A BIRD species in Africa has learned a new trick - posting sentries who watch for predators while the others in the group eat.
It represents a rare example of genuinely co-operative behaviour demonstrated by birds.
The pied babbler lives in the Kalahari Desert in southern Africa and dines on scorpions and small snakes hidden under the hot sands. Researchers from the University of Bristol have discovered that while some hunt for food, others delay their meal by serving as lookouts.
The sentries chirp out a distinctive "watchman's song", explains team leader Dr Andy Radford who publishes the Biotechnology and Biological Sciences Research Council funded study this morning in the journal Current Biology.
The birds foraging on the ground don't need to see the watchman, but only to hear the song to know that it is safe and that no predators are about.
Having a sentry delivers significant advantages. The birds can focus on hunting and can gather more food in a shorter time, Dr Radford suggested.
The team confirmed the effect of the sentry song by replaying recordings to the babblers. Birds habitually glance up and cluster as they scan for predators but, when the sentry's tune was played, they looked up less often, spending more time in the open.
"These exciting results point to a great example of true co-operation," Dr Radford said.
"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."
The pied babblers also proved themselves to be remarkably obliging research subjects.
The bird groups under study were "habituated" to human presence. Humans can approach to within a metre without frightening the birds. The birds have also been taught to fly to the researchers in response to a whistle and weigh themselves on a small set of scales.
Just as some human sentries might decide to grab a few winks rather than keep watch, the researchers are curious about the trustworthiness of the bird sentries.
"We are now investigating whether sentinels differ in their reliability and how this might influence the behaviour of their group mates," stated Dr Radford.