HELP! 3 week old Gold Sex Link chick "twitching".

[rhemerick]

We have a 3 week old Gold Sex Link chick that suddenly started twitching her neck this afternoon. Is this normal or something that we should be concerned about?

 

[rhemerick]

This is a video of a chicken that is doing what our chicks are doing.

Does anyone know what this twitching head thing could be? Our chick is much smaller (only about 3-4 weeks), but she is doing the same thing with her head. And now this morning it appears that at least 5 out of 10 of our chicks (ranging in ages of 2 weeks to 4 weeks) also have the same head twitching. I looked at their pupils, and they seem to be fine. Only one of them had eyes that seem to shake a bit.