Why are my chickens picking each other's tail feathers out?

I know that winters here in WI get long, and my chickens would like to get out soon. I think they have started picking at each other. Their tail feathers have started getting sparse on some of them. They are in a room about 6 x 12, for 8 chickens. They have a second level of about 6x3 that they go up on also. Are they picking at each other because they are in too small of a place? Are there other reasons that chickens pick at each other? Do they pick at their own feathers? Is it maybe that they are missing anything in their diet?

I just got chickens last September, so I am somewhat of a newbie.

Thanks!

Sounds like cabin fever

I find that if I add more interesting things for them to snack on, They're less apt to pick.

One of their favorites is sprouted Black Oil Sunflower seeds, milo and oats.. I have 2 cheap plastic 1 gallon pitchers that i bought at the dollar store. 1 a drilled about 25 small 1/8 inch holes in the bottom,the other ones bottom left whole.
I add about 2 cups of wild bird seed mix, or just all oats, or BOSS or a mix of any of those.. add the seed to the soaking pitcher(the ones with no holes).. add water to cover all the way plus about 2 inches.. let it soak over night. the next morning, dump it all into the holed pitched and let it drain.. (My pitchers are stackable for storage purposes.. I just stack the holed one into the solid one, let it drain and do my morning thing. rinse the drained seeds, right in the holed pitcher..then Dump the drained seeds into the solid one and add water again, to cover about 2 inches over. let it set til evening. (You want them soaking in water for about 24 hours)
Drain again in the holed pitcher and leave them in there.. over night. rinse again in the morning, allowing the water to just drain through.. by the end of the 2nd day you'll see sprouting action.. namely little roots.. you can keep doing the rinsing and draining thing for another day and you'll begin to see green shoots curling around.. you can wait a while longer.. like another 6 hours. ( be sure to rinse and drain at least every 8 -12 hours though) and you'll see even more green.. You can feed these to your chickens.. You could even give them a handful at each stage of sprout until its all gone.. Get a couple of pitchers and have varying stages going continually..
Or, you could just throw some unsoaked seed to them for a treat..
My DH LOVES to snack on the BOSS once they've sprouted. he just pulls them out of the shell and pops them in his mouth. I think they have a VERY nutty sweet taste. I can understand why he likes that..

Sounds like low protien, I was having the same problem, and started mixing 50/50, 16% protein Layena with 30% Protein Purina Game bird, It has helped alot, they are starting to get some feathers back. Don't feed any corn or grain, this will dilute the protein content!

Horrible story, we have a Light Brahma/Black Jersey Giant young rooster name Solo. Broody bantam hen set and hatched 4, something got 3 so we rescues Solo. We raised him separate from any others and he has to be the sweetest thing ever. We bought some Barred Rock pullets (4) from a lady that were his age and we decide to put them together for companionship.
I work 12 hour shift, sometimes days and some nights. Work 3 day shifts that had me leaving and it being dark and coming home and it dark. Did my 3 12's and checked on Solo and found him hiding from the others. Checked him out and found they plucked all his tail feathers and had his wings picked raw at the bend.
While I was holding him one walked by and just grabbed a feather off him.
Never seen anything like it.
He's FREAKED OUT and separated again.
Poor ole Solo is starting feather back up and is now the prettiest black with silver streaks.