Close-calling regulates spacing between foraging competitors in the group-living pied babbler

Among the wide repertoire of vocalizations produced by social species, perhaps the most frequently heard are the ‘close’ calls (those of short duration and low amplitude). Despite their prevalence, and the many studies focusing on their function in primate societies, little work has been conducted on close calls in avian species. We used a combination of observations, supplementary feeding and playback experiments to investigate the function of one particular close call, the ‘chuck’, in group-living pied babblers, Turdoides bicolor. There was no evidence that the chuck call is used to recruit conspecifics to a food source or to reduce the likelihood of an individual becoming separated from the group. Instead, there was good evidence that it is used to regulate spacing between potential foraging competitors. Although the chuck call is not used aggressively to deter competitors that attempt to share an individual's foraging patch, it appears to indicate the forager's current position and thus minimize the likelihood of another group member approaching closely in the first place. Foragers increased their call rate in larger groups and when their nearest neighbour was closer (i.e. when foraging competition was potentially higher), and playbacks of chuck calling caused individuals to stay further away from the speaker than did background noise. Maintenance of spacing was beneficial because individuals suffered a decrease in foraging efficiency if they shared a foraging patch. This study represents one of the first experimental tests of the function of avian close calling.9 page(s