Early start, early finish – Migration strategy of tropical Barn Swallows (Hirundo rustica)

Abstract

Abstract Background Bird migration patterns exhibit high diversity and are influenced by various ecological factors. While birds breeding in tropical regions exhibit multiple migration patterns, research on these patterns, particularly in the Indomalayan realm, is still scarce. Thus, conducting studies on tropical bird migration and comparing them with similar studies in temperate regions can enhance our understanding of bird migration behaviour and its underlying mechanisms. Methods We tracked the migration routes of a breeding population of Barn Swallows (Hirundo rustica) in the northern tropical region of Asia using light-level geolocators. From 2021 to 2023, we deployed geolocators on 92 breeding swallows (44 males and 48 females) and retrieved geolocators successfully from 23 individuals (9 males and 14 females). Results The studied breeding swallows all exhibited migratory behaviour and were local summer residents, mostly wintering on various islands in Southeast Asia, with a migration distance of 5,275.78 ± 916.52 km (mean ± SD, n = 20). They migrated from February to March in spring and from June to September in autumn, with migration timing earlier than that of temperate populations. The studied population of Barn Swallows adopted a seasonal migration strategy of "easy in autumn, hard in spring", bypassing the ecological barrier of the South China Sea in autumn and tending to directly cross the South China Sea in spring. The wintering distribution of female Barn Swallows was more scattered compared to males, and no protandry phenomenon was observed in the studied population. Conclusions In the study site, tropical breeding Barn Swallows exhibited an intra-tropical migration pattern; compared to temperate Barn Swallows, the tropical population adopted an "early start, early finish" migration strategy. Considering both summer and winter residents of the Barn Swallow utilise the study area, it is speculated that Barn Swallows in East Asia follow a chain migration pattern. The formation of their migration strategy may be limited by physiological factors such as moulting, and this migration strategy helps alleviate intraspecific competition.