It depends on the species and the individual swans. Outside of breeding season, swans are usually less territorial, not always, but usually. It also depends on the new swan's gender. If it is a female, it may be allowed to stay more readily than if it were a male. Finally, it depends on the age of the original swans, how long they have been at the habitat, how much food, shelter and other resources are available and the size of the habitat. The larger the habitat and more food from people, etc., the less likely that there will be a major push to remove the new swan from the habitat. The less available the resources of the habitat which could significantly affect the young, then the more probable that the newcomer will be chased from the area. The Regal Swan