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How can it be explained (in evolutionary and/or neuronal terms), that the spontaneous movements of birds and reptiles are seemingly "abrupt" and not so "smooth": Their spontaneous movements seem to start abruptly and end abruptly (while the spontaneous movements of mammals tend to be smoother: starting more slowly, ending more slowly)?

One line of (speculative) reasoning might go like this: Making smooth (= not abrupt) movements is "evolutionary better" (for the muscles and tendons - they don't wear out so fast). But it takes more computational power (= neurons) to perform smooth movements.

Hans-Peter Stricker
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    My experience with birds and mammals doesn't agree with the dichotomy you present. Can you provide objective data showing the movement patterns you suggest? – kmm Sep 18 '18 at 15:46
  • No, it's just a subjective impression (which might be backed by data of relative velocities and acceleration). – Hans-Peter Stricker Sep 18 '18 at 15:48
  • Exactly. My subjective impression doesn't match yours, so we each have n = 1. How can this question be answered? – kmm Sep 18 '18 at 15:57
  • Why do you say n = 1? We both have observed hundreds of birds, reptiles and mammals during our life time. (Only the level of exactness (= time resolution) could have been better.) – Hans-Peter Stricker Sep 18 '18 at 16:55

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Rats make rather abrupt movements too. I would expect the difference you observe in movement speed is caused by size more than it is caused by being a bird vs a mammal. But why size would matter?

An increase in the linear dimension $x$ lead to an increase in the cross-section of muscles $x^2$ and an increase in the volume (and hence mass) of $x^3$. The strength of muscles is affected by their cross section $x^2$. Hence as animal grow bigger, the ratio $\frac{x^2}{x^3} = \frac{1}{x}$ becomes smaller and so does their ability to create large acceleration of their body parts. Their movement become smoother and less abrupt.

You might want to have a look at this highly related post too.

Remi.b
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