Usually, a male swan will not fight with a female swan, but it is possible if he wants to be left alone on his pond/lake. Male swans will usually go on their own and may not be interested in re-mating if something happens to its mate.

Since you got the swan from someone else, they may not have had the swan sexed and took for granted that it was a male. There are 3 ways to sex a bird: Checking the vent/cloaca, DNA Sequencing (the most reliable) or waiting until nesting season to ascertain the gender of the bird(s) based on mating behavior, i.e., laying of eggs, etc.

Checking the vent of a young bird (under 1 year of age) is not the most reliable as the sex organs are not formed and what you think you have may not be what you actually have. If you are still unsure, we would recommend that you get both swans to an avian veterinarian or a veterinarian with experience in sexing waterfowl. If the swans are 5 years of age, vent/cloaca checking should be successful in letting you know the gender. If the aggression continues, you need to get the swan that is being picked-on out of the lake/pond or it could be seriously hurt or killed. The Regal Swan