Swans like all other birds, are in their migratory (winter feeding/resting grounds) mode. This means that aquatic and other vegetation is very scarce during the winter months. This field is allowing them to graze and get much need vegetative nutrition. Because of inclement weather, this field may freeze, get flooded or otherwise become inaccessible to the swans and other waterfowl. So, the swans will get their fill and then go back into the canal. The Regal Swan