Power in Numbers Avian brood parasites target particular bird species to raise their offspring, sometimes at great cost to the foster family. Feeney et al. (p. 1506 ; see the Perspective by Spottiswoode ) analyzed the global distribution of brood parasitism and found a correlation with the occurrence of cooperative breeders across multiple taxa. For example, Australian fairy wrens breed both singly and in cooperative groups, but the group breeders are better able to resist parasites than lone pairs, indicating that the prevalence of cooperative breeding may be a response to brood parasites.