We're not sure exactly what is occurring. Vocalizations can mean that the swans are looking for each other, they are stressed, warnings and then again, they can mean that cygnets have hatched. Has the male gone off for any period of time which would indicate that he might be visiting a nest?

Are you absolutely sure that these are opposite gendered swans? Can they fly? If these are captive swans (cannot fly and you have had them for some time), then this behavior would certainly be of concern as possibly a predator or some other problem has arisen.

If they can fly or if they are opposite gendered swans, they may have been juveniles and become separated on the migratory path. If the remaining swan is lost, he/she may eventually fly away to find or join the other flock. The Regal Swan