Why do birds move their heads abruptly?
Most species of birds have 2 foveas, the temporal fovea and the central fovea. 


  temporal fovea, which is like ours in the sense that it looks straight
  ahead and offers binocular vision (i.e. the temporal foveas of both
  eyes point in the same direction). But birds also have a central
  fovea, which points sideways and is, obviously, monocular (i.e., the
  central foveas of both eyes look in opposite directions).


So the bird has a choice of which fovea it wants to look through. 


  It can look straight ahead with its temporal foveas, to the left with
  the central fovea of its left eye, or to the right with the central
  fovea of its right eye. And this is not a hypothetical possibility:
  Birds actually do switch between foveas all the time! This is why they
  tend to swing their heads erratically in turns of about 90° (reference).