The old swan is a male and does not want a new swan (male) coming in and taking over its habitat. So, this is normal behavior and the new swan is probably a young male that was recently chased from its parents nesting habitat and told to go out and find your own habitat and mate. Unfortunately, the new swan found your swan's habitat and this is not going to work out. If this had been a young or even old female, there is a possibility that your male may have taken it as a mate and allowed it to stay. This scenario is occurring throughout the world during the mating/nesting season. This is nature and will work its way out. The young male will leave and your swan will be just as happy to enjoy his habitat alone should a female not appear. Even then, he may or may not take a mate. The Regal Swan