Why do birds twist their heads like "robots"?

Question

I don't know if others notice this, but why is it that when birds twist their heads, the motion is like that of a "robot" and not like other animals in which the motion of the head is "smooth?"

Answer 1

This motion has been named "cocking the head", and species other than birds, including humans do it.  I believe it is due to hearing a sound from a direction and moving your head quickly to line an ear up to that direction to hear the sound better.  Animals with mobile ears, such as horses, cows, and rabbits often "c**k an ear" towards a sound rather than move their whole head.  If the sound disturbs or interests them, they will then move their head so as to hear it with both ears.

Answer 2

It's just quicker