Take a look at Infectious Broncitis (IB,) Infectious Coryza, Mycoplasma Galliaepticum (MG) in this link: http://edis.ifas.ufl.edu/ps044
There are treatment options; culling is always the best option if you're sure you're dealing with a respiratory disease. Bloodwork or necropsy will verify what you're dealing with, then you can make the best decision possible.
It is always best to know what you are dealing with before treating with meds. This link will give you the necropsy information for each state, and you can call for info on how to preserve and send the bodies, cost, etc. It is: http://agr.wa.gov/FoodAnimal/AnimalHealth/statevets.aspx