Hey, just wanted to share my past experience with chickens and ducks as we kept them in the same pen. I was unaware up until today that Drakes have penises.
It sounds like you don't have a man in charge in your hen house or he would spur the snot out of the drakes.
I kept buff orphingtons at the time and had three roos who would gang up on the ducks. One, particularly wiry and fiery Orphington, Popcorn (bless him) would "ride" the ducks around the pen. He would grab the neck feathers and stick his feet into their backs and ride them around the pen until he came off with a beakful of feathers. Any duck that stole food from, or bullied any of the hens gave Popcorn a ride. Fergusson, our larger rooster, had the flog them away approach and would back up Popcorn anytime a hen was even looked at wrong.
When they free ranged, the ducks would head to the creek and the chickens would take to another part of the property.
I'm sorry to hear that one hen in particular is being bullied, but ducks are pretty tough creatures. I would favor the hen and try to keep her as safe as possible. If you can't have roosters to protect the hens while free-ranging, you may want to try penning up the ducks instead of free ranging them.
Even though ducks may be smaller than some chicken breeds, they are certainly heavier and meaner! Otherwise, more likely to harm your hens than your chickens are to hard them- from my experience.
I hope you get it sorted out!
