It started today, and then I kind of stopped worrying about it because I didn't see my baby chick do it. Now they are taking a nap on my leg and I see it happening again. Don't know the breed, their about 4-5ish weeks old. She like twitches her head all weird, it happens every now and then, I don't know what to make of it! Other then that she is happy, sleeping, eating, drinking, and everything else. Thank you! Another update: She's doing it a lot more now, while she's trying to sleep. It doesn't seem to be bothering her though, but the jerking is on and off non stop.