Abrupt movements of birds and reptiles vs. smooth movements of mammals 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.

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.