Abstract
Range shifts and phenological change are two processes by which organisms respond to environmental warming. Understanding the mechanisms that drive these changes is key for optimal conservation and management. Here we study both processes in the migratory Bewick's swan (Cygnus columbianus bewickii) using different methods, analysing nearly 50 years of resighting data (1970–2017). In this period the wintering area of the Bewick's swans shifted eastwards (‘short‐stopping’) at a rate of ~13 km/year, thereby shortening individual migration distance on an average by 353 km. Concurrently, the time spent at the wintering grounds has reduced (‘short‐staying’) by ~38 days since 1989. We show that individuals are consistent in their migratory timing in winter, indicating that the frequency of individuals with different migratory schedules has changed over time (a generational shift). In contrast, for short‐stopping we found evidence for both individual plasticity (individuals decrease their migration distances over their lifetime) and generational shift. Additional analysis of swan resightings with temperature data showed that, throughout the winter, Bewick's swans frequent areas where air temperatures are c. 5.5°C. These areas have also shifted eastwards over time, hinting that climate warming is a contributing factor behind the observed changes in the swans' distribution. The occurrence of winter short‐stopping and short‐staying suggests that this species is to some extent able to adjust to climate warming, but benefits or repercussions at other times of the annual cycle need to be assessed. Furthermore, these phenomena could lead to changes in abundance in certain areas, with resulting monitoring and conservation implications. Understanding the processes and driving mechanisms behind population changes therefore is important for population management, both locally and across the species range.
Highlights
Rapid global environmental change is affecting ecosystems, communities and species worldwide (Walther et al, 2002)
Estimating the first and the last locations based on this regression and calculating the great circle distance between them equals a north-eastward shift of 618 km, corresponding to a mean short-stopping rate of 12.9 km y-1
To analyse short-stopping one needs to fix the timing of winter in order to determine whether individuals change their location within this season, but this makes it by definition impossible to detect short-staying
Summary
Rapid global environmental change is affecting ecosystems, communities and species worldwide (Walther et al, 2002). For migratory species that visit geographically discrete areas during their annual cycle, differing rates of change and the unpredictability of favourable conditions among these areas can create severe challenges (Robinson et al, 2009). If rates of change are not correlated between the areas these animals frequent, it is impossible for them to predict the time and place with optimal conditions (Kölzsch et al, 2015). This can lead to a mismatch with optimal circumstances (i.e. peak food abundance) because the cues they use change at a different rate (i.e. temperature at staging sites; Both, Bijlsma, & Visser, 2005), or do not change at all It has been found that migratory species that are declining tend to be those that are showing weak or no phenological changes (Møller, Rubolini, & Lehikoinen, 2008), especially when migratory diversity within a species is small (Gilroy, Gill, Butchart, Jones, & Franco, 2016)
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