There is a possibility that both swans are very young swans and have no idea what they are doing and the egg is infertile. Sometimes, birds will drop eggs because they need to and not necessarily because they have nests ready. In the case of young birds, if they do not know what they are doing and have not yet built the nest, the egg still has to come out so this is the reason that eggs are seen lying in areas other than the nesting sites.

Another possibility is that the swans are actually two females which will still produce eggs during the nesting season, but obviously will be infertile. The mother swans know when the eggs are viable and whether they need to sit on them or build a nest.

The other swan may have left the area for another mate or habitat. Give it a couple of days to see if it returns. The Regal Swan