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In most cooperative vertebrates, delayed natal dispersal is the mechanism that leads to the formation of kin societies. Under this condition, the possibility that kin-based cooperative breeding is an unselected consequence of dispersal patterns can never be ruled out because helpers can only help their relatives. Here we show that a population of carrion crows (Corvus corone corone) fully fits the central prediction of kin selection theory that cooperative breeding should arise among relatives. On their territory, resident breeders are aided not only by nonbreeding retained offspring but also by immigrants (mainly males), with whom they share matings. Philopatry cannot account, however, for the high degree of genetic relatedness found between breeders and immigrants of the same sex that cooperate at a nest, indicating that crows actively choose to breed cooperatively with their relatives.