She probably would not incubate eggs that are not her own. Birds have a set number of eggs that they produce each time they lay eggs and they can count. There may be some type of odor, color or other identifying agent that allows birds to know which eggs are theirs. Female swans, even paired with another female, will go through the mating ritual and lay eggs although infertile. So, this is just part of nature and when she gets tired of sitting on the eggs and realizes they are not going to hatch, she will abandon them. The Regal Swan