Loss of back feathers...... what is going on??

[chicken_chik]

a few (2 hens, 1 rooster) are losing feathers on their backs?? I have gone out to look for mites/lice at night a few times, but I just don't SEE anything. I read that you want to distinguish between Northern fowl mite and red mite before treating because the treatment is different for each.

Does anyone have recommendations for a regular mite/lice dusting schedule? Or do you just treat on an as needed basis?
I put DE in the shavings in the coop, on the roosts, and in their dusting area regularly already.
They are "free range" most of the time. THey have only had to stay inside like 3 times this winter, due to really cold temps. Could it maybe be pecking?

Any ideas of what can be going on here with the feather loss on their backs?

 

[CMV]

You have some feather picking going on in your flock. Those feathers look way too clean to have a bug problem. Are the birds bored and/or crowded? What are you feeding?

 

[chicken_chik]

I was kind of thinking along those same lines. It started when they were kept in the coop during the few -20 days we had.
I am looking into vitamin supplements currently.

Feeding:
A layer ration mixed at the local co-op
Oyster shell supplement on the side

daily treats:
wheat bread
scratch with oats and sunflower seeds
lots of other fruits/veggies as I have them