Update: It's now been 48 hours for Cinnamon (the duck at the start of this thread). She is both better and worse. Today she has eaten on her own and has maintained her weight without additional feeding. She seems a bit worse neurologically. She pushes with her back legs when she wants to stand up and falls over backwards. Poop is normal and looks and smells the same as everyone else - often runny but sometimes as in the picture. I have received the suggestion to try a system flush (40:1 water to molasses dilution) in case she got into something at the pond two days ago. I am also cutting back on the niacin - I think she does have a deficiency, but maybe I am now giving her too much and that isn't good either.
I don't know if it's an "eye dropper" or "medicine dropper" but it has a bulb on top and contains 5 ml. I slide it past the back of the tongue going from the right corner of the mouth leftward (when facing the duck) and down. When I have the duck in the right position I can give 5 to 10 droppers or 25-50 ml at a time. This seems to fill her crop (without overfilling). But I only had to do it one day. Now she's interested in food again. Yay! Not sure how a tube system would work. I get the tube part and would be comfortable putting it in, but I don't get how the food will be put in the tube and sent down the tube.
Casportpony - Can you give me more details about how the food gets into the tube??
Amiga - Thank you for your support. I had to put the chicken down this morning and I miss her.