This is good to know. I occasionally have noticed the whole body tremble when I pick them up and I figure they may be nervous or chilled sometimes. The knee joint though seems something else entirely. In case it may help someone who read this thread later, my duckling previously showed signs of weakness in her legs. She would frequently sit back on her elbows to rest (or fall back to her elbows) instead of standing. That has stopped, but now she gets the vibration of the leg from time to time. She continues to eat, drink, poop, and grow, as well as grow healthy feathers and otherwise looks and acts completely normal. The shaking only happens when standing, not when sitting or swimming. It's like she reaches the point of exhaustion quickly. I get a shaky kneecap myself after a long hike or a run or something; I imagine it's similar.
