Morphological Sex Determination of East Asian Barn Swallows (Hirundo rustica) in Tropical Wintering Region

The Barn Swallow Hirundo rustica is a non-breeding, sexually dimorphic, and diurnal migrant that overwinters worldwide, including in Peninsular Malaysia. While numerous studies on Barn Swallows have been conducted, their ecology in wintering sites, particularly tropical regions, remains poorly understood. Notably, little information is available on the morphological sex determination of Barn Swallows, especially for the East Asian Barn Swallow H. r. gutturalis population. This population migrates through or winters in the Thai-Malay Peninsula, while breeding occurs from the eastern Himalayas to northeast Russia (Siberia), China, the Korean Peninsula, and Japan. This study aims to determine the best parameters for the morphological sexing of East Asian Barn Swallows and was conducted in Bentong, Pahang, central Peninsular Malaysia. Swallows were captured using a modified scoop net attached to a telescopic pole, and their morphological data were recorded. A total of 46 individual East Asian Barn Swallows (19 females and 27 males) were captured for sex determination. We confirmed the sex and subspecies of sampled individuals using a molecular approach. We observed that two of the seven measured variables—tail fork depth (the length difference between the outermost and innermost tail feathers; T6-T1) and the length of the outermost tail feather (6th rectrix; T6) were chosen as the best predictors for sex determination. According to the quadratic discriminant functions constructed, approximately 89.47% of females and 96.3% of males were correctly classified using a combination of both chosen predictors. These morphological determination findings represent baseline knowledge that can help to provide more accurate and convenient Barn Swallow sex determination in the field.