This defensiveness is because of their testosterone levels. They are now of the age to mate and are exercising their ownership/territorial space of the habitat. Usually, this behavior will dissipate, but unfortunately, it may not occur as fast to prevent serious injury or attack by a predator.

You need to capture both birds and bring them inside for a week. Once you remove the pond and the need to fight over it, the problem should either go away or at least be somewhat mitigated. It will also give the hormone levels time to go back to normal.

Place the birds indoors with sheets or straw on the floor so they cannot hurt their legs/feet on an abrasive or slippery surface. Feed the swans together by placing a mixture of 1/2 cracked corn and 1/2 poultry layer pellets submerged in a bowl of water. Closely monitor any defensive behavior. Completely dowse the birds on a daily basis so that they can preen and keep the uropygial oil gland (preen gland) healthy. This is extremely important to completely dowse the birds with water as if they do not maintain water repellency, they could drown once placed back onto the pond.

In a week, release both birds onto the pond at the same time. If you see any defensive behavior after this indoor time out, repeat the process. If the defensive behavior still exists, you may have to find one a new home. Having said this, we have seen this behavior in brothers many times and it usually works its way out. As long as no one is seriously injured in the fray, they will work it out themselves. Nature promotes dominant behavior so one of the males will eventually become the alpha male. As long as the other male accepts this, then things will calm down. Again, most of this behavior occurs once a year during the mating season. So, once you observe the behavior, you may need to keep them indoors during the mating time until the hormones settle.

Finally, if you must find the one a home, you will need to keep ducks and geese on your pond to keep the remaining swan from being lonely. You probably do not want to introduce another swan, (DO NOT introduce another male). Even if the new swan is a female, there is no guarantee that the swan will accept another swan on the pond. Good luck and let us know how this situation progresses. The Regal Swan