Worst case, may be botulism. Isolate the duck, give lots of fresh water.
Paraphrase from Storey's Guide:
You may only have 3 to 24 hours to help
Confine to a clean shady yard or building,immediately provide fresh drinking water with a laxative added, either 1 pint of molasses or 1 pound of Epsom salts per 5 gallons water
If it cannot drink on its own, "should be treated individually" (I think this means help it by getting the liquid down its throat somehow)
the addition of 1 part potassium to 3,000 parts drinking water or individual doses of 1 teaspoon castor oil have also been recommended as treatments
You could flush out the contents of the esophagus with warm water by using a funnel and rubber tube inserted into the mouth and severtal inches down the esophagus
Every effot must be made to locate the source of botulism, which is usually decaying plants and animals, maggots that feed on carcasses, or spoiled feed or canned food.
