Thanks for your prompt and detailed reply! Yes, it is the female who left – when they were together, I could recognize that the male swan is bigger, has a bigger knob and a thicker neck than the female swan. Uniquely, this male swan’s head is quite flat, making his head and neck together looks very much like a “7” so that I can still recognize it without comparison with the female. I will keep watching and let you know if the swan parent returns (hope it will!).