Cooperative sentinel behaviour in the Arabian babbler

Cooperative sentinel behaviour in Arabian babblers, Turdoides squamiceps, appears generally consistent with state-dependent models of individually selfish antipredator behaviour. We examined further detailed aspects of this cooperative behaviour, including the suggestion that by engaging in this behaviour sentinels advertise their status and gain social prestige. Chosen sentinel locations were higher, but no more exposed, than the best alternative locations within 25 m. Sentinels started off closer to the centre of the foraging group than when sentinel bouts were terminated. Change-overs between sentinels were nearly always due to previous sentinels terminating their own bout. On the rare occasions when bouts were interrupted by upcoming sentinels, physical contact or aggression was extremely rare. Dominant males tended to terminate sentinel bouts of other birds, interrupting especially the dominant females. Other than this, there was no effect of an individual's sex or dominance rank on any aspect of sentinel change-overs or location choice when acting as a sentinel. There were also no differences in any sentinel activity between simple family groups, and those more complex nonfamily groups within which individuals compete for reproduction. Rates of alarm calling did not differ between group members, although dominant males did make more territorial calls to neighbouring groups while acting as a sentinel. Therefore, we found relatively little evidence that individuals compete for the chance to act as a sentinel as a means of showing off within groups of Arabian babblers. Instead, our study confirms the primary function of sentinels as a system of cooperative vigilance. (C) 2001 The Association for the Study of Animal Behaviour.