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It is not normal for the male to be gone for long periods of time. However, are you constantly watching the nest? Could he be in the nearby area and you cannot see him? Have you not seen him for hours or days? Is there a nearby nest which is out of your sight? Are you sure you have a male/female swan pair?
These are just some of the questions that we have to consider before thinking something may have happened to him.
1. If you are not watching constantly, he may be in the area, but out chasing interlopers when you visit the area.
2. If there is another nest in the area, he could be constantly chasing and fighting with another male, be hurt or chased away. He could also be running two nests at the same time. This is a rare occurrence, but has happened.
3. Finally, if you have two females, they will still nest and lay eggs. Obviously, the eggs will not be fertile, and one of the pair may leave if a male swan becomes available. Again, this can occur, but is usually rare as most swans do pair for life unless something happens to the mate.
Hopefully, your swan will return shortly. However, the longer the separation, the odds increase that something may have happened to him. Yes, the female can take care of the cygnets on her own. The Regal Swan