My response is very different. I've always heard sneezing is not normal, and birds don't get colds, per se, but there are some nasty respiratory bugs that can run through a flock. I don't have personal experience--I just have healthy birds, apparently--but if I were you I'd post on the diseases forum and give them as much info as you can.
Really, as long is it only happens once or twice, and only to one or two birds, and there isn't discharge, there is usually no need to worry. It is true that there are many very serious respiratory diseases that chickens can carry, but sneezing alone is not usually reason to automatically assume something is wrong.
If chickens are eating or drinking, and they get a little dust over their nostrils, they will sneeze to dislodge it. Or if they are drinking from a water dish and get a little water of their nose, they will sneeze or shake their heads. This is totally normal, and alone is not necessarily a sign of a bird that is healthy or unhealthy.
As long as the chickens are healthy otherwise, the nostrils and beak are clear of any gunk or dirt, discharge, mucous, and breathing is normal, they are not lethargic or behaving strangely, they don't sneeze often or for no obvious reason, and they are eating, drinking, and pooping normally I would not rush to assume something is wrong.
However, if your chickens
are sneezing often or showing other signs of disease, there is reason to worry about respiratory disease, which is often serious and requires quick treatment.
