That looks like sneezing or snicking, a symptom of infectious bronchitis. IB is the most common respiratory disease in chickens, caused by a virus, and usually affects a large percentage of the flock. It lasts about 4-5 weeks, and sneezing and clear nasal drainage are the main symptoms. If you see thick nasal drainage, wheezing, or swollen eyelids, then you are dealing with something bacterial, such as MG or coryza. No antibiotics are needed with IB, but if you see more serious symptoms or secondary infections, get some Tylan 50 injectable to give orally. Dosage is 0.2 ml 3 times a day for 3-5 days.
With IB you may see a temporary decrese in laying or wrinkled egg shells. It can be more serious in young birds. The whole flock should be considered carriers for up to a year, so best not to hatch or bring in new birds for a year after the last bird improves.
I think my flock got it years ago, and when I would hatch new chicks, it would keep going around. After I waited a year, no one else was affected.
