Milder winters alleviate seasonal challenges for a migratory goose facing Arctic warming

Many migratory birds now reach their Arctic breeding grounds earlier in response to advancing springs and shifting nutrient peaks, either by departing earlier from non-breeding grounds or by travelling faster. For dark-bellied brent geese, there is limited potential to travel faster, as their spring migration is already among the fastest of Arctic geese and swans. Earlier departure would require reaching departure body mass earlier, either through a faster accumulation of energy stores during spring staging in the Wadden Sea or via adjustments earlier in the annual cycle. We examined long-term shifts in spring staging phenology and changes in winter and spring body mass trajectories of brent geese at the population level, with particular emphasis on the effects of winter temperature on body mass and spring body mass on departure timing. We used more than five decades of body mass measurements from individuals caught in the United Kingdom and France and in the Dutch Wadden Sea to reconstruct changes in winter and spring mass trajectories, respectively. These data were combined with over five decades of migration counts in the Netherlands and more than two decades of counts in Denmark to quantify changes in arrival and departure timing, respectively. We found that brent geese were heavier during and after milder winters and that their body mass trajectories flattened over the years: they no longer lose body mass during winter; during spring staging, fuelling rates have declined (in the so-called 'fattening phase'), while they also no longer lose body mass in the 1-2 weeks prior to departure ('plateau phase'). Brent geese have not shifted their spring arrival in the Wadden Sea but have advanced departure timing. Annual variation in body mass was not related to annual departure timing. These results suggest that milder winters have relaxed energetic constraints and improved body condition in brent geese throughout the non-breeding season. Our findings highlight the importance of considering the full annual cycle when assessing how migratory animals respond to global change, in particular those with limited capacity to adjust migration timing or speed.