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Provisioning calls given by nest attendants immediately before or while they feed nestlings have been recorded in many avian species. Although the adaptive significance of these vocalizations has rarely been examined, their most obvious function seems to be to stimulate begging in blind altricial nestlings. Bell Miners ('Manorina melanophrys'), cooperative Australian honeyeaters, give provisioning "mew" vocalizations that are relatively unusual in being individually distinctive and given during both feeding events and as attendants leave the nest. We show that mew calls stimulated begging during playbacks and that more calls were given if the brood initially did not beg, if the prey was of a type that was difficult to transfer, or both. This is consistent with mew calls being aimed at the nestlings to stimulate begging and, thereby, improve the efficacy of food transfer. However, mew calls were given in 52% of visits as attendants left the nest area, and the probability of calling in this context increased if another group member was nearby. Furthermore, mew call rate per visit increased throughout the nestling period as provisioning rates increased, despite nestlings being increasingly able to attend to other (e.g., visual) cues of impending food delivery. Together, these results suggest that mew vocalizations also have a role in communication between nest attendants. By improving the coordination and social cohesion of helpers and parents around a nest, mew calls may facilitate cooperative activities by the same group of birds away from the nest, such as during dangerous predator-mobbing events. |
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