One in five Bewick's swans migrating to Welney have been shot at on the way.

Declining numbers of Bewick's swans arriving to winter in The Washes could be partly down to illegal hunting of the birds on their 2,500-mile migration route.

While the Wildfowl and Wetland Trust (WWT) reserve at Welney has seen increasing numbers of migrating whooper swans, the Bewick's numbers have been declining and researchers are trying to establish the reasons behind the falling numbers.

It is a picture repeated nationally –with the whooper swan apparently doing well and the Bewick's in decline at traditional wintering grounds both in the UK and Europe.

More than one in five of the Bewick's X-rayed at the WWT headquarters in Slimbridge, Gloucestershire, were found to have lead shot in their bodies – and not through accidental ingestion. More than one in ten whooper swans show the same results.

'Historically hunting has been a problem. The numbers of swans containing shot have fallen over the years – but clearly it is still a problem,' said Julia Newth, swan research officer at Slimbridge.

While the whooper swan's 850-mile migratory route is mostly over the sea, the Bewick's flies over Russia and Eastern Europe before arriving in the UK for the winter. Both swans are protected for the entire length of their migration route, but both are still hunted.

The decline in Bewick's swans may be due to a number of combining factors, said Ms Newth, including the weather conditions at their Arctic breeding grounds in the Russian Tundra.

'We don't really know why the population is falling – it could be a combination of factors and research is currently being carried out now into what's going on,' said Ms Newth.

Record levels of both whooper and Bewick's swans arrived on the Washes this year although the harsh weather earlier in the winter is thought to have seen some of them relocate from other places to escape the snow.

More than 13,000 arrived in the Fens and 5,704 were Bewick's with the remainder whoooper – a higher number than generally recorded.

Between 1995 and 2005 the number of whoopers migrating to Europe rose from around 15,000 to 26,000 while the number of Bewick's declined by more than a quarter from 29,000 to 21,000.

The results of last year's Europe-wide swan census, carried out every five years, are expected to reflect the decline when the figures are released later this year.

Both Bewicks and whooper swans are preparing to leave the UK for their long flights back to their breeding grounds – but thousands are still currently at the WWT reserve at Welney.