I wanted to share some information that I recently discovered when 2 of my three week old Indian Runner ducklings developed weak and shaky legs. They were eating, drinking and walking but would quickly tire and sit. Based off my web searches I thought this might be an early symptom of Niacin deficiency. I was already adding brewers yeast to their feed since day 1, but I started adding Niacin to their water. After a week with no improvements I contacted my vet. He thought this was a Calcium deficiency not a Niacin deficiency. He suggested I crush up some Tums and add it to their greens. I crushed 3 tums and added it to their greens/water mix in the evening, and by the next morning I wasn't expecting much change but I was wrong. That morning my babies ate their entire bowl of greens without sitting once. The shaking is now minimal and they are moving around much more without any issues. I was surprised how fast I saw the results. For anyone experiencing this issue I would highly recommend trying the Tums.

Hypocalcemia definitely can be an issue! May I ask what feed you were feeding them?
I don’t know about any other BYCers, but I would say calcium deficiency seems to be the # 2 nutritional issue related to ducklings. There could be other things that I’m just not aware of.