Why do birds move so jerky?

Hmmm. Never really considered birds to move in a robotic fashion.

don't remember exactly why but its the same reason your eyes move jerky. Look in a mirror and try to get them to move smoothly. You can't do it.

Bird movement and behavior is complicated and can't be generalized. Get a pair of binos (7 or 8 x), a little birdwatching book and have a look for yourself. Woodpeckers boink their beaks hard enough into trees to make large holes (viz: Pileated), hummingbirds hover like littte Sikorskis, herons walk gracefully thru shallow water like delicate dancers, some birds feign injury to lead you away from nest filled w. eggs, phoebes bob head, wrens tilt tails up, roadrunners run rather than fly, wild turkeys lumber and fly a little; then lodge in a tree (they aren't called turkeys for nothing), chickadees can be tamed enuf to eat out of yr hand. Sorry for the flippant reception you received, Sam. Hang around.

Dear Sam,
I think there are many factors that, when combined together, create this effect. When these birds move, they do not have much of a ramp-up time. It would be like a car going from 0 to 60 instantaneously. Think of hyper fast twitch muscles in humans. Of course, these birds don't really stop and go in an instant, but the other side of this is how the viewer perceives motion. Our brains cannot process movements that are change so readily. So, to us, they appear jerky and robotic.

But, you are asking, what are the underlying mechanisms that make birds go from a steady state to a motion state (and vice-versa) so fast? Excellent question! I am not sure. Biochemistry and short signal transmission times in the nerve?

I think it's not 'jerky' as much as it's just faster. Birds have a very high metabolism and high temperatures. They do everything faster than us. Yes, there are surely better bio-chemical descriptions of their physiological processes, but on a basic level, that's it. Insects and spiders are that way, too. Mice, smaller mammals, etc.

I think the better question is why do we humans see and move so slowly.

I believe it is just the head that moves in a jerky manner, for vision purposes. See this article abstract:

Head movements and eye functions of birds

K. Dunlap and O.H. Mowrer

Available online 28 May 2007.
Certain land-dwelling birds, such as the pigeon and chicken, manifest in walking a characteristic pattern of head movement. The movement consists of quick forward thrusts of the head alternating with periods during which the head remains almost stationary, moving slowly backward with reference to the bird’s progressing body. This same pattern of head movement also occurs when the bird is carried by the experimenter, providing that vision is not excluded, that the speed is not excessive, and that the bird is not unduly frightened or excited. If the body of the bird is oscillated slowly back and forth through a distance which does not exceed the limits of stretch of the bird’s neck, the head will compensate for this bodily displacement and remain practically fixed in position. When the body is held at rest, “peering” movements may occur, which consist either of moving the head into various planes by means of flexing the neck and then thrusting the head away from the body in a different direction, or of simply changing the angular position of the head with reference to the neck and the rest of the body. Each of these three patterns of head movement has its own particular significance, but all of them may be said, in general, to have either the function of maintaining a motionless visual field or of obtaining a new visual field. During this latter function the nictitating membrane usually flicks across the eye and eliminates distinct vision while the head is being shifted, thereby functioning in a manner comparable to the shutter on a movie camera. Studies of head movement resulting from bodily rotation and of the general relation of the visual and vestibular mechanisms in the production of both head and eye movements are to be reported later. (PsycINFO Database Record© 2006 APA, all rights reserved)

And another:

What are birds looking at? Head movements and eye use in chickens

Marian Stamp Dawkinsf1

Animal Behaviour Research Group, Department of Zoology, University of Oxford

Received 7 August 2001; revised 2 October 2001; accepted 17 December 2001. ; Available online 21 June 2002.
Abstract

Using video recordings of hens, Gallus gallus domesticus, as they approached different kinds of objects, I examined how change in object distance is associated with a change from lateral to binocular viewing. The birds tended to view distant objects laterally while they preferentially viewed objects less than 20–30 cm away frontally; this was true whether they were looking at another bird or at an inanimate object. However, as well as switching between lateral and frontal viewing, the hens also swung their heads from side to side with movements so large that the same object appeared to be viewed with completely different parts of the retina, and even with different eyes, in rapid succession. When confronted with a novel object, the hens walked more slowly but continued to show large head movements. This suggests that, unlike mammals, which gaze fixedly at novel objects, hens investigate them by moving the head and looking at them with different, specialized, parts of their eyes. Many aspects of bird behaviour, such as search image formation, vigilance and visual discriminations, may be affected by the way they move the head and eyes. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.