A niacin deficiency can be caused by an amino acid deficiency, called tryptophan. Again, we’re talking about poultry, though chickens are less affected than turkeys, ducks and geese.
Most supplemental feeds for poultry will have sufficient niacin (or tryptophan).
Niacin is also converted to niacinamide. Niacinamide is used in part of the treatment for discoid lupus and lupoid conditions.
I had a few cases of suspect lupoid onychodystrophy which I can maybe go into here. I say suspect, because it's based on clinical judgement. The diagnosis is made by histopathology - which requires amputation of P3.
The cases were young dogs around the age of 4-5, that came in for "broken nails". Broken nails itself is not the issue, but how the nail looks and sloughs off with inflammatory exudate between the nail and the nail bed. It's more common than I expected and really reminds me of laminitis in horses. I tried to do a Google Scholar search once to see if there was any relationship between lupoid onychodystrophy and laminitis and there isn't much, other than they are both inflammatory conditions of the "nail". Maybe you know of a pathologist who can look into that for me??
Either way, I start those patients on high doses of EPA/DHA, niacinamide and possibly tetracycline/doxycycline. And sometimes pain medication.
Back to chicks and ducks! The condition in chicks is called "black tongue" and it causes decreased egg production in hens. Here's a little blog post from a hatchery on niacin deficiency which has a few images for you to browse.
The chickens and ducks may have issues with their hock joints where they will have splayed legs.
There is also the PoultryDVM site for more information on chickens!
Again, I don't have any personal stories about chicks or ducks to share, but niacin seems to be fairly easy to supplement for those poultry farmers.
Also note that a chick with leg issues, may have differential diagnoses to think about!