Many migratory birds are known to respond to climate warming by shifting their distribution towards higher latitudes or elevations. However, previous studies used ring resighting or count data, making it difficult to examine in detail how individuals respond to annual climatic variation, and especially to temperature throughout their autumn migration.
Bewick’s swans in flight by Frederick Fleet.
The winter range of the NW-European Bewick’s swan in the countries around the North Sea has shifted more than 350 km closer to the breeding grounds since 1970 due to “short-stopping”: the ceasing of autumn migration at increasingly north-eastern latitudes and longitudes. Here, we used large amounts of multi-year GPS tracking data to examine at the individual level how autumn-migratory movement and annual winter distance from the breeding grounds relate to temperature.
GPS tracks of the swans’ autumn migration revealed that individuals migrated larger distances with lower temperatures, and especially so with beneficial winds early in the season. Later in the season, the effect of wind disappeared and swans only travelled further with temperatures below freezing. Temperature thus drives movement away from the breeding grounds through different mechanisms during early and late autumn, with the frost effect only becoming apparent later.
Overview of the study.
When looking at multi-year tracks, we found that individual swans are highly variable in their annual winter distance. There was a strong relationship between winter temperatures and winter distances: for every 1 °C increase in mean winter temperature, swans spent their winter on average 118 km closer to the breeding grounds. Our study shows that range shifts in long-lived migratory birds can be driven by flexible responses to changing environmental conditions at the individual level. At the same time, individuals were highly consistent in their annual breeding location. Whether this means that the Bewick’s swan is less flexible in adapting to climate warming in the breeding range, is a subject of further research.
The full article can be found here