First, spend some time with them, to see who is sneezing, and how often. Is the same one sneezing every few minutes? Are they eating at the time? How is the coop ventilation? Have you used something like DE or other in the coop lately? Is it dusty, or damp, or have a ammonia odor?
All chickens may sneeze now and then, especially if they are eating chicken feed, and getting some into their nostrils. Environmental things may also cause an occasional sneeze.
The most common respiratory disease is infectious bronchitis, which causes sneezing every few minutes. There may be aome congestion and moisture around the nostrils which will attract dirt. It spreads through the whole flock over weeks to months. There is no cure since it it caused by a virus, but will go away after a few weeks. They remain carriers for 5 months or more.
If you later see bubbles in one eye, nasal drainage, hear crackles or wheezes, or see gasping, it would be good to get some testing to see if it is mycoplasma (MG) or coryza, or another virus called ILT? Tylan50 injectable fiven orally or Tylosin generic powder in the water can help if it is MG. But I would only treat chickens who had symptoms, not the whole dlock.
