Characterizing partial migration using GPS tracking: high individual consistency in greylag geese

Migration is a critical ecological strategy throughout the animal kingdom and can be either fixed or flexible at the species, population, and individual levels. In partially migratory species, where only some individuals migrate, individuals may show consistency in their migratory behaviour. Greylag geese Anser anser are considered partially migratory, with known population‐level flexibility in migration strategy. We used GPS tracking data from 21 wild greylag geese over seven years (2018–2025) around Lake Neusiedl, Austria, to investigate partial migration and the individual consistency of movement behaviour across years. To do so, we developed a multi‐metric framework combining 95% annual kernel density estimates, net squared displacement (NSD) profiles, segment‐level distance and speed, and model‐based classification with the ‘migrateR' package. As part of this framework, we introduce a novel summary metric – the duration‐normalized area under the NSD curve – which integrates how far and for how long an individual displaces from its breeding ground into a single, individual‐comparable value. Based on these metrics, we found that 20/21 geese were migratory, with only one studied individual remaining a year‐round resident for their entire tracking duration. Four individuals changed their movement behaviour between migratory and resident across years. Migratory individuals varied significantly in their movement patterns, overwintering across a broad latitudinal gradient, from upper Germany to Sardinia. These findings highlight the coexistence of plasticity and consistency in greylag goose migration and offer valuable opportunities to investigate the ecological and intrinsic drivers shaping movement behaviour at both individual and population levels.