I have 10 chickens. 1 of them is shaking her head almost continuosly for the last few days- getting more noticable in the last day. I don't know if its any worse than it was a few days ago, but I'm noticing it more. The shake is like a twitch. It is one 'pop' (a quick flick) and a little bit of a shiver after, over and over. She has no other symptoms and none of the others have any unusual symptoms. Any ideas? She is 21 weeks, as are all of my chickens, no one is laying yet, she is an EE.