My roo is a little more expressive than that. He pants like a dog when he's happy, and his idea of a smile is to hang his beak open just the tiniest bit. I hate to brag, but there's something about Buster. He can also change his expression simply by altering the light in his eyes, if that makes sense to anyone.
Yonaton, thank you for your post and pictures. It clears up everything! I printed it for my co-worker who has chickens too. Now we'll know exactly what a hungry chicken looks like!