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?
Answer
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).
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