Mutual ornamentation, sexual selection, and social dominance in the black swan

We investigated the adaptive significance of a sexually monomorphic ornament in the black swan Cygnus atratus. Both sexes grow curled feathers on their wings (range 7–22 curled feathers per wing), which are displayed prominently in a range of social interactions. The number of curled feathers increased until the birds reached sexual maturity (at 2 years of age) but did not vary with age thereafter. We found evidence for both sexual and social functions of the ornament. Paired, mature individuals of both sexes had higher numbers of curled feathers than unpaired, mature birds, and individuals paired assortatively with respect to curled feather number, suggesting the feathers may be involved in mutual sexual selection. More ornamented individuals were dominant in agonistic interactions with birds of the same sex and pairing status. Highly ornamented pairs were also more likely to maintain extended tenancy of preferred cygnet feeding areas, which resulted in improved offspring survival. The curled feathers thus appear to function as a signal of social dominance, which is highly correlated with reproductive success and is therefore a reliable signal of parental quality in mate choice.