Hello Lesley, I'm Dr. Bob.
I'm sorry to read of the problem you're having with your little flock.
Judging from your description of the sequence of events and the condition of your birds, I suspect that the habit of feather eating may have established itself in your flock. This problem often starts out with one bird pecking at the area around the vents of other birds, and spreads through he flock with several, or more, birds being the primary culprits.
You can tell who the main offenders are by the fact that they themselves are not affected.
Spend some time watching the flock to see if you can spot the main offenders, then separate them from the rest.
Often an underlying protein deficiency in the ration plays a big part in the development of this behavior. You can overcome a protein deficiency by feeding a handful of dry cat food (any brand is fine) for every five birds daily for two weeks, then every other day as needed, to prevent the resumption of this habitual behavior.
Overcrowding is possibly another factor, and expanding their pen area or thinning your flock can be helpful.
If you should have further questions, please let me know.
Best regards,
Dr. Bob