Safe selfish sentinels in a cooperative bird

1. Sentinel behaviour involves a unique cooperative system of dedicated look-outs which protect members of their social group from attack by predators. Using detailed observations from groups of Arabian babblers, Turdoides squamiceps, we tested the original theoretical suggestion that cooperative sentinel systems are simply the result of individually selfish state-dependent patterns of behaviour. 2. Sentinel effort and the number of sentinel bouts per individual per hour were greater for males than females, and both increased with individual dominance status within the group. 3. Sentinel behaviour was unaffected by group social structure, in terms of patterns of relatedness and the number of potential breeders. 4. Total group sentinel effort increased with group size, while the effort per individual decreased irrespective of sex and dominance rank. Simultaneous sentinel behaviour by two or more birds was very rare, but increased with group size. 5. Sentinel effort followed seasonal fluctuations in food availability, but not peaks of raptor migration through the study site. 6. Body mass was greater for males than females and was positively related to dominance rank. Overall, body mass explained much of the variation in individual sentinel effort both within and between birds. However, data on individual foraging performance could not be related to changes in body mass and thus to differences in sentinel effort. 7. Sentinel behaviour in groups of Arabian babblers therefore confirms many of the predictions arising from state-dependent models of cooperative, yet individually selfish, sentinel behaviour.