Effectiveness of Morphological Sex Determination in the East Asian Barn Swallow (Hirundo rustica gutturalis) on Spring Migration

Information on sexual dimorphism helps explain a species' evolution in sexual selection and conservation issues such as sex-specific response in environmental changes. The Barn Swallow (Hirundo rustica) is a well-known sexually dimorphic species in which males have longer tail streamers than females. However, when compared with the European-African or American subspecies, little information is available on morphological sex determination in the East Asian subspecies H. r. gutturalis, especially outside the breeding season, when opportunities for molecular sexing are often limited and morphological clues may be the only ones available for sex determination. We collected morphological data on H. r. gutturalis during the northbound spring migration at a stopover site off the Korean coast. Two of eight measured variables - streamer length (the difference in length between the 5th and 6th rectrices; T6-T5) and length of the white patch on the outermost tail (6th rectrix; T6) - were selected as the best predictors for sex determination by stepwise discriminant analysis. Quadratic discriminant functions based on these variables showed that 92.4-93.5% of females and 82.9% of males were correctly classified. Our results provide baseline information that will benefit more accurate sex determination of the East Asian Barn Swallows, especially during the early months of a calendar year in non-breeding and stopover areas.