I notice that when birds are perching, their head motions - particularly looking around or going after food - are all at about the same speed, and they stop on a dime. (I don’t know how else to describe this type of motion.) It’s jerky. A motion may even be broken into several stages, but each stage is a discreet movement at a constant speed with a sudden stop. There doesn’t seem to be any slow motion movement, and there doesn’t seem to be any gradual slowing down. It reminds me of the way ants move. When they stop, they stop instantly. They don’t just slow down. Is this predominantly a behavioral quirk, or is there something about their musculature and nervous system that leads to that?
Or, am I just not seeing the motion very well?
I’m not aware of any significant difference in the way birds move compared to other animals of similar size.
CC, I’m a bird owner/servant, and I know what you mean. I think there are a few things going on.
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Balance. You’re seeing the motion of someone trying to balance on a narrow, often moving, perch. Compare the movements of the Olympic gymnasts on the balance beam when they stumble.
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Mass. Birds are so light that they have much less momentum when they move. Stopping instantly is easier for them.
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Confirmation bias. We notice the times when move in a jerky way, and not the times when they move more sinuously. Like when they dance. (videos)
I’ve seen squirrels and mice show the same type of movements described in the OP. It makes sense, as Tom suggests, that this is easier for smaller animals to do. I think feathered and furry animals may have their neck movements somewhat hidden by their coverings. Perhaps a naked bird wouldn’t appear to move quite the same way.