Climate change affects migratory birds in time (phenology), space (use of stopover sites, staging and wintering areas) and numbers (numerical change). We assessed these long-term changes by using data from the monitoring of the phenology and size of the Pannonic flyway population of Arctic-breeding greater white-fronted geese (GWfG, Anser albifrons) in regular weekly counts in Hortobágy National Park (east Hungary) between 1989 and 2019. We found that the flyway population size increased considerably from < 2000 to > 30,000 in 30 years, more so in the spring than in the autumn migration. Since 2007, an increasing number of GWfG have overwintered in the study area, which had not been used for wintering before. A positive relationship between December peak counts and mean temperature suggested that milder winters played a role in this increase. However, there was no long-term trend in mean temperature of any month over the study period, indicating little direct, local impact of climate change. Because land use and management also changed little in the study period, we conclude that the increase of GWfG results from the general increase in flyway population size, which was recently reported in other studies. The establishment of the new wintering area is probably based on this increase and made possible by milder winters since 2007. Our long-term study thus provides evidence of climate change-related modifications in migration phenology, spatial re-arrangement of the wintering grounds, and large-scale increase in population size detectable at the local scale in an important staging area.