Mutualism benefits and the evolution of an interspecific sentry call in associations between Sociable Weavers and Fork-tailed Drongos

Abstract Mutualisms, where organisms benefit from cooperation, are common in nature, but payoffs are dynamic and mutualists can incur costs as well as benefits from their association. Nevertheless, selection can favour the coevolution of behaviour that increases benefits. Such coevolution can potentially result in new mutualistic behaviours. Our study considered associations between Fork-tailed Drongos (Dicrurus adsimilis), birds which use both false alarms and aggression to steal food from other species (kleptoparasitism), and their most frequent host, the Sociable Weaver (Philetairus socius). While drongos apparently gain and weavers lose from this association, we investigated whether weavers derive foraging and predator protection benefits, and whether drongos increase foraging opportunities through sentry call behaviour. When with drongos, weavers increased their foraging time and reduced vigilance. Experimental playbacks further demonstrated that drongo sentry calls attract sociable weavers, as well as increasing their foraging time and decreasing their vigilance. Weavers also resumed foraging after an alarm more quickly when sentry calls were made, but sentry calls do not appear to improve drongo false alarm success since sentry calls did not increase the likelihood weavers fled to subsequent drongo false alarms. Consequently sentry calls benefit weavers via foraging payoffs and drongos via weaver attraction and potentially by increasing opportunities for kleptoparasitism. Results demonstrate that despite costly deception between mutualists, individuals nevertheless derive benefits which may be enhanced by the coevolution of a mutually beneficial vocal signal, specific to interspecific communication.