SEXUAL SELECTION AND TAIL-LENGTH DIMORPHISM IN SCISSOR-TAILED FLYCATCHERS

Abstract

Abstract Scissor-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. Assort...