It is a winter habitation option that few would hesitate over: the Siberian tundra or the glorious Gloucestershire wetlands.

But flocks of Bewick’s swans appear to have plumped for the former, prompting fears that their great migration might never be seen again.

Concerns have been raised by the Wildfowl & Wetlands Trust centre at Slimbridge where hundreds of the swans would normally have arrived and be settling for the winter months after a summer in Siberia. None has been sighted, leading conservationists to suggest that climate change has made the Arctic so warm that they are happy to stay put.

The Royal Society for the Protection of Birds (RSPB) said that if temperatures continued to rise the birds might lose their “collective memory” of their winter home, denying Britain’s birdwatchers one of the year’s most impressive sights. About 8,100 swans usually winter in Britain. The majority, about 6,000, go to East Anglia, about 300 head for Slimbridge and others are seen on the Severn estuary, the Nene Washes, Cambridgeshire, and Martin Mere in Lancashire.

The swans were due at Slimbridge on October 21, although they have been late before. In 1969 they did not return until November 6, and in 1981 and 1982 until November 3. The most recent year that they arrived after October 21 was 1995, when they were spotted on October 27. Andrew Parker, who works at Slimbridge, said of the late arrival: “It is being put down to climate change - a lot of people don’t necessarily believe in it, but that’s what it is. It has been a lot warmer for a lot longer, not just here but where they have been. They have simply stayed put for longer and there hasn’t been the necessity to come back. We’ll have to see what develops over coming years.”

Grahame Madge, an RSPB official, said that in previous years there had been a decrease in the number of Bewick’s swans returning.

“There is a fear the birds might lose their connection to the site. There is a collective memory in Europe’s 23,000 Bewick’s swans. The population remembers from year to year where to go, but if it shifts eastwards that collective memory could be lost,” he said.

Once they decide to depart from the tundra on their 1,860-mile (3,000km) flight to Britain the swans fly through Estonia, Denmark, Germany and the Netherlands, often spending a fortnight in each country.

The British Trust for Ornithology attempted to calm twitchers’ panic and blamed the late arrival on the availability of good feeding in the Netherlands. There is at least one positive to be drawn from their nonappearance, however: an early arrival traditionally heralds a cold winter.

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