Poop/pasty butt 18 week old chicken

[NickVoll34]

So I’ve looked around and can’t find quite what we are experiencing. Last week we found our Barred Rock dead, comb and waddle were quite pale but she ate and drank normally and was actively free ranging. Today our 18wk Black Australorp seemed a bit off, noticed she seems to be sneezing/coughing has a very dirty butt and her nasal passages seem to have some type of debris, maybe dirt? She also definitely has diarrhea (yellowish runny excrement). We have her quarantined in the house in her own enclosure any thoughts?

 

[Yardmom]

Sorry to here about your hens. How is the rest of your flock doing?

 

[NickVoll34]

Sorry to here about your hens. How is the rest of your flock doing?

Thank you, the other 5 seem to be doing fine, no issues with them for now. When I get home I’ll check all of them again and make sure that nothing has changed