Bare spots on wings - odd need advice

[Whitsr]

My duck has this bald spot on where her wing bends. First picture just to show location. Second picture is a close up. Both wings.
Any idea what it is?

 

[oldhenlikesdogs]

Could be rubbing on something. What does the pen, set up look like?

 

[DukesDucks]

That looks like the bald elbow spots on my dog's legs which are caused by the way he lies on the floor. If they don't look sore, I would guess it is the same for your duck.