From individual movement to a changing population distribution in Bewick’s swans

By Hans Linssen, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam

The recent decades have seen the Bewick’s swan wintering distribution in Europe slowly shifting in northeastern direction: where the population was centered in the UK in the ‘70s, numbers of wintering swans are now largest in northern Germany.

Although we know that the shift is connected to a warming climate, which makes increasingly northeasterly areas suitable for wintering, we know very little about the choices individuals make that drive the population shift. In this project, running from 2021 to 2025, we use a combination of GPS tracking, existing tracking data, field observations and various modelling techniques to examine individual Bewick’s swan movement strategies within and between wintering and migration sites. Which factors drive individuals’ choices of wintering area, and how large is the role of temperature? Bewick’s swans have started foraging increasingly on maize rests, but what influence do diet choices have on migration timing? We aim to track both juvenile swans and their parents. In that way, we can find out to what extent wintering area and migration timing are copied between generations and remained faithful to throughout an individual’s life, or to what extent they are flexible and responsive to year-to-year or month-to-month variation in environmental conditions. Taking together these aspects of year-round individual movement, we will have a better, more mechanistic understanding of how the current population shift arises.

Seven swans newly equipped with GPS trackers are released on their night roost. Photo by Hans-Joachim Augst

Spring (top) and autumn (bottom) migration tracks of mother swan and her three juveniles. During their first spring migration, the juveniles separate from their mother in the White Sea, Russia, but they stay with each other for the summer. During autumn migration, the young split from each other as well and go their own way into the wintering area.