This is the classic sign of botulism. In times of hot weather, too much rain after a drought, harmful bacteria are stirred from the bottom of the muck in which the swans eat. They ingest the deadly toxin "botulism" produced by the clostridia bacteria.

The swans and other waterfowl lose their ability to control their central nervous system and neck muscles causing them to be unable to swallow, hold their heads out of water and they eventually drown or die from the impending paralysis. The Regal Swan research staff worked with Dr. Wade G. Gardner and his son, Dr. Geoffrey R. Gardner as well as Pfizer Animal Health to pioneer the use of a vaccine to mitigate swan deaths due to the deadly toxin. Additionally, these two veterinarians, and our other swan veterinarian Dr. Christopher W. Brown and Fort Dodge Animal Health also pioneered the use of a vaccine to mitigate the swan deaths from West Nile Virus. So, there are ways to prevent swan deaths, but only if the swans are vaccinated and that botulism is indeed the culprit in this case. You might try to get the carcass necropsied to ensure that there is not another underlying cause. The Regal Swan