Hi! This happened to all 3 if my ducks last summer, not long after I had an insect treatment done around the perimeter of the house. It had been raining for days & ultimately the only thing anyone could come up with was they got some of the poison. I brought them to a vet & they couldnt find a definitive answer.
They were flaccid, unable to walk or stand, and could hardly hold up their heads. But, they were alert. Totally a neurological thing.
It sounds to me your duck “got into something”. At the vet hospital all they could do was hydrate them and monitor after doing blood work etc. I would recommend bathing your duck bc if anything is in the skin it can be removed, preventing further absorption.
Let us know how it goes!
Marlene