Nest predation is the main cause of reproductive failure in birds, yet the factors that drive predation pressure, as well as the avian strategies to minimize it, are poorly understood. There is a well-known commensal relationship between ants and birds nesting in acacia trees, but the direct benefit in terms of avian reproductive success has not been tested properly. We used artificial nests to compare success and survival probability of nests placed in Hinds' acacia trees (Acacia hindsii) associated with ants (Pseudomyrmex spp.) with those of nests placed in trees without ants. Nesting success and the probability of daily survival were greater in acacias than in antless trees. All cases of nest failure were due to egg predation, but none resulted from wren activities, as has been reported in previous studies. The results of this experimental study indicate that the presence of ants in acacias may enhance avian reproductive success by reducing the probability of nest predation.