function of sentinel alarm calls in the Arabian babbler

In various group-living species, from time to time one of the group members acts as a sentinel and utters alarm calls when detecting a predator. Hearing these calls, other group mates usually react by quickly changing their behaviour. Uttering alarm calls is an anti-predator mechanism with several possible functions. In this study our aim was to assess the function of alarm calls in the Arabian babbler Argya squamiceps, a cooperatively breeding songbird. We observed 10 groups in natural conditions and recorded 1000 reactions of 55 foraging group members to 268 sets of alarm calls uttered by the sentinels. The most frequent reaction of the group members to the sentinel's alarm calls was to stop foraging immediately, to fly up to the closest treetop and to start calling alarm calls. In all the recorded cases, the sentinel remained in place and continued to call. The alarm calls continued as long as the raptor could be seen, and then ceased gradually, after which babblers flew down and returned to foraging. We conclude that the main function of alarm calls in the Arabian babbler is pursuit deterrence. In particular, while foragers benefit from the information content of the sentinel's alarm calls and react on their basis, the raptor is the primary audience addressed by the calls uttered by the sentinel and the foragers who join it.