I've had the new girls a few weeks they are between 1 yr and 2 yrs old (older than my flock). We let her out to free range with the rest and she will eat grass but I have not seen her eat feed or treats we throw out every once in a while. And she will only come out for a little while eat some grass then go back Into the coop and just stand in there. Her poops have just been wet with only yellow to the color no solids at all. She no longer gets on the roost anymore and you can tell which nesting box she sleeps in because all of the shavings are neon yellow in there. I don't have any antibiotics but if I were to find someone who never finished a round could I use that? And how would I give it to her? I have never had to treat my chickens with anything. they are all new layers infact she probably has only layed a handful of eggs before she started getting like this and she ate just about every egg of hers for some reason. I did give her a bath when this started thinking maybe she was egg bound but I'm thinking it's not as simple as I hoped. None of the other members are affected by whatever it is so it must be specific to her and not something spreadable. I do have monostat I could use incase it is vent gleet however I've been reading something else usually causes vent gleet so giving her monostat won't fix the actual issue is that true? And I didn't respond much to everyone else because I was hoping someone experienced the same thing and could give me an exact answer but I'm realising it's more complicated than that. Someone said possibly bird flu and told us to move the entire coop and run and get her away from the others however I don't think it's that because she's been acting like this long enough I'd expect the others to all start getting sick by now.
I do not have a vet on the island infact we dont have an avian vet anywhere nearus
I hope you get answers! 
