Experts have told of their surprise after witnessing a rare "divorce" between a pair of swans at a Gloucestershire wildfowl sanctuary.
The Bewick's swans have returned to winter at the Wildfowl and Wetlands Trust centre at Slimbridge - but both have brought new partners.
It is only the second time in more than 40 years that a "separation" has been recorded at the centre.
Staff have described the new couplings as "bizarre".
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, they said.
During the past four decades 4,000 pairs of Bewick's swans have been studied at Slimbridge, with only one previous couple moving on to find new partners.
First suspicions of the rare event were raised 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.
The pair's arrival led conservationists to fear the worst for Saruni.
But shortly afterwards Saruni arrived at the wetlands site - also with a new mate, Surune.
And after observing them, the experts discovered the old relationship had ended and new ones had begun.
Julia Newth, wildlife health research officer at Slimbridge, said the 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 in close proximity on the lake at Slimbridge.
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 said: "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.