Hello, could your girls have mites or lice and worms??? This combo can kill chickens from all that I have been reading. One of my girls was soooo sick a few weeks ago. She was in the hospital (guest shower) for a full seven days before I let her go out with her litter mates. Below is a link to my journal which I have not updated lately, but I still have all of my girls. Email me if you wish....
https://www.backyardchickens.com/forum/viewtopic.php?id=97371
https://www.backyardchickens.com/forum/viewtopic.php?id=97371