Detour for the inexperienced? Migration count data suggest mostly juvenile Greater Spotted Eagles appear in coastal peninsulas in China

Soaring bird migration often relies on suitable terrain and airflow; therefore, route selection is vital for successful migration. While age and experience have been identified as key factor influencing migration route selection among soaring raptors in the African-Eurasian Flyway, how they shape the migration route of soaring raptors in East Asia is still largely unknown. In this study, we investigated potential variations in the routes and timing in autumn migration of juvenile and older soaring birds, using count data of Greater Spotted Eagles (Clanga clanga) from two coastal sites and two inland sites in China. From 2020 to 2023, we recorded a total of 340 individuals, with the highest site averaging over 90 individuals per autumn, making it one of the world's top single-season counts and thus a globally important site for this species. We found that 82% and 61% records from coastal sites were juveniles, significantly higher than inland sites (15% and 24%). Juveniles at all four sites exhibited markedly earlier median passage time than non-juveniles, with brief overlapping in their main migration periods. Both coastal sites are located on the tip of peninsulas stretching southwest, requiring long overwater flights if crossing the Bohai Bay or Beibu Gulf, which would be energetically demanding and increase mortality risk. Experienced individuals may have learned to avoid such terrain and subsequent detour, while juveniles are more prone to enter these peninsulas due to lack of experience and opportunities for social learning, or following other raptor species that are more capable of powered flight. Our findings highlight the importance of age and experience in migration route selection of large soaring birds.