Sexual Selection and Tail-Length Dimorphism in Scissor-Tailed Flycatchers

Abstract

AbstractScissor-tailed Flycatchers (Tyrannus forficatus) exhibit elongated tails in both sexes, and sexual dimorphism in tail length. At Fort Sill, Oklahoma, during 1991 and 1992, Scissor-tailed Flycatchers exhibited sexual dimorphism (male–female) in tail length (1.48), with more moderate sexual dimorphism in wing length (1.09) and beak length (1.04). Based on an analysis of museum specimens, immature birds (<1 year of age) of both sexes in their first calendar year exhibited significantly shorter tails than adults (measured in the field). Furthermore, tail length was highly variable among both sexes relative to other morphological traits. Male tail length was correlated with early clutch initiation by the male's mate and, in 1991, with larger clutch size. Similarly, female tail length was correlated with early clutch initiation, and, in one year, larger clutches. Longer-tailed females also tended to arrive earlier on the breeding grounds in 1992, the only year for which such data were available. Assortative mating by tail length was observed. Those findings support the hypothesis that tail length is a sexually selected trait in this socially monogamous species, and that female tail length may be correlated with measures of female quality (e.g. early arrival and breeding, large clutches). However, confounding effects of age on tail length make it difficult to distinguish among various hypotheses for evolution of elongated tails in this species.