Chicken acting strange

[kyobona]

Hi there,
I have a couple of chickens and they are all friendly. They happily lay down on my lap and they love to do softly patted. But for some reason, when they are sitting happily. My chickens will slowly start to open their mouth and leave it open for a while then slowly start it. Does anyone know what type of behaviour they are displaying?

 

[Mahonri]

Could be they are warm.... it's a way of cooling.

Or... gapeworm??????

 

[kyobona]

A way of cooling, that sounds about right. Thanks for your help