An adult Bewick's swan spreads its wings at Slimbridge Wildfowl and Wetlands centre.

The first Bewick's swans of the year to make the winter trip from Arctic Russia to a British bird sanctuary have been captured on camera.

Some 13 of the small, elegant swans landed back at Slimbridge Wildfowl and Wetlands Trust (WWT), in Gloucestershire on Sunday and around 300 will eventually touch down.

News of their arrival - which traditionally signals the start of a cold snap - comes as the number of swans wintering here continues to fall.

Slimbridge staff discovered on Sunday that a baker's dozen of Bewick's swans had landed at the charity's reserve.

Although their arrival was later than usual, their return was a surprise because the winds were not only high but the "wrong type" - blowing from entirely the wrong direction.

First into Slimbridge was Dario - a regular since 1999 and, lately, the swans' traditional "squadron leader". He was followed by Dylan and Deena, who are sponsored by the Queen.

Over the coming days, some 3,500 swans will land in Britain to spend winter at the charity's reserves, which include WWT Welney in Norfolk and WWT Martin Mere, Lancashire.

The swans' arrival will also trigger the next stage of a major new international study by WWT and others into why fewer Bewick's swans are reaching Britain now than in 1979.

Julia Newth, WWT's wildlife health research officer, said: "We are intrigued. We are confident there will still be an impressively large number of Bewick's swans to see at WWT Slimbridge and WWT Welney this winter, but it is a fact that numbers are dropping at traditional over-wintering sites, both in the UK and in northern Europe.

"The suspicion is that this may be due to falling birth rates, habitat loss, pollution and threats such as illegal hunting. Climate change may also be a factor, with warmer continental weather shortening migration routes."