Hi Jo: The female could have taken the cygnets to another area to search for food or protect the cygnets from a predator. Usually the male will stay with or at least in close proximity to the family. However, we have seen males go about their business after several weeks of initial babysitting. Now, having said this, how big is the loch (are there nooks and crannies that you might not see or be able to access and search that she could have taken the cygnets), what and how many predators are there that might have attacked the family, could a human have taken them, are you sure that this is the dad? A larger male could have entered the area and chased the family to another area. Any or all of these scenarios could be possible. The Regal Swan