I've noticed not just with my Silkie chickens but also with the quails that when they hear certain tones/sounds or my singing to them (haha) they start yawning and shaking their heads all in unison. I try to keep them entertained by playing relaxing or at least what I thing is relaxing music to them and they'll do this. Needless to say, when I see their reaction I immediately change the music or stop singing. Lol. Last thing I want to do is cause them any discomfort or distress. So...what exactly does that yawning mean?? Is that a bad thing?
) - their beaks are usually open and they make a big sneezing noise and shake their head at the same time. I don't know for certain, but I imagine something went down the wrong tube. As long as this isn't a perpetual thing with yours (as in they do it for hours straight), it's probably just crop adjusting or 'chicken sneezing'.