Male swans may or may not repair with another female after losing their mate. Usually, they do not. Australian Black Swans produce cygnets twice a year. Since you are not in Australia, his summer and springtime is skewed. So, his aggressiveness, which we would rather call defensiveness is due to heightened testosterone levels. This will go away in 1- 2 weeks and his normal behavior will return. Unless, his mate's demise was caused by humans (you did not indicate cause), but if a human was unkind to him or his mate, he would certainly be defensive around people for that reason.

If you just dropped the female swan (presumably you did have her tested to insure she was actually a female ( not a young male which you would not know unless it was a year old or DNA tested) into the pond without a proper introduction, then this could be the problem. Swans must be properly introduced with time to acclimate. A pen should be built, 1/2 on water and 1/2 on land with zero entrance. The pen should be completely enclosed so that predators cannot climb over or dig under the pen to access the swan. A feeder should be placed in the pen and the new swan placed in the pen for approximately 2 weeks. This will allow both swans to get to know each other, allows the new swan to get used to the new habitat and you. You can use this time to also observe for any signs of aggression. If so, you will need to find the new swan a new home or see if the seller will take it back. If no aggressive behavior, then you should be able to get her out of the pen in 2 weeks. Just have a boat or other rescue equipment available and ready just in case.

Even if you introduce 2 new swans to the pond, you should use the pen. Otherwise, the birds may walk from your pond to other areas which is not safe. All swans need to know their boundaries and habitat before releasing them.

We would suggest that you leave him by himself, give him plenty of food and love, speak to him and see if his behavior changes. We do not know where you are located in the U.K., but if in England, we would suggest calling Wendy Hermon at swan Support to see if she may find him a good home should you decide not to keep him. But, again in all fairness to him, your pond is his home and he needs to remain even if alone. He will do just fine. The Regal Swan