Yes, the swans know when the eggs are no longer viable. As long as the birds are still sitting on the nest, it means that at least some of the eggs are viable. If the swans start kicking the eggs out of the nest or abandoning the nest, then it is safe to say that their breeding session will be over this year.

We have had the same type of erratic weather in Florida and have noticed swans double clutching as well as abandoning cygnets and eggs. The Regal Swan