Northward range expansion in spring-staging barnacle geese is a response to climate change and population growth, mediated by individual experience

All long-distance migrants must cope with changing environments, but species differ greatly in how they do so. In some species, individuals might be able to adjust by learning from individual experiences and by copying others. This could greatly speed up the process of adjustment, but evidence from the wild is scarce. Here, we investigated the processes by which a rapidly growing population of barnacle geese (Branta leucopsis) responded to strong environmental changes on spring-staging areas in Norway. One area, Helgeland, has been the traditional site. Since the mid-1990s, an increasing number of geese stage in another area 250 km further north, Vesteralen. We collected data on goose numbers and weather conditions from 1975 to 2017 to explore the extent to which the increase in population size and a warmer climate contributed to this change in staging area use. During the study period, the estimated onset of grass growth advanced on average by 0.54 days/year in each of the two areas. The total production of digestible biomass for barnacle geese during the staging period increased in Vesteralen but remained stable in Helgeland. The goose population has doubled in size during the past 25 years, with most of the growth being accommodated in Vesteralen. The observations suggest that this dramatic increase would not have happened without higher temperatures in Vesteralen. Records of individually marked geese indicate that from the initial years of colonization onwards, especially young geese tended to switch to Vesteralen, thereby predominating in the flocks at Vesteralen. Older birds had a lower probability of switching to Vesteralen, but over the years, the probability increased for all ages. Our findings suggest that barnacle geese integrate socially learned behaviour with adjustments to individual experiences, allowing the population to respond rapidly and accurately to global change.