But experts at the Wildfowl and Wetlands Trust centre at Slimbridge have witnessed a rare "divorce" between two Bewick's swans - with both parties bringing their new partners to winter at the site.

It is only the second time in more than 4,000 pairs of Bewick's swans studied over 40 years at Slimbridge, Gloucestershire, that a separation has been recorded.

It is not unheard of for the birds, which usually mate for life, to find a new mate but it tends to be because one of the pair has died.

So when male swan Sarindi turned up in the annual migration from Arctic Russia without his partner of two years Saruni and with a new female - newly-named Sarind - in tow, conservationists feared the worst for Saruni. But shortly afterwards Saruni arrived at the wetlands site - also with a new mate, Surune.

Julia Newth, wildlife health research officer at Slimbridge, said the "bizarre" situation had taken staff by surprise.

She said swans tended to have "real loyalties to one another" and long partnerships. "As long as they are both still alive, they will try to stay together. If they have a change of mate it is perhaps because of mortality, not necessarily through choice," she said.

In this case, however, both swans and their new partners are now over-wintering on the lake at Slimbridge and things are a little awkward. Ms Newth said the old pair had not acknowledged each other with any signs of recognition or greeting - even though they are occupying the same part of the small lake.

As for why they may have split, she suggested: "Failure to breed could be a possible reason, as they had been together for a couple of years but had never brought back a cygnet, but it is difficult to say for sure."

Bewick's swans are the smallest and rarest of the three species found in the UK and each individual can be identified by their unique bill pattern.