I am by no means expert on anything, but I've nursed several sick chickens back to health. The feathers falling out says mites or lice to me, but again, I have no idea what I'm doing half the time. LOL There are some helpful photos that you can look at and then examine your birds and see if you find evidence:
http://www.dummies.com/how-to/content/poultry-external-parasites-lice-and-mites.html
From what you're describing, I'd try treating for mites/lice first plus some nutritional boosting up. You might also be dealing with coccidia, but the feather loss is a red flag for creepy crawlies. Clean the inside of the coop as much as possible - scrape and remove feces, dump out nest boxes, etc. Dust everything with mite powder.
They sell mite powder that you can sprinkle around in the coop area at farm supply stores. You can also use it directly on the birds but you have to make sure it doesn't get in their eyes and that they don't inhale it, of course. We've done a pillowcase powdering where we sprinkle dust into a pillowcase and the put the pull the pillowcase up over the chicken, wrapping it gently around the upper neck, to keep the powder from coming out, and then fluff the pillowcase as much as possible to get the dust all over the bird's body.
If you can, giving everybody a warm soak with Epsom salts in the water is awe some - if they have lice, and you submerge all but the neck, you'll get lots of lice off in one fell swoop. For some reason, Epsom salt baths seem to help them just feel better overall, no matter what's wrong with them. You have to be able to keep them nice and warm, and draft free afterwards, though, so I don't know if you are set up to be able to do that at this time of year. Some people blow dry the chicken after the bath, but I've not gone that far with it. LOL
Use Nutri-Drench or else the vitamin/electrolyte powder mix made for poultry in the water. Also try feeding plain yogurt, as well as scrambled eggs. They will usually dip their beaks in yogurt but you can gently make them do it if they don't seem interested. If I have a chicken that refuses scrambled eggs, I know I have a very sick chicken - they usually can't resist scrambled eggs.