When we humans look around, we pan smoothly from one side to the other. Birds on the other hand seem to point their head in one direction for a while, then abruptly point their head in another direction and stay in the new position for a while. Then they continue with the abrupt movement. Why is that?
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).
Many birds, especially birds of prey and owls, have eyes that are proportionally much larger than that of similar-size mammals. Most birds cannot move their eyes, as with larger eyes there is little or no room for the required musculature.
This, in addition to having two foveas as
@The Last Word mentioned, necessitates that a bird move its head to change viewing angle. The relatively rapid movement of a bird's head is analogous to that of the more subtle, but still rapid movement of your eye as you view different subjects within a frame of vision without moving your head.
Birds have more vertebrae (13-25) in their neck to provide additional flexibility, which provides for their quick head movements. Some birds, such as owls, can turn their head 270 degrees. Other birds have a wide field of vision because their eyes are located on the sides of their head. This degree of flexibility or field of vision provides birds with good compensation for being unable to move their eyes.
These characteristics, combined with the physiology of two fovea, make the abrupt, seemingly unusual movements of birds' heads more easily understood.
For additional insight, consider the book Bird Sense: What It's Like To Be a Bird by Tim Birkhead or Manual of Ornithology: Avian Structure and Function by Noble S. Proctor and Patrick J. Lynch; both of which I own and can recommend.