Duck losing feathers at base of her neck!
I have four hens. I don't have any drakes.
The hens will rip neck feathers out/ make little cow lick whorls in neck feathers when they're mounting each other.
I have one flirty hen that invites all the other hens to mount her. They've pulled feathers out of her neck and several places on her back and chest. White feather grew back to replace the missing feathers.
As log as no one is bleeding or being picked on outside of their mounting games I leave them be.
Although I do crack a rib laughing at them when two hens are standing on each other standing on top of my flirty duck.
I'm not a duck expert by any stretch of the imagination.
Mounting is normal and neck grabbing is normal with or without mounting. It's how they express relationships and connections within the group :)
I'm nto sure what picked on behaviors would be. We have lots of experienced duck keepers that could describe bullying behavior.