Migration pattern of a population of Barn Swallows (Hirundo rustica) breeding in East Asian tropical region

Birds exhibit a high degree of migratory diversity, which is influenced by various ecological factors and life history strategies. Conducting studies on tropical bird migration, of which research is scarce, and comparing it with temperate birds can enhance our understanding of bird migration behaviour and its underlying mechanisms. In this study, we explored the migration behaviour of a breeding population of the Barn Swallow (Hirundo rustica) in Zhanjiang, southern China, a region located in the northern tropics, using light-level geolocators. From 2021 to 2023, we deployed geolocators on 92 breeding swallows and retrieved geolocators successfully from 23 individuals. These swallows all exhibited migratory behaviour, and wintering on various islands in Southeast Asia. They displayed sex differences in their wintering locations. All males concentrated in Borneo, while females primarily chose Borneo but also dispersed to the Philippines, South China Sea, and Vietnam for wintering. The studied swallow population adopted a seasonal migration pattern of “indirect in autumn, direct in spring”, bypassing the ecological barrier of the South China Sea in autumn and tending to directly cross it in spring migration. Moreover, the distance and duration of autumn migration was significantly longer than those of the spring migration. Compared to temperate Barn Swallows, the Barn Swallow population breeding in Zhanjiang adopts a pattern of “intra-tropical migration” and initiates autumn migration earlier. The formation of their migration pattern may be limited by ecological and physiological factors.