Are you sure that the swan is not eating at night or at other times that you may not be observing? If the temperature is cold, the swans (especially young ones) tend to stay in the water as the water temperature is warmer than the outside temperature. Swans will tuck their beaks under their down feathers to maintain body warmth. If the young swan is truly not eating, then this would be a warning sign that the swan is not doing well and too many days of not eating can lead to dehydration, starvation and then death. If you can capture the young bird and get it to a veterinarian for an examination, this would be prudent in this situation. The Regal Swan