Decline in Bewick’s swans prompts Fenland researchers to study birds’ behinds

AS hundreds of Bewick’s swans migrate from Fenland to Arctic Russia, researchers are hunting for reasons why the breed is in decline - starting by looking at their behinds.

The Wildfowl and Wetlands Trust, who have a centre at Welney, have been ‘abdominal profiling’ - recording the size of the area between the swans’ legs and tail to judge how much fat they have build up over winter.

The Ouse Washes hold the largest roost of Bewick’s swans in the UK. Their numbers are falling sharply and scientists are hoping to rule out a shortage of suitable food at their UK wintering sites as a reason for the decline.

By observing the swan’s behinds, researchers can determine whether they have been able to find enough food to survive the 2,500 mile journey back to breed.

Julia Newth, researcher, 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.

“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.

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“We know so much about Bewick’s swan behaviour, having followed the lives of many individuals visiting Slimbridge since 1964. It is really critical that we learn why their numbers are decreasing.”

Counts from this winter indicated that numbers peaked in February, with 6176 birds using the Ouse Washes and the feeding grounds in the surrounding area.

The numbers of Bewick’s swans wintering in Europe have declined sharply since 1995 - dropping about 27 per cent from 29,000 to about 21,000 in 2005.

It is suspected that the swans are being affected by habitat and weather changes at their breeding grounds. Other known causes of death include collisions with power lines, lead poisoning and illegal shooting.

Mrs Newth added: “Results from this year’s observations will be compared with data collected previously during the 80’s, 90’s and last winter.

“It will help rule out that the reduction in overwintering swans is due to changes in habitat at UK wintering sites - and give an indication of how the swans are responding to environmental change.

“Researchers will be able to see if the population’s ability to gain condition over winter and their feeding behaviour has changed in recent years. This in turn will really help in making conservation management decisions for the future.”

Every five years ornithologists across Europe complete a co-ordinated swan census to determine the total number of birds in each swan population.

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