The first video looks like niacin deficiency. I added brewer's yeast to my pekins food when they were babies and it wasn't enough, sadly, I ended up with one that had splay leg and one who had her foot turn in slightly. The vitamin B complex works miracles - it gives a quick boost and can catch their body up. I used it for a couple months and that along with hobbles corrected my ducks issue. I don't have enough experience to say if the current problem is a progression of the deficiency or something else, but if it was me, I would get her on the liquid vitamin B ASAP. You can't overdose them, so if all the ducks get it they are fine. You want to make sure the one in need gets enough though, so if she will take it over treats, or you can put her in an area with a small water and food dish that has the vitamin added, that would be great. I think they say 1 ml is the dose (you can check this on the "sticky" topic about niacin deficiency at the top of the duck forum) but I just a teaspoon or so in a small water dish and dribbled over food once or twice a day to make sure enough was consumed. At this stage you probably don't want her having to walk that way much, so maybe making a special area for her where she can see the other ducks, but is close to her food and water, would be best for the time-being, at least during the day, so she doesn't hurt herself stumbling around. I kept my ducklings on the vitamin B for a couple of months, and still occasionally give them a little bit now and then. I think you would see an improvement in a week or two, to know whether it's working. I hope the best for your duck!
