By Victoria Gill
Science and nature reporter, BBC News
A slim bird (top) has a concave bottom but a well-fed bird's backside bulges
Bigger bottoms are definitely better - for swans at least.
Scientists at the UK's Wildfowl and Wetlands Trust are measuring the size of Bewick's swans' behinds, to see if they have the fat reserves to survive their long migration to Arctic Russia.
The swans' population is in decline and the researchers want to find out if a shortage of suitable food at their UK wintering sites could be the cause.
The birds are just about to embark on their annual journey.
Researchers at the WWT's wetland centre in Slimbridge, Gloucestershire, are trying to gather the measurements before the birds set off on their migration.
Between 1995 and 2005, the number of Bewick's swans wintering in Europe fell from 29,000 to about 21,000.
The team says that these measurements should "rule out" a shortage of suitable food at their UK wintering sites as a reason for the decline.
Scientists and trained volunteers are recording the size of the area between each swan's legs and tail, which is where they store fat they build up over the winter.
The birds need these extra fat reserves to survive the 4,000km migration to their Arctic breeding grounds.
WWT researcher Julia Newth said: "In a slim bird, the bum will look slightly concave, whereas a well-fed bird will have a double bulge."
Athough these vital statistics are yet to be analysed in full, Ms Newth said that the observations suggested that most birds were embarking on their migrations 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," she added.
WWT researchers have been observing this species since the 1960s. They think the swans could be suffering from changes in their habitat and in the weather at their breeding grounds.
Other known causes of death include collisions with power lines, lead poisoning and hunting.
Bewick's swans are smaller than the more familiar mute swans, which live in Britain all year round.
Just before they migrate, the birds' behaviour and vocalisations change. The birds become very alert, bobbing their heads and "chattering" at a distinctive pitch.
Bewick's swans become more alert just before they set off on their migration