Tail length and sexual selection in a monogamous, monomorphic species, the Roseate Tern Sterna dougallii

Both male and female Roseate Terns (Sterna dougallii) have unusually long outer tail feathers, and males tend to have longer tails than females. We examined whether these tail streamers may have evolved as a result of sexual selection, using data from a 15-year study at Bird Island, Massachusetts, USA. Data on tail length were analyzed for 2,515 terns, of which 745 were of known sex. Tail length was positively correlated with predictors of reproductive success, such as laying date, body mass, and age, and thus can act as an indicator of mate quality. The increase in mean tail length with age appeared to result from a combination of growth in relatively young terns and differential survival among older terns. The mean duration of pair bonds was short at 1.73 years. A female-biased sex ratio is present in this population, and we demonstrated that short-tailed females are not preferred mates: females paired to males had longer tails than those in female–female pairs or other multi-female associations. In male–female pairs, tail lengths of mates were correlated, but this may have resulted in part from the correlation in ages. These observations are consistent with the hypothesis that tail streamers are used by both sexes in mate choice. In contrast to our results for tail length, tail symmetry was not significantly related to indices of individual quality and was not significantly correlated between mates.