Extra-pair paternity explains cooperation among neighbours in a bird

In many social animals, females mate with multiple males, but the adaptive role of female extra-pair mating is not fully understood. This data set tests whether male pied flycatchers (<em>Ficedula hypoleuca</em>) engaging in extra-pair copulations with neighbouring females were more likely to assist their neighbours in predator mobbing. It shows that extra-pair sires joined mobs more often, approached predators more closely, and attacked predators more aggressively than males without extra-pair offspring in the neighbouring nest. Our results support theoretical predictions suggesting that extra-pair mating incentivizes males to assist in nest defences.