Well... With the information given, I'd say seperate the bird from the rest. Put her inside with a lamp in one corner so she can choose to be warmer or not. If she's not drinking, use a dropper to drop water in her mouth or give her some fruit flavored (but NOT grape) Pedialite from the children's section. I'd be more worried about her not drinking than not eating.
If she's of laying age, make sure she's eating lay mash and not a lot of scratch. Scratch is like candy to them and should be used as a treat only.
Check to make sure she doesn't have parasites, like mites, on her.
Have they been wormed?