They can be adjusting their crops (yawning). Or it can be gasping from a respiratory disease, or gapeworm. Gapeworm is kind of rare from what I have read, and so it is most likely to be just her adjusting her crop, I am guessing.
Yawning can also be the sign of a throat infection. Had a large peachick last year that was yawing, depressed and not eating anything but soft food. Took him to the vet and she prescribed Cephalexin 250mg twice a day for 14 days.