Do you know what the protein percentage is in the food? I've heard that sometimes ducks will pick at another's feather for extra protein, although in this case, I'm pretty sure she's being bullied.
I would suggest maybe keeping both of the younger ones separate overnight, if that's possible...
Hi, and Welcome!
She's very young to be in with the older ducks. That definitely looks like she's being picked on, to me.
Put her in a dog crate or something similar at night, to prevent this happening.
How many ducks do you have, and what are their genders? Also, how big is the coop they...