https://www.backyardchickens.com/t/375884/duck-has-problems-with-walking
https://www.backyardchickens.com/t/1013885/my-duck-isnt-standing-or-walking
https://www.backyardchickens.com/t/831608/pekin-duck-cant-walk
The first two are people with ducks who had a niacin deficiency. :idunno