Welcome to our flock! There are many respiratory diseases that chickens can get. Not treating can lead to secondary illnesses and death. It would be best to start them on antibiotics. There are many posts and links on this site to help you try to figure out the illness, what to treat with and the dosages, methods and egg/meat withdrawal times. Use the search bar at the top of this site or google respiratory diseases in chickens.
Mycoplasma (MG or MS,) coryza, infectious bronchitis, ILT, and others can cause those symptoms. Do you see any swelling around an eye, eye bubbles, or smell a bad odor? Most of those respiratory diseases make carriers of your flock, and can be chronic. Tylan, oxytetracycline, and Gallimycin are some of the antibiotics commonly used to treat symptoms. Here is a good link to read about those and other diseases: http://edis.ifas.ufl.edu/ps044