Same age, 1 is smaller?

[NewAtThis8]

I have two ducklings same age- 5/6 weeks and both are currently being treated (on day 4) for niacin deficiency. The one on the left has it more severe, and is currently not walking. The one on the left is also noticeably smaller than their sibling on the right. Could it be due to the niacin OR, is this a gender specific thing? I noticed right duck has what could be a drake feather? But thought it came through later down the line at like 12 weeks…

 

[Quarterhorses1]

It could be the deficiency or gender. My duck was tiny compared to her sibling too

 

[HollowOfWisps]

With Runners the females are usually smaller as ducklings compared to males. Or at least every one I have had I noticed the male to female size difference.