This is normal behavior. They both are tired of watching the nest and the mother has lost approximately 30% of her energy resources. She needs to eat and regain energy. This is her main focus for awhile. One unsuccessful nesting season usually is not a concern. Weather, stress predation or change in the habitat can be reasons for eggs not to hatch. Especially, the late winter that occurred this year. Eggs do not do well in extreme changes, i.e., hot cold hot. As long as the swans were successful in breeding prior to this year, everything should be fine. The Regal Swan