Hi Suzie:
Usually, the female will be with the cygnets and not the male. Are you sure that this is not the female? Once the cygnets go into the water, the male will hang around at night or throughout the day, coming and going, foraging around the habitat and returning if needed to protect the family. The female loses approximately 30 % of her weight/energy reserves while sitting for such an extended time. The mother swan may be out foraging and/or resting nearby to catch up on eating and resting. You may have caught her resting and not seen her with the cygnets later. Observe the family for a couple of hours, at different times to see if the female returns. Please let us know how this works out. The Regal Swan