Swans get bum deal on migration from Norfolk
Researchers will be closely watching the rear end of swans as they leave the Ouse Washes and head back to Russia to breed.
For they now believe the size of the birds' bottoms could determine whether they have built up sufficient reserves over the winter for their long migration.
Declining numbers of Bewick's swans have been a cause for concern and scientists are hoping to rule out a shortage of food in their UK wintering grounds as a potential problem.
Trained observers will be looking at the size of swans' behinds as they leave The Fens over the next couple of weeks to see if they are carrying enough weight for the journey.
More than 6,000 birds were using the Ouse Washes in February and they will soon be flying 2,500 miles to the Russian Arctic for breeding.
The observers have been abdominal profiling, recording the size of the area between the swans' legs and tail where they store fat. Researchers can then determine whether the swans have been able to find enough food to survive the journey.
Julia Newth, a researcher for the Wildfowl and Wetlands Trust, which manages a reserve a Welney, said: 'We need to eliminate the possibility that the swans are suffering while they winter in the UK. In a slim bird the bum will look slightly concave, whereas a well-fed bird will have a double bulge. Our observations in the field certainly suggest they are leaving for the journey with big healthy behinds. It is really critical that we learn why their numbers are decreasing.'
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The numbers of Bewick's swans wintering in Europe have declined sharply since 1995, dropping about 27pc from 29,000 to about 21,000 in 2005. A number of factors could be affecting the swans' survival and breeding success, including habitat and weather changes at their breeding grounds, lead poisoning and illegal shooting.