Some species are so completely monogamous that, once mated, the pairs stay together for the rest of their lives.
This level of faithfulness is rare. Humans strive for it but we and many other species tend to practice serial monogamy: pairing with one mate, then breaking up and pairing with another.
For tundra swans (whose subspecies include Bewick's swans) their pairings are truly "Til death do us part." Swans are so wedded to their one mate that a widowed swan may not choose a new mate for a very long time -- if ever.
So it was with great surprise that staff at the Wildfowl and Wetlands Trust Centre at Slimbridge, UK noticed that a pair of Bewick's swans had apparently divorced.
Bewick's swans nest in Siberia. The western group spends the winter in Denmark, the Netherlands and the British Isles, returning to the same site year after year. Because each Bewick's swan has a unique yellow and black bill pattern, naturalists at Slimbridge are able to identify the individual swans who come to their refuge.
That's how they found out that Sarindi and Saruni had split.
Last fall Sarindi came back to Slimbridge with a new mate so naturalists feared the worst - Saruni was dead. But Saruni arrived with her new mate and there they were, all four birds on the same lake and the former couple not acknowledging each other.
No one knows why this pair went their separate ways but it's such a rare occurrence - only the second time in over 40 years - that it rated its own headline in the BBC News. Swan divorce.
(photo of Bewick's Swan showing its distinctively marked bill, by Adrian Pingstone, from Wikipedia in the public domain)