GPS-tracking provides novel insights into the age-specific migration behaviour of an endangered migrant seabird in the Afro-Palearctic flyway

Abstract Background Migration is a complex behaviour, requiring individuals to integrate genetic, environmental and social information to reach distant goals, with important consequences for individuals, populations and communities. However, despite its ecological importance, our understanding of the development of migration behaviour across an organism’s lifetime is lacking for most taxa. Recently, developments in bio-logging have enabled us to track juveniles of many species during their first migrations, improving our understanding of migration ontogeny. The Baltic Lesser Black-backed Gull ( L. f. fuscus ) is a long-distance migratory seabird inhabiting the eastern Afro-Palearctic flyway that has undergone recent, dramatic declines. However, determining the drivers of these trends is hindered by poor understanding of fine-scale space use, particularly during the non-breeding period. Methods Here we fitted GPS-tags to a total of 70 adult and 39 juvenile Baltic gulls from 19 Swedish colonies to examine age-related differences in movement behaviour during autumn migration. Results We observed clear differences in the autumn migration behaviour of adult and juvenile Baltic Gulls. Adults tended to migrate further than juveniles, with a much higher probability of crossing the Sahara. They also completed their migrations faster despite similar flight speeds between age classes, with observed differences in overall migration speeds resulting from longer and more frequent stopovers in juveniles. Moreover, migration routes showed equivalent directness between age classes, with a notable lack of exploratory movements by juveniles. Conclusions GPS-tracking of adult and juvenile Baltic Gulls revealed little evidence of exploratory behaviour, providing limited support for the exploration-refinement hypothesis during neonatal migration. Combined with latitudinal differences in migration destinations between age classes, our work suggests that learning and development of this long-distance migration requires multiple annual cycles to complete. Additionally, we identify several key stopover and wintering sites used by Baltic Gulls during the non-breeding period, facilitating the identification of potential threats across the species’ life cycle.