You need to contact a wildlife/waterfowl rescue facility immediately and have the swan removed from the area and its feathers bathed in Dawn Dish Soap.

If the swan has petroleum or food oil on its feathers, it knows it cannot preen itself. This means that the oil from the swan's uropygial oil gland (preen gland) cannot be spread on the feathers to keep them clean and water repellant. The bird will drown or get sick if it goes back into the water because of its lack of water repellancy and inability to keep dry and warm. If the bird refuses to go into the water, it will starve, lose its feathers due to lack of cleanliness and finally, with the summer temperatures, the bird may die from dehydration and heat related illness. Please let us know how this turns out. Thanks. The Regal Swan