There are no guarantees that the two swans were male and female. Do you know for sure if they were sexed? Can the birds fly?

The swan that came back may have been the swan that was actually sitting on the eggs. Mute Swans, unless they are side by side, are very difficult to determine if they are male or female based upon general observation. With the exception of size and maybe (although we have seen some exceptions to this also) the size of the knob on the bill, you usually cannot determine the sex just by looking at the swans. The birds would need to be sexed.

Female swan pairs will lay eggs even though they are not fertile. If there is something wrong with the eggs or the swan senses that they are not fertile, they will abandon the eggs. Additionally, if a predator is in the area, the swans may abandon the nest knowing that an threat is in the immediate area. Just because swans do not stay together, does not necessarily mean that something bad has occurred. It could just be that they found another habitat or mate, especially if they can fly. Since we do not know too much about the habitat, the swans themselves, it is difficult for us to try and determine what may be occurring. The Regal Swan