Numerous studies have reported a reduction in vigilance by foraging individuals when they are less vulnerable to predation; for example, in larger groups and in the centre of a group.

It is commonly assumed that foragers assess the presence and position of other group members visually, but many social species produce frequent ‘close’ calls while foraging.

In a new study published in Biology Letters, researchers from the universities of Bristol and Cape Town used playback experiments to show that foraging pied babblers (Turdoides bicolor) are less vigilant in response to the close calling of more individuals and individuals on either side of them, compared to calls of fewer individuals and calls on one side of them.

Dr Andy Radford, from the School of Biological Sciences, said: “These results suggest for the first time that foragers can use vocal cues to gain information on group size and their spatial position within a group, and adjust their anti-predator vigilance accordingly. Individuals should therefore keep both their eyes and ears open at all times.”