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.
…
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.