A gradual migratory divide determines not only the direction of migration but also migration strategy of a social migrant bird

Migratory divides separate populations of migratory animals, facilitating the evolution of intraspecific differences in migration strategies. The optimal migration theory suggests differing migration strategies for birds using different flyways (with different habitats), but the knowledge regarding the impact of the flyway to the individual migration strategies is scarce. We used satellite tracking and neckband resightings to unravel the structure of the migratory divide between two European flyway populations of greylag geese Anser anser. We modelled satellite tracking data using Gaussian processes to study migration strategies of birds using different flyways. The mean posterior probability for an individual to migrate along the Western Flyway decreased gradually from 0.98 to 0.06 within the Baltic Sea coast, revealing a gradual migratory divide. In addition, migration strategies differed between the flyways. The birds using Western Flyway migrated earlier in autumn, performed longer annual migration and made a clear stopover during migration, whereas the birds using Central Flyway flew directly to their wintering sites. The gradual migratory divide that also divided migration strategies provides exciting insights to ecological and evolutionary factors behind migratory divides. Gaussian processes enabled modelling of detailed migration strategies, encouraging their future usage in movement ecology.