Unfortunately, this cygnet has some type of neurological compromise. If it has aspergillosis, it will be dead in hours. Your best bet is to get it to a vet and have it put out of its misery. Even more bad news, is that if it is this infection, the rest of the cygnets may meet the same fate. You need to keep them separated from the sick one, but also get them to the veterinary hospital with an experienced avian/waterfowl/poultry specialist.

Four other pointed questions: Are they hand raised or still with the parents? Were they raised on hay or straw? Are you feeding them a mixture of cracked corn and poultry layer pellets? Are there any weeds, plants, flowes or something sprayed (chemical, fertilizers, insecticide or other toxin they could have contacted or ingested?

Hand raised cygnets or those raised on hay have a greater chance of fungal infections such as aspergillosis, not always, but usually. Hay does not stay as dry as straw and can become moldy etc., which can produce certain types of infections.

Cracked corn has an essential nutrient, Vitamin A which helps prevent neurological problems due to poor diet.

Finally, moldy food, poisons such as chemicals or other such toxins from certain types of plants can also cause this behavior. In any case, a thorough exam from the veterinarian of the healthy swans and a necropsy of the sick cygnet may provide you with answers so this does not happen again. However, with nature, there are no guarantees. We wish you and your cygnets the best. The Regal Swan