Yes you need to remove the carcass as soon as possible. Predators can be attracted to the area along with flies and other pests. Harmful bacteria can also be spread, so removing and burying the carcass quickly is a good thing. The remaining swan should be fine. To rush out and get another swan would not be advised, especially if you don't know what killed the first one, I.e. predator, poison, illness, harsh winter, injury or age. There is also no guarantee that the remaining swan will allow another swan in the habitat. The Regal Swan