4 sick hens...Help!

I've got 4 hens, all about a year old. They've been acting lethargic the last 2 days, keeping their eyes closed and not leaving the roost. Tonight one sounds like a cat purring and one seems kind of out of sorts, she tried to jump up one level of the roost and hit the back wall and stumbled. Also, their chest bones seem too prominate when I pick them up like they are thin. Any advice would be appreciated.