Not being able to physically examine her to ensure the swan is not injured, ill or having eaten something toxic, we will assume that this may be normal. During the molting season, one parent will molt while the other parent does not. This allows one parent to be able to protect the young and even escape a predator. The molting swan will stay away from the family so a predator is not lured to the family. As for the lethargic behavior, mom has been sitting on a nest for 35-45 days, losing 30% of her energy reserves due to not eating as much during the nesting period. Additionally, if the temperature is extremely high (summer weather) swans will slow down and even control their eating because it takes too much energy. The Regal Swan