Strange duck behaviour- advice please

Advice please.

This is my first time keeping ducks. I have two female khaki Campbell ducks who are 20weeks old and not yet laying. Over the past week I have noticed one of them pinning the other to the floor for a few seconds at a time either by sitting on top of her whole body or by just holding her neck to the floor. The other duck does not seem too upset and crouches down to allow this to happen. There is no pecking or chasing. Both ducks appear well and getting enough food though I have started putting feed down in two separate bowls just in case one gets bullied away from the feed. They share a small coop at night but are completely free range in the day so I can’t imagine they are bored or stressed by their environment.

Is this some kind of dominance behaviour as they are reaching maturity or is something else happening? Do I need to do anything or will they sort it out themselves?

It could be that you one of your female duck thinks it is a male.
Maybe they have enough bulk calories in their diet but they are missing some random mineral.
It could just be them figuring out the social pecking order.

It could be that one of your female ducks actually IS a male!

It's a dominance thing. I had some female khaki campbells that did it, too, and even some of my other female ducks do, too. It LOOKS like they're trying to mate, but really just asserting dominance. If your ducks are anything like mine, after a while they'll figure themselves and their hierarchy out, and not do it as often...that is, unless one IS a male, and then you'll see a lot of that behavior, especially in the spring...

As for what to do, you can try to break it up when you see it happening (just walking over to the ducks will make them stop usually), or just let it run it's course. If they're not nipping at each other or holding each other down constantly, they should figure things out quite safely within probably a month.

Yep, nothing unusual at all. Most animals do this to do as Nicole mentioned; to set the pecking order. (Sorry for the pun, but it actually works in this case).

Thanks everyone!

I have some teenager Khaki females and I've seen some of the same behavior and I *know* they're females.  I try not to intervene as it is just like children learning to settle their differences - it is a "life skill". I will intervene if: 1. a bird submits but the aggressor doesn't appear to be accepting the submission, 2. a bird submits and them tries to run away and the aggressor doesn't let them, or 3. if it develops into a full-blown fight that goes long enough for someone to get hurt. The other day my youngest and middle adult drakes (Muscovy) had a tiff, but later they were hanging as if they were best friends. I had watched the altercation, and it broke off when the younger one tried to run and the middle one gave a short chase before letting him go. That's what I want to see - small squabbles that get worked out and with the rest of the group coalescing on the excitement acting as a distraction rather than a cheering committee.
If I find I have an actual bully on my hands, I name it "Dinner". Those signs are more like: A) the "not letting her run away" behavior, B) taking a swipe at a bird minding its own business, C) chasing the victim away from food constantly, and D) just general harassing a bird constantly. In this situation, step one is to examine the victim carefully - flocks tend to chase off a sick bird. Step two is for the human to act as "Top Duck" and be assertively dominant towards the aggressive bird. Step three is to separate the aggressive bird into something like a dog crate where it can see its better behaved companions having fun and getting treats. I only actually use the "Dinner" option as a last step.
Learning to "speak duck" happens through watching the ducks interact with each other as well as with their humans. They have personalities and preferences which we can only learn through observation. For me, that's the part of animal keeping I enjoy the most - entertainment with benefits.

My muscovies do this. Both male and female will do it. The females will also walk in a circle hissing around the lone adult male and then fight. It does help generally keep the peace to spread out their food so everybody can get some, just like humans.

With animals, it is not always easy to tell mating behavior from dominance behavior. It may be just as others have said, and one duck is asserting her dominance over the other. However, homosexuality does exist in the animal kingdom; sometimes, two males or two females will pair off and mate; among geese, who mate for life, these same-sex couples form as lasting a bond as any others.