We do not know where you are located, but there could be several things occurring.

First, swans do not belong in salt water. Therefore, it could have gotten sick from the salt and possibly died. If it knew that
the salt was making it sick or there was not an appropriate food source it may have flown to another area. The two swans may be young birds and the other failed to leave or was too weak to leave.

Second, are there any predators in your area, i.e., alligators, coyote, bobcat, fox, etc.? Predation could be a major issue if the swans did not know the area.

Third, everything is fine and it will show up in the next few days. If there is an area of brackish or freshwater in a certain section, they should be fine. The Regal Swan