Scientists have been studying the backsides of Bewick’s swans as they turn tail for their breeding grounds, to learn more about why their numbers are falling.
Experts are hoping that bulging rears will show that the sharp declines experienced by the species are not the result of a lack of food at their wintering sites in the UK.
The swans have been leaving the UK where up to 8,000 over-winter between October and March, and heading off on their annual migration to the Russian Arctic for the summer.
But the numbers coming to Europe to spend the winter have dropped sharply by more than a quarter in a decade, from 29,000 in 1995 to around 21,000 in 2005. Experts believe that a number of factors are affecting the birds’ survival, including habitat and weather changes in their breeding grounds.
The swans are also at risk of collisions with power lines, lead poisoning and illegal shooting.
Trained observers at the Wildfowl and Wetlands Trust (WWT) at Slimbridge, Gloucestershire, and Welney, Norfolk, have been studying the swan’s bottoms as they depart to conduct “abdominal profiling”.
By recording the size of the area between the birds’ legs and tail where they store the fat they have built up over the winter, the researchers can see if the swans have found enough food during the last few months to get them to their breeding grounds.