I have a four month old rooster who I've recently noticed shakes his head in a quick, jerky way every time I speak to him. I've watched him listening to other sounds and he doesn't do it, and neither do any of my other chickens shake their heads this way. Its just the one guy whenever I speak. I've even tried speaking more high or low pitched and he still does it. But with other sounds, nothing. I've checked his ears and there is no sign of illness, injury, mites etc. He is generally very healthy, never had any problem. So what's the head shaking all about?