The duck can't keep its mouth shut, so it can't eat. It should be duck serositis. You can use duck cream, which is Chinese medicine. Swimming may be too hot for ducks. It is also possible that the food is flowing backwards and it will slowly adjust itself. If he still doesn't eat, don't make him eat. See if the duck's mouth is swollen. If it is swollen, it may be a mouth injury. Tiaolai mutton tou