There are several upper respiratory type of infections including allergic reactions also ( like to new bedding, sawdust etc.).
Coryza...Cull them if they have a stench like rotting flesh noted when you get close to their heads. Been there before and it was a horrible disaster to my flock.
If it is bronchitis, there are some other meds you could use, Sulmet or gallimycin, Tylan
BUTTTTTTTTTT....I'd try to de-lice and boost their nutrition first. Poly vi-sol vitamins, one dropper full, everyday for a few days can do wonders. Also Vet Rx is a good natural respiratory reliever.