Eli5 Why do birds have such twitchy movement?

Question

Compared to mammals, birds seem to have this jerky, almost robotic quality to the way they move. I was wondering if there’s any explanation for this

In: 1836

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You know how eyes turn to look at something pretty much instantly? Well birds eyes don’t move. At all. Their eyes are completely fixed into their skulls. So they need to be able to twitch their heads around as fast as an eye moving.