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.
"We" don't, always. In addition to the factors in other answers, there's a simple matter of size and inertia at work. If you look at small mammals like squirrels or mice (and presumably small monkeys, though I've never observed one for any length of time), you'll notice that ALL their movements seem to be quick and "jerky", because their heads and other limbs are small, and so have little inertia. (It's a matter of scale that's not limited to animals. Consider how small engines or fans can spin much faster than large ones - motorcycle vs diesel truck, computer cooling fan vs wind turbine, &c.)
Larger animals like humans (and dogs, horses, &c) CAN move their heads quite quickly, as when they're startled by something, but it takes effort and is rather uncomfortable. (Try it!) But virtually all familiar birds have small heads, though it'd be interesting to look at something like a pelican or toucan that has a large beak.
As i understand it:
Birds can't move their eyes around the way we can (excepting cormorants and maybe some others), and the result is that they don't have stereoscopic vision when they don't move their heads. More than that, their field of focus is very small if their heads are kept still. That last is also true for us.
The chickens we raised have their eyes even more on the sides than pigeons. They would look at us with one eye ---- then the other eye, then the first eye again, etcetera. This gave them a wider field of view and a stereoscopic view of us and their environment.
Try: Look at a letter somewhere near the center of your screen. Without moving your eyes or head, how many words can you make out up, down, left, right of your point of focus. If you can't move your eyeballs (as birds can't) you can only get the wider view by moving your head.
Plus, pigeons are prey animals and therefore have eyes on the sides of their heads which lets them see movement a lot further behind than we can. (https://en.wikipedia.org/wiki/Field_of_view) We have a horizontal field of view of about 210 degrees and "Pigeon FOV is around 340-degrees horizontal and about the same 135-vertical degrees as humans, but their vertical field is even more asymmetrically oriented toward the ground."(https://www.quora.com › What-does-a-pigeons-field-of-view-look-like)