Flyway population increase and emergence of new wintering grounds with climate change in an Arctic-breeding goose.

Changes in climate, land cover, hunting disturbance, and food availability are often cited as drivers of bird migratory patterns. We examined long-term changes in the Pannonic flyway population of the Arctic-breeding Greater White-fronted Goose (GWfG, Anser albifrons) in regular counts conducted in Hortobágy National Park, eastern Hungary, between 1989 and 2019. During 31 years, GWfG counts increased from fewer than 2,000 to more than 30,000 individuals, with a stronger increase in spring than in autumn. The rate of increase exceeded that of the entire Pannonic flyway population. Spring counts rose more rapidly in the early years (1989–1997), whereas autumn counts increased faster more recently (2007–2019). Overwintering began in the mid-2000s, and winter counts have increased sharply since 2007. Mean temperature influenced GWfG counts positively in winter and negatively in spring, while the number of frost days in winter and snowy days in spring had negative effects. GWfG counts decreased less from December to January when temperatures did not drop or increased. These results indicate that milder winters have played a major role in the long-term increase and appearance of overwintering. In addition, small changes in land cover and slight increases in hunting disturbance may have also contributed to the increase of the GWfG counts at the study site, whereas food availability on nearby croplands declined and may have had an opposite effect. Our study shows that the disproportionate increase in staging and overwintering GWfG is primarily driven by climate change, especially by the rising frequency of unusually mild winters.