Jerky movement of head by birds

Adding to several other observations on Nature that have intrigued me, on which I had posted earlier, I like to mention another that has come to my notice a couple of days ago...

It is the following...

Our house compound  has a few trees that give us shade in summer. They attract several species of birds and animals like squirrel. They chirp especialy in the mornings and evenings and move around jumping from branch to branch. Observing their movements  a peculiarity struck me.

They turn their heads in a jerky fashion. Its never a smooth motion like ours, humans.

Why that is so? Nature surely has a reason to endow these creatures with such pattern of head movements. Seldom do you find this peculiarity among animals except in Sqirrel.

I propose that such a behavior is because their eyes are on both sides of their head which severely restricts the possibility of full coverage of the visual world in front of them. The reason is that the central portion of the field of vision is occluded because of the positioning of the eyes on a nearly parallel plane on both sides of the head unlike the great advantage that we humans have as a result of positioning of both eyes on the same plane at right angles with the direction of view.

The latter style of positionig covers almost 120 degrees of view of the scene in front with the individual cones of vision overlapping for nearly 30 degrees of each cone of vision. In fact as a result of this overlap we get additionally a 3-d vision of the scene which eables us to know the distance of each object in front of us. (Most probably birds are not able to get a 3-d view for the reason that the cones of coverage of the two eyes do not overlap.) The fact that there is a blind patch in front of them they need to move their heads FAST to ensure that there is no threat from the blind area. With the Sqirrel also this is the case. But most other animals have their eyes positioned in such a way that they can look straight ahead with the visual cones overlapping (to a lesser degree than for humans). Owls, monkeys have their eyes in the same plane as humans.  Most other animals have their eyes in converging planes ( with the meeting line being in front) but sufficient to give a total frontal view. So they don't need to have a jerky movemnt of head. Recall how a cow or a lion moves its head. I'm not sure of the Owl, though...

I would like to know whether any other explanation is plausible for this phenomenon...

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