Nature will not allow a species to unnecessarily sit and watch a nest if there is no change for viable young. This effort takes too much time, costs the parents, especially females a lot of energy and body weight and more importantly places them in greater harm from predators. So, if she is still coming back to the nest and hiding the egg, she knows that there is still the possibility that it will hatch. Usually swans, depending on the species can stay on the nest from 30 - 45 days. Incubation does not occur based upon a human count, but from the swan's perspective. Usually, incubation begins when the last egg has been laid. The Regal Swan