Data from "Earlier spring departure mediated by higher spring body mass after milder winters in a migratory goose facing Arctic warming"

1. Many migratory birds now reach their Arctic breeding grounds earlier in order to keep pace with advancing Arctic springs and shifting nutrient peaks, either by departing earlier from non–breeding grounds or by travelling faster. Both strategies affect the accumulation of energy stores, called fuelling. For dark–bellied brent geese, travelling faster seems no option, as their migration to Taimyr is already among the fastest of Arctic geese and swans. Earlier departure to their Siberian breeding grounds, on the other hand, would require fuelling adjustments, either through faster fuelling at spring staging sites or via adjustments earlier in the annual cycle. 2. We investigate to which extent brent geese have changed their spring staging phenology and fuelling trajectories in the Wadden Sea at the population level, with particular emphasis on changes in initial body mass in relation to winter conditions and fuelling rates. 3. We used over five decades of body measurements from individuals caught in the Dutch Wadden Sea to reconstruct long–term changes in fuelling trajectories, and combined these with migration counts in the Netherlands and Denmark to estimate the population’s spring phenology. 4. We found that brent geese have not shifted their arrivals in the Wadden Sea in early spring, while they have advanced the timing of departure from this key staging site when correcting for wind support and spring advancement. Furthermore, we found that after milder winters in France and the United Kingdom, brent geese in the Wadden Sea are heavier, and that their mean body mass additionally increased over the years, while fuelling rates in the Wadden Sea did not change. 5. Higher body mass in early spring, largely facilitated by warmer winters at temperate non–breeding grounds, thus broadens the potential for Arctic migrants to align arrival and breeding with advancing Arctic springs. Our study shows that phenological flexibility is shaped across the entire annual cycle, and that long–distance migrants with limited opportunities to alter migration timing or speed can still respond to global change by enhancing flexibility that originates on non–breeding grounds well before fuelling at staging sites.