We have 10 birds in a pen that consists of a 3ft x 6ft under coop space plus a 10ft x 6 ft attached run. I feel like they're a bit crowded but I haven't been able to expanded it yet. The original plan was that there would be 12 birds in an 8ft x 16ft run with attached coop by now but life happened.
Are you saying they have a coop? Or that they shelter under a coop but without having access to it? (This being summer, that could be fine, but I'm just trying to figure this out.)
So 3x6 =18 and 10x6=60, making 78 square feet total for 10 birds, which is 7.8 per bird. A bit smaller than the recommended 10 square feet per adult bird in the run, but they are not quite adults yet either, so I agree that it's a bit small.
Giving them more things to do can help. For example, if you have a compost pile, you could shovel it into their run--digging through it for goodies can keep them busy for quite a while! (Or you could just start piling all new compost-type material in their run, for a similar effect.)
Any advice on how to return my flock to a peaceful status would be appreciated.
More space or less birds would be the obvious long-term fix.
(Permanently removing the one that was picked on, or permanently removing the bullies, would probably help quite a bit.)
In the short term, an additional run and/or some distractions in the pen might help. Or just housing several birds separately, so there are less birds in that amount of space.