Abstract: Migratory birds should adjust their spring migration timing to keep pace with advancing Arctic springs and shifting nutrient peaks. Many species now reach their Arctic breeding grounds earlier by departing earlier from non–breeding grounds or by travelling faster, both strategies with a bearing on the accumulation of energy stores, called fuelling. For dark–bellied brent geese, however, the potential to travel faster is limited: their 5,000 km spring migration to Taimyr is among the fastest of Arctic geese and swans. Earlier departure to their Siberian breeding grounds would therefore require fuelling adjustments, either through faster fuelling at spring staging sites or through adjustments elsewhere in the annual cycle. We combined over five decades of body measurements from individuals caught in the Dutch Wadden Sea to reconstruct changes in long–term fuelling trajectories, and paired these with migration counts from the Netherlands and Denmark to estimate the population’s spring staging phenology. We found that brent geese have not shifted their arrivals in the Wadden Sea in early spring, but 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 did not change. 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.

Contains input and created data (as Excel and shapefiles) and code used to analyse the data (in RStudio; order indicated by number). Migration counts are available at trektellen.nl (for the Netherlands) and dofbasen.dk (for Denmark). Catch/body mass data can be obtained from submit.cr-birding.org, and tracking data from movebank.org, or upon request from the authors.