Sexual selection and tail-length dimorphism in Scissor-tailed Flycatchers

Auk, The, Jan 2001 by Regosin, Jonathan V, Pruett-Jones, Stephen

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 (

SCISSOR-TAILED FLYCATCHER (Tyrannus forfi- catus) are common summer residents in the south-central United States and northeastern Mexico occurring in most open habitats, including agricultural and urban areas, where there are perches for feeding and trees and shrubs for nesting (Fitch 1950, Regosin and Pruett-Jones 1995, Regosin 1998). The species' social organization is best characterized as social monogamy, with males defending large, dispersed territories, and individual males pairing with single females (Regosin and Pruett-Jones 1995). Although the female alone carries out nest building, incubation, and brooding of young, both males and females feed nestlings (Regosin and Pruett-Jones 1995).

Scissor-tailed Flycatchers (and Fork-tailed Flycatchers [T. savana]) of both sexes exhibit dramatically elongated tails relative to other kingbirds of the genus Tyrannus, and that elongation is not explained by interspecific variation in body size (J. Regosin unpubl. data). Furthermore, tail length in both Scissor-tailed and Fork-tailed flycatchers is highly sexually dimorphic. In this study, we examined sexual di- morphism and breeding phenology in Scissortailed Flycatchers in an effort to document the relationship between tail-length dimorphism and the species' social organization. Here we present a quantitative analysis of tail-length dimorphism, and examine variability in tail length as it relates to various aspects of the species' life history and mating system. Our data are consistent with the sexual selection hypothesis for the evolution of tail-length elaboration and sexual dimorphism in this socially monogamous species.

METHODS

This study was conducted from March to August during 1991 and 1992 on the range of Fort Sill Military Reservation in Comanche County, southwestern Oklahoma, in the mixed plains biotic district (Blair and Hubbell 1938). The study area consisted of four noncontiguous zones covering approximately 8 km^sup 2^ of mesquite (Prosopis juliflora) savanna dominated by little bluestem (Andropogon scoparius) and 1 km^sup 2^ of landscaped area with mowed grass and planted trees including hackberry (Celtis reticulata), American elm (Ulmus americana), and honey locust (Gleditsia triacanthos). An extensive system of dirt and paved roads runs through the study area. For a complete description of the study area, see Regosin and Pruett-Jones (1995).

Nests were located through extensive searches on foot and by car. Birds were captured with mist nets set around nest trees during egg laying, incubation, and brooding periods. Birds were banded with a numbered US. Fish and Wildlife Service aluminum band as well as with a unique combination of three plastic, colored bands. Morphological measurements of body mass, flattened wing chord length, tarsus length, and tail length were made for each individual that was captured. All linear measurements were taken on both sides of a bird's body in 1991, but only tail length was measured on both sides in 1992. In 1992, measurements of beak length, beak width, and beak depth were also taken. In our analyses, we used measurements of left tarsus, left wing chord, and maximum tail length, unless otherwise noted.

Sex of some females could be determined at the time of banding ti rough the presence of a vaculari- zed brood patch. To determine sex of other banded individuals (or of unbanded birds), we relied on behavioral characteristics: singing and display behavior, nest building, copulation, egg laying, and incubation. Male Scissor-tailed Flycatchers have never been observed to construct nests or incubate eggs, and females do not engage in singing behavior or perform tumble flight displays (Fitch 1950, Smith 1966, Regosin and Pruett-Jones 1995). Only those banded birds for which sex could be determined were included in our analyses; banded birds for which we never observed display or reproductive behavior were excluded.

Because immature, hatching year (HY) birds in their first calendar year could not be captured and measured in the field, we obtained wing, tail, and tarsus length measurements from specimens at the American Museum of Natural History (AMNH) that had been collected between September-December in Texas, Mexico, and Guatemala. Specimens of HY individuals and birds in their second calendar year (SY) can be identified by shape and extent of notching of the outer (P10) primaries (Pyle et al. 1987). That criterion is reliable for individuals measured through March, but is unreliable for identifying SY individuals returning for the first time to the breeding grounds (but see Pyle 1997). At the time of the study, there were no known plumage criteria for ageing older, after-second-calendar-year (ASY) individuals.