Lay SummaryPreening one’s partner is important for staying together and sharing offspring care. Grooming or preening a partner is widespread in primates and birds. We tested the idea that this behavior (allopreening) in birds plays a role in maintaining partner relationships. Bird species that allopreen were more likely to share parental duties and more likely to breed with the same partner in consecutive breeding seasons, compared to species that do not allopreen.Twitter: @ElspethKenny