This is my first small backyard flock of ducks, and we got 2 Blue Swedish and 2 Welsh Harlequins. They are all now 4 weeks old but our one Welsh is much smaller than the others and seems to have trouble walking. She sits often (leading to her getting run over by her siblings) and when she does walk it seems labored, lifting each leg high, hunched over, and pigeon toed. After a quick internet search we decided to give her some niacin drop directly and have been doing so for about a week now with no signs of improvement. No worsening of her condition, but no better. She still eats and drinks, although less than the others, and will walk when needed, but after a minute you can see her getting shaky. We just switched their food to Purina Duck Grower yesterday from the chick/waterfowl starter they were on to see if that helps. Any ideas what could be causing this or potential remedies? Could it just be bad genetics?