Abstract
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 250km further north, Vesterålen. 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 0.54days/year in each of the two areas. The total production of digestible biomass for barnacle geese during the staging period increased in Vesterålen but remained stable in Helgeland. The goose population has doubled in size during the past 25years, with most of the growth being accommodated in Vesterålen. The observations suggest that this dramatic increase would not have happened without higher temperatures in Vesterålen. Records of individually marked geese indicate that from the initial years of colonization onwards, especially young geese tended to switch to Vesterålen, thereby predominating in the flocks at Vesterålen. Older birds had a lower probability of switching to Vesterålen, 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.
Highlights
Many organisms currently face rapidly changing environments due to global warming (Blunden, Arndt, & Hartfield, 2018; Lowry et al, 2013; Tilman et al, 2001), which force them to adjust their be‐ haviour if they are to survive. This is especially true for long‐distance migrants, which depend on spatially separated environments that may change in different ways and at different rates
We investigated the performance of spring‐staging barnacle geese (Branta leucopsis) in Norway
The numbers of barnacle geese staging in Vesterålen increased rapidly from the 1990s onwards (Figure 3), and within 15 years after the first flocks of spring‐staging barnacle geese colonized Vesterålen, the numbers surpassed those in the traditional Helgeland area
Summary
Many organisms currently face rapidly changing environments due to global warming (Blunden, Arndt, & Hartfield, 2018; Lowry et al, 2013; Tilman et al, 2001), which force them to adjust their be‐ haviour if they are to survive. We studied the temporal relation between the popu‐ lation sizes of staging barnacle geese in both staging areas and the local environmental conditions, and to what extent this rela‐ tion has been mediated by the annually changing choices of new recruits in the population, and by older individuals that change their migratory strategy later in life To this end, we first quanti‐ fied annual numbers of barnacle geese both in Helgeland and in Vesterålen from 1975 to 2017, using counts of numbers of geese during spring staging as well as annual estimates of total flyway population size in winter (Griffin, 2018; Owen & Norderhaug, 1977; WWT, 2017). These data allowed us to assess whether the age composition in both staging areas differed, and whether the prob‐ ability of switching staging area between years changed with age and over time
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