Advice needed

Hi everyone,

I am new to BYC and was referred here by my chicken-loving older sister, who thinks there may be someone out there who can give me some advice. I have had 6 chickens (4 Barred Rocks and 2 Americaunas) since April. They had been happy and healthy up to this point, but I noticed that my largest Americauna the other day had what looked like something sticking out of her vent. She was acting a bit "off" too. I continued to observe her when I noticed one of the Barred Rocks peck her in the butt twice as she made a feeble attempt to get away. I have had her in the house since. Her appetite is not great. She is somewhat listless, but does make noises occasionally and walk around her pen. She has allowed me to wash her butt with a warm water spray from the kitchen sink and mild soap. It doesn't seem to be helping however. Her backside is not improving. She also has a lot of feathers missing in her rear.
I am attaching a picture I took with one hand today while I was holding her with the other after giving her a "sitz bath" in the sink. I've given her yogurt and hard boiled eggs. She takes a bit of those, but not with the usual gusto. I searched online and read that I should worry about an egg stuck up there. I gently probed her vent after the soaking today but did not feel anything although I did not go too far. I spoke with my animal science pre-vet daughter earlier and she had me worried that what she had might be zoonotic...she thought I should get some "bird biotic" (doxycycline).
I would really appreciate any advice you could give. Thank you!

 

Thank you so much dawg53! I don't see her as thirsty really nor do I know yet how to assess a crop (my sister's coming for Thanksgiving and I'm sure will show me...) however I am going to add some cider vinegar to the water right now. It certainly can't hurt.

 

Thank you all for your kind wishes! I am ordering some Nustock right now!