Welcome to BYC. Sneezing occasionally can be from dust in the coop or feed. More frequent sneezing could be a sign of a respiratory disease, such as infectious bronchitis, a virus. A clear runny nasal draiange is another sign. If you see worse symptoms, such as gasping, wheezing, foamy bubbles in the eye, or facial swelling, that could be mycoplasma (MG,) coryza, or trach (ILT.). For a virus like bronchitis or ILT, antibiotics are usually not effective, but Tylan and tetracycline drugs are used to treat MG and coryza. Make sure they have good overhead air circulation with windows covered by wire on opposite ends of the coop. No direct drafts on chickens though. Keep bedding dry and free of dust or mold. That can help to prevent respiratory diseases.
