From my duck book. Storey's Guide to Raising Ducks. Dave Holderread.
I hope this helps and gives you some ideas.....
A common mistake is to presume that a health problem cannot be diet related because only part of a flock is showing symptoms. Remember every birds digests and assimilates nutrients and toxins differently and it is normal for diet related symptoms to vary widely in a flock of birds, from individuals that show no symptoms to others that experience severe symptoms or death.
Some common materials that are known to be toxic to ducks include commercial fertilisers, salt, lead (from birds picking up pellets or nibbling on leaded paint) herbicides, pesticides, baits for rodents, slugs and snails, leguminous plants and their raw seeds, cotton seed meal, and leaves of tobacco and rhubarb. Other plants that are suspected of causing illness or death in ducks include fox glove, potato vines, potatoes that have turned green, and egg plant leaves.
Botulism.
Botulism commonly known as limp neck is usually fatal and can affect birds of any age. A few hours after eating poisoned food, birds may lose control of their leg, wing and neck muscles. In some cases, body feathers loosen and are easily extracted. Dying birds may slip into a coma several hours before expiring. Botulism normally kills in 3 to 24 hours, although in mild cases birds may recover in several days.
Causes.
This deadly food poisoning is caused by a toxin produced by Clostridium botulinum bacteria, which are commonly found in soil, spoiled food, and decaying animal and plant matter. botulism strikes most frequently in dry weather when levels in ponds and lakes drop, leaving decaying plants and animals exposed for ducks to eat. Maggots that feed on decaying carcasses often carry the botulism toxin. Ducks can also contract this toxin from spoiled feed or canned food from the pantry.
Prevention.
Burn or bury carcasses of dead animals and clean up rotting vegetation. Do not let your ducks feed in stagnant bodies of water or give the birds spoiled canned goods or feed.
Best of luck.
OL