Understanding the evolution of social behavior, especially apparent cooperation, requires knowledge of genetic relatedness among individuals. Evolutionary theory demonstrating the potential effects of kin-structure in populations on the spread od social traits like cooperation and reciprocity has been well developed. It is proposed that even apparently altruistic acts (like helping others to rear young rather than reproducing oneself) can be beneficial to the donor. This explanation holds when the potential benefits of such behavior (the enhancement of reproduction of the actual breeders weighted by the relatedness of the cooperator to recipients) more than balance the costs (young that could have produced in an independent breeding attempt). This analysis depends on accurate assessment of genetic relatedness between cooperator and young produced, which in turn depends on precise knowledge of exactly who begat whom. For ten years the principal investigator have studied several populations of wrens (Campylorhynchus nuchalis, C.griseus, and C. fasciatus) the practice such cooperative breeding in South America. These birds live in extended family groups on permanaent communal territories, a habit that is widespread among tropical birds. These species are an especially revealing test case of the above evolutionary ideas cooperation so effectively increases the number of young that can be produced. Pedigrees are available for as many as 11 breeding seasons in groups of wrens involving a total of more than 1600 individually recognizable individuals. Cooperation in breeding enhances reproductive success five-fold, as unaided breeding pairs almost always fail. Young birds usually remain as helpers on their natal territories, meaning they are usually helping their presumed parents produce siblings. Based on this assumption, cooperation benefits these helpers. Our extension of this work has two aims. Using newly- developed DNA fingerprinting techniques, the PIs will establish whether the presumed breeders are the only birds reproducing in these family groups, and whether putative nonbreeding helpers are aiding in rearing siblings. This test will now be based on conclusive evidence from the genetic material itself rather than on fallible inferences based on behavioral observation. The extent to which parentage is shared within and between groups will be documented, and analytical models and computer simulations will quantify the potential importance of parentage shared beyond a principal pair in social groups. The project will continue field experiments examining the processes of competition for productive breeding positions and group defense against predators, the suspected mechanism of the advantage of breeding in a group.