niacin deficiency. After giving the duckling brewer's yeast for a bit, the customer reported that it is now in good health.
The only reason I knew about this is because of a problem I had a while ago. One night I was trying to lock up the ducks and it was taking forever to round them up. I noticed something strange. A Rouen seemed to be caught in chicken wire and all the other ducks were standing as far away from it as possible. I ran over to rescue the duck and then the Exorcist level creepiness began. The Rouen that I thought trapped was actually having really bad seizures. It was turning its head around like an owl and somersaulting backwards. All the other ducks were terrified and quacked in alarm, getting as far away as possible, trampling each other pressing against the opposite side of the chicken wire. I had never seen such a thing before and out of kindness for the seizing duck and all the other ducks that were freaking out, I chopped it. Then I started doing some research... And found that I probably could have cured the duck with niacin. Maybe it would have worked, maybe not. Now I'll never know.