It is not airborne as far as I know, it's in the soil like @ChickCrazed
said, but I think they can give it to one another by steping in the poo and and spreading it around that way, so I would think that even tho you other chicks are in the same room but NOT THE SAME PEN, they should be fine, make sure you wash you hands and anything else that may come in contact with both groups every very good each time you or they touch anything in the sic pen including the pen itself.
I love your feather planter, that is so cool looking.
How log have you had her/him? Your strain would be different then her strain so the immunity would be different. Would treat the group he/she was with too if it were me. Thats 1 reason I quarantine EVERYTHING for 30 days.