Snotty nose and watery eyes.
There are quite a few respiratory illnesses in chickens, the common ones seen in the US being mycoplasma (MG,) infectious bronchitis, ILT, coryza, and aspergillosis. Testing by your local extension service, or having a necropsy done on a refrigerated body by your state vet could identify the illness. Secondary infections are common. Antibiotics such as Tylan, oxytetracycline, sulfa, and erythromycin are sometimes used to treat them. Most diseases can make your flock carriers for life, which can infect any new chickens, so closing your flock is wise. Here is a good link to read about those and other common diseases: https://edis.ifas.ufl.edu/ps044
Stress can bring out illness. Are there many chicken or poultry farms in your area? Testing is available through local extension agents or by calling your state vet with questions. I would rather know exactly what I was treating, since medications can differ with each disease. Some are viruses or fungi which don't respond to antibiotics. Secondary infections also can complicate a mild illness.