There could be several reasons for the nesting failure and disappearance of the mate. One, a predator attacked the nest and possibly scared the mate and it took off, or it was also attacked.

If the nest is intact and there is no sign of predation, then the mate may have temporarily left the area.

In nature, if one or both of the swans are infertile (unable to produce cygnets) then the swans will try to find another mate so that it can do what nature intended, carry on the species.

The other possibility is that the mate took off (after spending all of the time in a specific area) because it needed a break. It may still be in the area, just around a bend or other side of the habitat which you cannot see. Give it a couple of days and the mate may return. The Regal Swan
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