You need to get the cygnet to an avian veterinarian immediatley, and even then, this case does not seem promising. What you are describing is neurological based. The question is what caused the neurological problem in the first place. This could be a number of causes:

1. Toxin Something the bird ingested (only found by

Toxicology test)
2. Illness Aspirgilliosi (Fungal infection) or a bacterial or

a viral infection (only can be determined by blood

test).
3. Nurtritional (Lack of Vitamin A or other required vitamin

or mineral nutrient)
4. Internal Injury (Attacked by another swan or predator)

Is the bird eating and pooping? If it has ceased either one of these functions, and even then, you may not know exactly how much nutrition it is getting, the situation is dire.

We would suggest that an x-ray be taken to insure that the cygnet has not gotten something stuck in its throat, does not have any fractures or is bleeding internally. If tube feeding can be accomplished with a Vitamin B-12 shot, it may enhance the bird's feeding and provide some immediate nutrition requirements.

The coloration you describe seems to be that of a cygnet or juvenile swan which could mean that it was nearly drowned by a larger, older swan or predator causing internal injury.

Again, we have seen this type of situation many times, and the outlook has not been good. A bird can exhibit signs that it is improving or stabilizing only to go downhill so quickly that nothing can be done.

If the bird cannot stand or walk and continually falls over, euthanasia would be the kindest thing that could be done for this suffering bird. Without the ability to be mobile, the swan does not have a chance to survive in the wild and would require intense rehabilitation and assistance by humans to allow it to survive. This would only be survival and not quality of life. The Regal Swan