Maybe it's a bird thing (although I can't see chickens doing it) but the parrots I grew up with and my parents macaw will do that to music, they really go to town. lol
My two females (a buff, who does the most head bobbing, and a Cayuga) always bob their head up and down since like 2 weeks old. They seem to do it a lot when they are happy and excited, like when I come bring them food or fill up their pool or let them out, or when they see I have treats...