Bewick's swans are a migratory water bird that breed south of the Barents Sea in European Russia. They fly thousands of kilometers every year to find suitable wintering areas in Western Europe.
However, their migration patterns have changed significantly in recent decades due to climate change.
According to a recent study by researchers from the University of Amsterdam (UvA) and the Netherlands Institute of Ecology (NIOO-KNAW), Bewick's swans fly less far during their autumn migration when the weather is warm.
They used GPS data from 25 years of tracking individual swans to show that the birds are very flexible in their choice of wintering areas from year to year.
The study found that cygnets (young swans) fly further to the southwest during autumn migration on colder days.
Later in the autumn, this sensitivity decreases and the swans only move deeper into their winter areas when the temperature drops below freezing.
"When it is warm, they stay in place," said Hans Linssen, a PhD candidate at UvA and one of the authors of the study.
The researchers calculated that Bewick's swans hibernate on average 120 km closer to their breeding areas when the winter temperatures are on average one degree Celsius warmer.
When it is two degrees warmer, the average distance is 240 km, and so on.
This means that climate change has led to a shift in their common wintering areas over time.
"The core area used to be in southern England, now it is northern Germany. The birds now simply stop migrating earlier in the autumn," said Linssen.
Temperature is not the only factor that affects the migration of Bewick's swans. The study also looked at the effect of wind and food availability on the migratory behavior of the swans.
The researchers found that the birds flew longer distances when they had the wind at their backs, but that influence disappeared during the autumn.
This suggested that they only use the wind as a tool to reach their winter destination, while the temperature can determine where that destination is.
Food availability also influences the migration of Bewick's swans but in a more complex way.
The researchers found that food abundance in potential wintering areas had both positive and negative effects on the migration distance of the swans.
On one hand, more food means more energy and less competition for the swans, which could encourage them to fly further. O the other hand, more food also means less need to move around, which could discourage them from flying further.
The researchers concluded that food availability has a weaker effect than temperature on the migration distance of Bewick's swans and that its effect varies depending on the season and the age of the swans.
They suggested that future studies should investigate how food availability interacts with other factors such as predation risk and habitat quality to affect the migration decisions of Bewick's swans.
Implications for conservation
The study by UvA and NIOO-KNAW provides new insights into how climate change affects the migration of Bewick's swans, and how individual swans adapt to changing environmental conditions.
The results have important implications for the conservation of this species, which is classified as endangered by the International Union for Conservation of Nature (IUCN).
One implication is that climate change may reduce the connectivity between different populations of Bewick's swans, as they become more dispersed across different wintering areas.
This could affect their genetic diversity and their ability to cope with future environmental changes.
Another implication is that climate change may expose Bewick's swans to new threats in their wintering areas, such as habitat loss, human disturbance, hunting, and disease. These threats may reduce their survival and reproduction rates, and thus their population size.
The researchers recommended that conservation efforts for Bewick's swans should take into account their individual flexibility and their sensitivity to temperature.
They also suggested that conservation managers should monitor the distribution and abundance of Bewick's swans in their wintering areas, and identify and protect the key habitats that they use. By doing so, they hope to ensure the long-term survival of this magnificent migratory bird.
