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 bums 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 Bewick's 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 2,500 miles back to their breeding grounds.
The aim is to rule out a shortage of suitable food at wintering grounds in the UK as one of the causes of the bird's falling numbers.
Julia Newth, WWT researcher, said: "We need to eliminate the possibility that the swans are suffering while in the UK.
"In a slim bird the bum will look slightly concave, whereas a well-fed bird will have a double bulge.
"The analysis has yet to be bottomed out but our observations in the field certainly suggest they are leaving for the journey with big healthy behinds.
"We need to do further work to see whether their body conditions have changed over the years, and, if so, whether this is connected with the decline in numbers seen in recent years."
Using results from this year's observations and data collected during the 80s, 90s and last year, experts hope to rule out the possibility that the reduction in overwintering swans is related to changes in habitat at UK sites.
The research will also give an indication of how the swans are responding to environmental change and help drive conservation efforts in the future, she said.