If the weather is starting to warm, the swans know this and are beginning to prepare for the mating season by building a nest. Also, if the temperatures are extremely cold during the daytime, the swans may stay in the water at night and use the reeds/grasses for warmth during the daytime.

More than likely, they are preparing for the nesting season and are not quite ready to lay eggs. If she does lay eggs too early, extreme cold weather could destroy the eggs or the female may just abandon the eggs. Then, they may or may not try to nest again. The Regal Swan