A few of my hens have feathers missing at the base of their tail on their back like that, as well. They have a 400 square-foot run(for 9 hens), more than enough space in the coop, and they free range daily. But, they did start pecking Feathers a little around the same age yours are. I think once they all start laying eggs and get their mama bird sassy pants on, the pecking order shenanigans step up a little bit.
Think, Sorority Sisters who have to live together all the time. They have enough space, and it’s all sunshine and rainbows at first, but once Stacy gets a stain on a borrowed sweater from Janet, and Lucy starts dating Julies ex boyfriend, tensions mount and they get a little catty. Just some good old fashioned family drama, IMO. As long as the pecking stays minimal like this and they done sustain any real injuries(like broken skin, bleeding), it’s best to let them work it out.
I have had a little success using “Peck-No-More” if one girl seems to be receiving the brunt of the bullying. Non toxic and chickens don’t like the taste. Purple, smells like grape soda.
Doing the math, your run allows about 11 square feet per chicken. I think a lot of resources would say that that’s “enough”, but kind of like the “3-4sqft per chicken in a coop” rule, 10 sqft per hen in a run is more like a minimum allowance, rather than “optimal”. Would it be at all possible for you to expand their run?