If the swans do not leave soon, they probably will not be able to fly in the impending severe winter storm. Swans are made for cold weather, but if there is no open water, they can starve, they can drown if they should fall through thin ice by not being able to climb on top and predators can climb onto the ice and grab the swans. As long as there are good sized open pockets of water to eat, drink and even stay in at night (water will be warmer than the outside temperature) as long as they don't get caught in the ice, they should do fine. Many swans live on the tundra in much colder weather than what we even see in the Northeastern U.S.

If the swans know that the pond is going to freeze and have a few days notice, there is a good chance that one morning you will awaken and the swans will be gone. Fluctuations in barometric pressure can forewarn animals and birds of impending weather systems. This warning gives them ample time to evacuate. The Regal Swan