There could be several reasons for these deaths. Neurological problems resulting in the swans being unable to hold their head/neck erect and sudden demise could be the following:

1. Nutritional deficiency. You might try to add cracked corn as this food source provides a critical resource for Vitamin A. Lack of Vitamin A can cause neurological problems.

2. Insecticides/Pesticides/Fertilizers/Agriculture run-off. Due to their long necks, swans can get into areas that other waterfowl may not be able to reach. If some are feeding in an area that others are not, you might closely monitor the particular area and rope it off so that the swans cannot get into these areas.

3. Botulism. Again, swans can reach into areas that other waterfowl may not be able to reach. Botulism is a condition that affects the neurological system and is caused by the bacteria (Clostridium botulinum) which is found in non-oxygenated areas such as the soil. You can have the swans vaccinated against this condition, but if they are able to access an area (especially during times of drought) that they usually cannot access, a bacterial overload may still override the vaccine. Again, you need to closely monitor which areas that the swans are accessing and cordon that area off if the other birds do not feed in that area to prevent any future poisoning.

4. Parasite/viral/bacterial infection that cause neurological issues. You need to have the swans checked for infection by a licensed veterinarian. If you do not have a swan specialist, you might check a local university that has a veterinary program to have the swans tested. Their blood chemistry as well as temperature needs to be examined to determine if an infection is responsible.

If you cannot get the surviving swans tested, you can see if the university or other entity can possibly conduct a necropsy (autopsy) of the dead bird to see if an answer can be obtained.

Again, this sounds like some type of poisoning, especially the sudden inability to hold their heads/neck erect, lack of appetite and sudden demise. The Regal Swan