Knowledge about intraspecific and individual variation in bird migration behavior is important to predict spatiotemporal distribution, patterns of phenology, breeding success and interactions with the surrounding environment (e.g., human livelihoods). Such variation is key to adaptive, evolutionary responses, i.e., how individuals respond spatiotemporally to the environment to maximize fitness. In this study we used GPS location data from one to three full annual cycles from 76 Greylag geese Anser anser to test the hypothesis that geese originating at five latitudinally separated capture sites in Sweden have different migration strategies. We also assessed individual consistency in movement strategy over consecutive annual cycles. We used the scale-independent net squared displacement modelling framework to quantify variables of autumn and spring migration for geese from each capture site: distance, timing, and duration. Our results demonstrate a positive correlation between migration distance and latitudinal origin. Geese from the northernmost site on average migrated farther south and about 15 times as far as the short-moving or resident geese from the two southernmost sites. Movement strategies of individual geese varied considerably both within and among capture sites. Individual consistency in movement strategy from one annual cycle to the consecutive was high in geese from the northern sites moving the farthest, whereas the resident or short-moving geese from the southernmost sites generally showed lower or no individual consistency. These changes have come about during a time span so short (i.e. ca 35 years or 8-10 generations) that it can unlikely be explained by classical Darwinian between-generation adaptation. Consequently, and given that young geese follow their parents during their first migration, we presume an important role of within-family, inter-generation change as a driver behind the large-scale changed migration habits in Swedish Greylag geese.

The study is based on location data obtained 2017-2020 from 76 Greylag geese with GPS-equipped neckbands, made by either Ornitela (OT-N35 and OT-N44) or Made-by-Theo (Theo Gerrits), focusing on breeding and molting adults and their unfledged goslings. Handling protocols were approved by the animal ethics committee for central Sweden and fulfilled the ethical requirements for research on wild animals (decision Dnr 5.8.18-03584/2017). The GPS location dataset was handled and analysed according to the framework of net squared displacement modelling. For detailed methods, see the focal paper or Bunnefeld et al. 2011. A model-driven approach to quantify migration patterns: individual, regional and yearly differences. - J. Anim. Ecol. 80: 466–476.