Climate change and contrasting plasticity in timing of a two-step migration episode of an Arctic-nesting avian herbivore UNCORRECTED PROOF Climate change and contrasting plasticity in timing of a two-step migration episode of an Arctic-nesting avian herbivore

Greenland white-fronted geese Anser albifrons flavirostris wintering in Britain and Ireland migrate over the sea for 700–1200 km to stage 3–5 weeks in Iceland in spring, continuing a similar distance over the sea and Greenland Ice Cap to West Greenland breeding grounds. During 1969 to 2012, the geese advanced the mean departure date from Ireland by 15 days, during which time also they attained threshold fat stores earlier as well as departing in fatter condition. Over that period, Iceland spring-staging geese shifted from consuming underground plant storage organs to grazing managed hayfields, which provide fresh grass growth despite sub-zero temperatures, when traditional natural foods are inaccessible in frozen substrates. In 2012 and 2013, geese arrived three weeks earlier to Iceland, in fatter condition and accumulated fat significantly slower than in 1997–1999 and 2007. Although geese accumulated sufficient fat stores earlier in Iceland in 2007, 2012 and 2013, they departed around the same date as in 1997–1999, prolonging spring staging by three weeks. Plasticity in winter departure dates is likely due to improved winter feeding conditions (enabling earlier departure in better condition) and a novel predictable food resource in Iceland. Greenland white-fronted geese attained threshold fat stores in Iceland earlier, but remained rather than departing earlier to Greenland. Despite arriving earlier in Iceland, arrival dates on the breeding areas have not changed since the 1880s, presumably because of relatively constant cool springs and heavy snowfall in West Greenland during recent years [Current Zoology 60(2) : – , ].