Experimental evidence of reciprocal altruism in the pied flycatcher

Although human behaviour abounds with reciprocal altruism, few examples exist documenting reciprocal altruism in animals. Recent non-experimental evidence suggests that reciprocal altruism may be more common in nature than previously documented. Here we present experimental evidence of mobbing behaviour, the joint assault on a predator in an attempt to drive it away, as reciprocal altruism in the breeding pied flycatcher (Ficedula hypoleuca). Given a choice, pied flycatchers assisted in mobbing initiated by co-operating neighbours and did not join in mobbing when initiated by conspecific neighbours which had defected from necessary assistance 1 h before. The results suggest the birds followed a 'tit-for-tat'-like strategy and that mobbing behaviour of breeding birds may be explained in terms of reciprocal altruism.