We are assuming that nothing has happened to the male and it is still in the area. If so, then probably the cygnet is a female and is not seen as a threat to the family. Usually, the cygnets are chased and not allowed to sit this close to the nest, so it is not a typical situation.

However, if something happened to the male, the female swan will take the cygnet (male or female) as a mate. Yes, swans will pair up with swans of the same gender if there is not an opposite gendered swan nearby. This pairing occurs while the cygnets are juveniles. Also, if something were to occur to the male, the female will typically, not always, but usually re-pair with another swan. If something were to occur to the female, most male swans will not re-pair with another swan. Again, not always, but usually.

Female swans will produce eggs throughout their lives during nesting season, regardless of the presence of a male swan. Although the eggs are infertile, the female swans will still lay the eggs.

If male swans are paired, they will nest, but obviously, will not produce eggs.

If there are only two swans in your area, it means that the cygnet is now the mate of your female swan. If there are three swans, then eventually the cygnet will leave as it matures to find its own mate and habitat. The Regal Swan