Why do chickens yawn and shake heads with certain sounds?

[RubiaSexy]

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?

 

[DobieLover]

I've never seen a chicken yawn.
I've seen them open their beaks and and stretch their necks up and move their heads a bit to adjust their crops.
I've seen a few birds open their beaks when they've got something stuck in their throat they are trying to cough out.
I've seen pictures of birds suffering from acute respiratory illnesses with their beaks agape gasping for breath.
Can you take a video of this behavior, upload it to YouTube or Vimeo and post a link here?

 

[CalBickieMomma]

I'm thinking along the same lines as @DobieLover. Probably crop adjusting (looks like a yawn, with the neck stretching). Does the noise sound like a sneeze? My girls do this when they scarf down their food too fast (as if I starve them ) - 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'.