Re-introducing a chick that was "attacked"

[yeye5]

I have a cute little barred rock chick that I got from a friend when each of us had similar aged chicks. She did great in the coop I put her in, which had 3 other chicks the same age. It seemed she was accepted immediately.
Oddly after approx 10 days, the others began pecking the feathers off of her tiny head! I saw this one morning but she seemed otherwise in good spirits. By the time I returned home a few hours later her body language was totally different. If it's possible to say about the emotional state of a chick, she looked downtrodden and depressed.
I removed her from the coop, chose a chick from another coop, similar age but larger, and put them in a brooder in the guest bathroom.
*My thinking was that if she and the larger chick bonded she might have someone to stand up for her when they got back to whichever coop I decide on (I still don't know which...)
If anyone can answer this: why is it that this kind of thing happens?
And, practically speaking, I'm wondering about the slow growth of her feathers.(Chicks all get medicated starter,and I supplement her every so often with dried mealworms bcs feather growth takes extra protein).
Originally most of her head feathers were gone. She's grown back many but the areas above her eyes and back behind her ears (like a strip of space) are still entirely bald.
I know I can't reintroduce her/them until all of her feathers grown back in.
Any thoughts or ideas about the area she's not regrowing feathers?
I'm concerned that the "attackers" could have damaged the follicles to the point they will not grow in.
Is this chicken mama paranoia and I just need to get a grip?

Thanks in advance.

 

[ChicketySplit]

Would you share her age? Any possibility of mites or fleas? You have 3 chicks in one coop, and how many in the other? How much older are the chicks in the other coop, and what are the sizes of the coops each in square feet? This info may help, as well as pics, if possible.

 

[yeye5]

Her hatch was in the later part of May so she is approx 3 months old. I've had many more chicks in each pen and never had problems...but I'm willing to consider this if necessary. She was in "The Condo" with 3 other similar size and similar age chicks.
In the other coop where I chose her companion from, there were originally 3 chicks, removing him (or her?) made that 2 in that coop.
I'd have to get outside with better light too measure the coops to be accurate (I don't estimate space well at all!).
Also I can post pics of her size compared to her buddy's size and she is tiny even though he is less that a week older!
I took pics of both sides of her head and hopefully they've arrived in my email by now.
'Or I'll go back to the drawing board and try again tomorrow!

 

[yeye5]

Also, I have checked her thoroughly many times and see no evidence of mites or fleas. I have used a bit of DE in their environment just in case.

 

[Mrs. K]

The thing that surprises me is she is getting along with the one you paired her with?

I am wondering if there is not something wrong with the chick, you said she is not as big as she should be? Sometimes I have heard where chickens can sense a defect or disease, and ostracize or attack that bird. However, there is often a discrepancy between the sizes of birds even within the breed.

Space is usually the other culprit in these cases. What was enough space when they are tiny chicks often times becomes not enough space when full grown.

Feathers do grow back rather slowly. If she is bright eyed and active, and eating well, I would not worry about it.

 

[yeye5]

She gets along with everyone. It's the others that started by getting along with her then "changed their minds". I selected a chick from the other coop so s/he didn't have any memory of the bullying she got...not sure if that matters.
The group of chicks I selected her from who all hatched from the same clutch (my friend's coop) varied slightly in size but I'd she she was petite but well within the normal range. The coop she moved into also had chicks her size. And in a 10 day period would there be enough growth to signal them to feel a cramping in space? Hmm, I just don't know...but it was weird.
I took pics after posting yesterday (the way I know to post pics here is to send them from my phone to my email/laptop and post from there). I just resent them bcs they didn't show up. A picture's worth a thousand words so I'm thinking if you all see her it might be more obvious.
The chick she's with--same hatch dates (within 2-3 days of each other in my coops) is giant compared to her. No problems there...they are best friends.

 

[yeye5]

Visible in the 1st 2 pics, her side side is much more naked than her left side (pic 3)

 

[aart]

And in a 10 day period would there be enough growth to signal them to feel a cramping in space?

Chicks grow fast.
Crowding is the number one cause of aggression.

 

[yeye5]

Well I don't have the picture loading process down yet. Above is her left side. Here is her right side.

 

[yeye5]

@aart thanks for your response. This is certainly something I'll be more aware of now that I have this feedback.