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Birds announce their sentry duty to help comrades get a good meal

17 April 2008

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."



Click on the thumbnails to view and download full-size images.

Note that these images are protected by copyright law and may be used with acknowledgement.

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A Pied Babbler forages under the surface of the sand for small snakes and invertebrate prey such as scorpions. (2.5MB) (Copyright Andy Radford)
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The study population has been habituated so that researchers can walk within a few feet of the group without altering their behaviour. (2.2MB) (Copyright Matthew Bell)
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While on duty sentinels give a little call called the watchman's song to tell the rest of the group that they are in position and looking out for danger. (1.4MB) (Copyright Andy Radford)
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The birds have been trained to come into the researchers in response to a whistle, at which point they will also jump on the scales and weigh themselves. (2.1MB) (Copyright Andy Radford)
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A sentinel takes up its position on a piece of deadwood to look out for danger. (1.3MB) (Copyright Andy Radford)

Notes to editors

This research is published in Current Biology: Hollén et al., Cooperative Sentinel Calling? Foragers Gain Increased Biomass Intake, Current Biology (2008), doi:10.1016/j.cub.2008.02.078.

Dr Radford’s research is funded by around £300K of fellowship funding from the UK’s Biotechnology and Biological Sciences Research Council (BBSRC).

BBSRC funds up to 10 David Phillips Fellowships each year to support early career scientists with world-class potential.


The Biotechnology and Biological Sciences Research Council (BBSRC) is the UK funding agency for research in the life sciences. Sponsored by Government, BBSRC annually invests around £380 million in a wide range of research that makes a significant contribution to the quality of life for UK citizens and supports a number of important industrial stakeholders including the agriculture, food, chemical, healthcare and pharmaceutical sectors.

External contact

Dr Andy Radford, University of Bristol

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