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

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April 2008

Dr Andy Radford
"Many foraging animals face an important trade-off between collecting enough food, and therefore minimizing the starvation risk, and looking out for danger, and therefore minimizing the risk that they themselves are eaten. Now one way of getting round this problem, particularly if you live in a group, is to split the duties between group members. So, for example, one individual could dedicate itself to looking out for predators, while the rest of the group is foraging. This is what is known as sentinel behaviour, and that’s akin to the behaviour of lots of human groups, where they post guards or look-outs, while the remainder of the group is focusing on other activities. And in lots of social bird and mammal species we find exactly this behaviour."

"However, there is one particular aspect of sentinel behaviour that has received far less attention and that is what’s known as the watchman’s song. And this is the idea that the individual on sentinel duty gives a little vocalization, often throughout the period that it’s on duty, announcing its presence to the rest of the group – the individuals on the ground who are foraging. And what we were interested in was what might be the benefits to both parties of the sentinel giving these little calls, this watchman’s song. And we conducted the work on a particular bird species in southern Africa called the pied babbler. These are little black and white birds that live in groups that range in size from three to 15 individuals, but usually have a group size of six or seven on average. And we investigated what impact this vocalization might have on the foraging group members, and any particular changes in their behaviour under conditions where there was a sentinel on duty giving this little call. The foraging group members might be responding either to a visual cue – a sentinel being perched high up in the tree – or to the vocal cue – the watchman’s song."

"We collected sound recordings of sentinel calls from different individuals, and we also did sound recordings of background noise to act as a control, and that’s important just to prove that any response we find is not just because we’re playing a noise out of a speaker. We went to several different groups and played back either the sentinel call or the background noise in turn, and then we looked at the behaviour of the foraging group. And we did these playbacks from a speaker that we’d placed 2.5m up on a pole to mimic the position of a natural sentinel. And then we examined the behaviour of the foraging group members in response to the sentinel calling and the background noise. And what we found was that foraging group members spread out more widely, they ventured out into the open more, they spent less time being vigilant and they looked up less often when we played back sentinel calling, compared to when we played back background noise. And, as a result of these changes in behaviour, they gained an increased foraging success, so more food intake per unit time, when we played back sentinel calling compared to background noise."

"So what this little experiment allows us to say is that the forgaing group members are responding to the vocal calls, the watchman’s song of a sentinel, not just the visual cue presented by a bird perched high in a tree. So it is the watchman’s song that they are responding to."

"You can look at the possible benefits to sentinel behaviour from the perspective of both the sentinel and the foraging individuals. By responding to the sentinel call, the watchman’s song of this individual on duty, foraging group members change their behaviour in such a way that they gain increased foraging success: they get more food per unit time. But what might be the benefit to the sentinel itself of giving the watchman’s song? Well, the most likely possibility is that they’re benefitting because of the increased foraging success of the foragers. It might be benefitting because, if it’s a member of a larger group, it’s safer from predation itself. Larger groups are more likely to be able to defend themselves against predators, and they’re also more likely to be able to defend their territory against neighbours who are trying to usurp them. The other potential benefit that the sentinel might be gaining from the increased survival of the group members is because in these co-operatively breeding groups individuals are often closely related. And therefore the sentinel benefits because it shares large numbers of genes with these foragers who are getting more food and therefore potentially surviving better. The giving of the watchman’s song is likely to be maintained because of cooperative benefits. It’s likely to be a truly cooperative behaviour because the benefits to the sentinel of doing so result from the benefits gained by other group members, rather than selfish benefits gained immediately by itself."

"So this work was made possible because we work on a habituated population of birds in the Kalahari desert. By habituated I mean that we are able to walk within a few feet of wild groups without their behaviour being adversely affected in any way. The habituated population was initially set up by Dr Amanda Ridley of the University of Capetown back in 2003. Each group has been trained by a whistle. So we walk out into their territory, give this whistle and if the birds are in the right mood, they come flying in to us. At that point, we place a scale on the ground, and the birds have been trained to jump on and weigh themselves. This is important, and very unusual, because in most vertebrate studies, you actually have to capture individuals in order to get measures of body condition or weight."