There may be several thing occurring. First, are you absolutely sure it is his mate and not one of the siblings. Male parents can become very aggressive toward their young, especially time for the kids to leave the nest and the new breeding season begins. With the weather patterns such as suddenly cold and then a brief warming trend, makes the swans think that it is breeding season and time to mate.

This may also have been occurring. He may have tried to mate and she was not ready to sit on eggs in the dead of winter. He may have looked like he was trying to drown her, but actually trying to mate.

If he was trying to drown her, which we do not doubt what you were seeing, only offering various possibilities, there may be something wrong with her and he is trying to put her out of her misery. We would suggest that you get her to an avian veterinarian to make sure that there is not an underlying problem such as illness or injury. She still may have incurred an injury after the fight with him, so it is better to be safe than sorry and have her checked out.

He may be chasing her from the habitat as well as eventually chasing the 2 sons and try to mate with his daughter. Yes, this does happen, so the other option is to remove all of the young from their habitat and find them a new home. This is especially true if the juveniles are more than 8 months of age as the parents will begin to aggressively chase them from the habitat and keep them from entering the water and eating.

Finally, it just may be that he had a bad day. It happens. Before placing her back into the lake, have her checked by a a veterinarian and then place her in a pen (1/2 in water 1/2 on land with feeder inside). The pen must be enclosed on the top so that predators cannot access the pen and kill the swan. Leave her in the pen for about a week and see if there is any further aggression towards her. If there is, you need to find either the male or the female a new home. If there is no outward aggression, then introduce her back into the lake and have a boat, canoe or kayak immediately available to intervene if there is aggression once she is let out of the pen. If there is no signs of aggression, then things should settle back down. Let us know how this transpires. The Regal Swan