A flock of Bewick’s swans which had begun their migration from the UK to the Arctic tundra have turned back due to Storm Darcy.

Twenty birds had set off from Slimbridge Wetland Centre last week but 12 reappeared at the site just four days later.

Eleven had previously left from the centre and one new Bewick’s swan, which staff have now named Darcy, tagged along.

Each year, Bewick’s swans fly 4,000km to the UK to escape the harsh Russian winter and journey back there in early spring to breed.

They are triggered to leave by the lengthening days and rarely have to abort their mammoth trip, a behaviour known as reverse migration.

Kane Brides, research officer at the Wildfowl and Wetlands Trust (WWT), said: “Arctic migrants like the Bewick’s swan are used to chilly weather and given the extremes of climate they experience, are very adaptable as a result.

“However, freezing conditions reduce food availability and blizzards reduce visibility for migration.

“With the easterly wind direction against them for their onwards migration to Russia, they are very sensible to sit this out.

“The Bewick’s swans at Slimbridge are lucky they have a comfortable B&B to shelter at until the cold weather period passes.”

The reserve at Slimbridge is maintained to ensure the habitat is right for the visiting swans, which are fed by staff three times each day.

Bewick’s swans at the centre are the subject of one of the most intensive wildlife studies in the world.

Researchers can identify each individual swan by the unique pattern of yellow and black on its beak.

The study, started by WWT founder Sir Peter Scott, has been running continuously for 50 years and has recorded the lives of almost 10,000 swans during that time.

Since the early 1960s, WWT has expanded its swan research and linked up with researchers throughout the swans’ range in northern Europe and Russia.

Together, they have managed to secure international protection for a chain of wetlands along the way that are vital for the swans to feed and rest.

The number of Northwest European Bewick’s swans has dropped by a third in recent years, with less than 21,000 remaining.

Bewick’s swans are endangered in Europe and protected from hunting by law in each country they fly through.

Despite this, a third of live birds caught and x-rayed by researchers are found to be carrying shotgun pellets.

WWT is working with scientists, hunters, indigenous groups and young people to protect the birds from illegal hunting.