Are you absolutely sure you have a male and female? Many times swans will be sexed at a very young age through the process known as probing. This is the least reliable method as the genitalia may not be completely mature and false identification can occur?

Other possibilities include an immature male with hormones gone amock and he doesn't know what to do. Did he ever accept the other swan? If it isn't hormonal, has the habitat changed (something added or deleted) such as plants or ornaments? Have new neighbors with children or pets moved nearby, any presence of predators in the area? Any disruption in the habitat could cause sudden aggressiveness including injury or illness.

The only options you have are to give one of the swans away or fence the habitat so he cannot get to her and see if he will eventually accept her. There are no guarantees that either this pair or another pair of swans will pair-up, so getting other swans would just add to the confusion. We would definitely have them re-sexed just to make sure

that they are opposite gendered. If the one swan is constantly chased and prevented from entering or exiting the water, bathing or feeding, it can become stressed and be injured or succumb to illness. We would strongly suggest to keep them separated until things settle.. The Regal Swan