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Abstract

A new study finds that a social bird responds to the position from which it receives information. Nigel Williams reports. A new study finds that a social bird responds to the position from which it receives information. Nigel Williams reports. For many species, information has to be gleaned the hard way — from experience of interactions with the environment — and the lessons can be tough. But for social species, there is another option — listening to and looking at their group members. Such ‘public information’, however, comes with pitfalls — how reliable is it? A new study reported in the Proceeding of the Royal Society series B (published online) reveals that one species of social bird not only gleans information from other individuals on ‘sentinel’ duty, but reacts to where they believe it to be. Andrew Radford at the University of Bristol and colleagues in Bristol and Cambridge have worked with groups of the pied babbler in Southern Africa. What the researchers found was that group members of the pied babblers were able to perceive the position of sentinel birds — a task undertaken by all the members of the group at times — which greatly helped their ability to forage when the birds perceived the sentinel to be high up in neighbouring foliage. Information about a sentinel could be obtained by foragers in two ways: first, through visual monitoring, which is how social information is generally assumed to be gathered; second, from visual cues because, while on duty, sentinels of several species produce regular quiet vocalisations known as the ‘watchman's song’. Research has shown that foragers use these calls to detect the presence of a sentinel but they might also use them to gain additional information, such as the sentinel's height. The researchers used recordings of a sentinel's ‘watchman's song’ played back at different heights to observe the differences in behaviour. And indeed they found that the higher a song was transmitted, the more confident foragers were in extending the distance and duration of their activities. When sentinels moved to a higher position, their probability of detecting predators is likely to be greater, the researchers believe. Their group reduced their vigilance, spread out more widely and were more likely to venture into the open, the team said. “Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a substantial increase in biomass intake.” As the babblers in this study were accustomed to human association, weighing them on scales on which they jumped was relatively easy, said Radford. “This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information,” the researchers say. Sentinel behaviour, where an individual adopts a raised position, scans for danger and gives alarm calls to warn foraging groupmates of predatory threats, has evolved in a number of bird and mammal species, the researchers say. “In the presence of a sentinel, foragers can benefit from both lower predation risk, because sentinels tend to detect predators more often and from further away than do individuals on the ground, and a lowered starvation risk, because they can spend more time foraging and can do so more efficiently,” they believe. Although individuals are known to respond differently to alarm calls given by callers of differing reliability, no study has investigated whether foragers monitor features associated with sentinel reliability and adjust their behaviour when these change, they say. Because pied babblers are preyed upon by a variety of terrestrial mammals, snakes and raptors, “the movement of a sentinel to a higher perch would potentially reduce the vulnerability of foraging group members, and thus explain their increased spread and use of open areas, as well as their reduced vigilance.”