GPS tracking data reveal the annual spatiotemporal movement patterns of Bridled Terns

Understanding the spatiotemporal patterns of animal movement is a central theme in the growing field of movement ecology. The Bridled Tern (Onychoprion anaethetus) is widely distributed across tropical and subtropical latitudes; however, knowledge of its annual movement is based only on relatively scarce recoveries of ringed birds. Studying the annual movement of pelagic seabirds is important to identify the impact of changes of marine habitats on them. We examined fine-scaled movement patterns of the species from two colonies in southern China by using GPS-GSM transmitters in 2018–2022. Twenty-three terns bred in Xichiyu Islet and Qilianyu Islet while wintered in four different sites: Gulf of Thailand, Natuna Islands, The Sulu Sea, and Makassar Strait. Bridled Terns made small detours and employed a fly-and-forage strategy with frequent stopovers to forage during autumn migration, but took more direct routes and reduced the need for stopovers during spring migration. Distance of migration between breeding and winter sites was significantly longer in autumn (average 3635 ​km) when compared with spring (2777 ​km). Ten birds with whole-year tracking data used the same breeding sites both years and four birds with tracking data of two consecutive years returned to the same wintering area, indicating that Bridled Terns are highly faithful to their breeding and wintering grounds.