We are not sure of your location, but if you live in the U.K., you might try to contact RSPCA or Wendy Hermon at Swan Support in Datchet, U.K. There is a possibility that the female swan may have been captured and taken to a rescue facility for treatment which could explain why she was missing for several weeks. This absence may have provided time for the male to re-pair with another female.

The above contacts may be able to shed some light on the female swan's absence and may also be able to intervene to see if they can sort things out with the three swans. The Regal Swan