OK guys... I think we have some separate issues going on here, so I will try to address them all.
Are your birds molting?
Have you checked for parasites? Mites can be very hard to see and must be looked for after dark with the aid of a flashlight. Mites look like tiny, tiny moving grains of sand. You can see them especially around the vent area. Mites can lead to feather loss, irritated skin and (in extreme loads) death. Lice are fairly easy to spot and the links from ddawn should prove helpful in identifying which external parasite you may have. Bugs are a pain and need to be dealt with ASAP. There are lots of posts here that tell you how to deal with them.
If a bug check is clear then I'd look at feather picking from the coopmates. This is a tough habit to break because it could be caused by a number of factors- dietary, stress reaction, or behavioral. Increasing the amount of protein in their diets may alleviate this (and will help aid in feather re-growth if they are molting). If they are too crowded they can turn to feather picking (and worse- cannibalism), so make sure they have enough space in the coop and run. Boredom can also be a factor. Chickens get bored too, especially during the long winter. Offering some diversions such as flock blocks and hanging vegetables and fruits in the coop/run for them to pick at can keep them busy. Scattering scratch in with some shavings/dried leaves is also a great way to keep them busy. If none of these work to stop the problem then there are a few products on the market to stop this behavior- pine tar (don't recommend it), Hot Pick/No Pick, and pinless peepers. I would also try a little hands on time with the birds to see if the behavior is related to one or two birds. If a couple birds are doing most of the damage then maybe they need to be removed from the flock.
I hope this helps. Good luck.