Soldiers on sentry duty in hostile territory keep in regular radio contact with their colleagues to assure them that all is well and that they are safe to carry on their manoeuvres. New research funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and published in Current Biology today (17 April) reveals that this is also a feature of the bird world and is very likely to be a rare example of truly cooperative behaviour.

Researchers from the University of Bristol, led by BBSRC David Phillips Fellow Dr Andy Radford, have demonstrated that by giving the distinctive ‘watchman’s song’, individuals scanning for danger as sentinels ensure that their group-mates can focus on foraging, and so capture more food. Dr Radford said: "These exciting results point to a great example of true cooperation. The unselfish behaviour of the sentry is probably rewarded down the line by the improved survival of group mates, which leads to a larger group size. This increases the sentinel’s chances of survival when the group is under attack from predators or having to repel rivals from their territory. It’s a win-win scenario!"

The new work shows that the foragers respond to the watchman’s song alone, whether or not they see a sentinel sitting in a tree. In response to playbacks of recordings of the call, the foraging individuals spent less time looking out for predators, looked up less often, spread out more widely, and spent more time out in the open. This means that they have more time for foraging, are less likely to lose track of prey, have more foraging patches to choose from and are less likely to encounter patches that have already been depleted. As a consequence of these changes in behaviour, foragers had greater foraging success.

The work to study the watchman's song was carried out by observing a bird species called the pied babbler, which is found in southern Africa. Pied babblers live in groups of, on average, 6–7 individuals and operate a sentinel system while they forage for prey such as scorpions and small snakes found beneath the surface of the sand. The study population of 12-20 groups living in the Kalahari Desert was habituated five years ago, so that the birds fly in to the researchers in response to a whistle and weigh themselves on a small set of scales. Observers can then walk within a few feet of the birds to observe their behaviour and monitor the prey that they catch.

Dr Radford said: "Decision making in response to vocal cues is an important behaviour in social birds, and by studying it we can discover much about the way that different groups of animals develop language use. We are now investigating whether sentinels differ in their reliability and how this might influence the behaviour of their group-mates."