Gimpy Duck

After I got the ducks (they were 9 days old), we noticed one was a bit gimpy.  Well, Gimpy got progressively gimpier, and my best poultry book has gone entirely AWOL, so I had to look up online what could possibly be causing him to walk on his heels with his useless webbed feet up in the air.  The two options I came up with online are either infections in the joints (which would be more believable to me if it were not a bilateral problem), or a niacin deficiency, which ducks are apparently prone to suffer.  So the duck is getting niacin supplements.  And I'm giving it to the rest of them in smaller amounts, just in case (there are two more ducks out there that are clumsier than the rest in their walking - they step on their own feet because they are so pigeon toed).  They are only going to be with us for about two more weeks, and then they will be in the freezer, so I am not sure that is enough time to see if the niacin improves the condition or not.  I just feel bad for the gimpy duck.  This isn't going to prevent me from roasting and eating him.