How do they know?

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Have you ever watched a flock of birds flying around in complete "unison"? I mean, the entire flock makes exactly the same moves, at exactly the same time! Near where we live, there are lots of Starlings, and we often see large flocks flying around, and it's truly amazing to watch! I can't help wonder how they know when to do what! It's as though a Master Choreographer was coaching them as they swooped and swirled and soared this way and that way, in near-perfect unison. What do you think? How do they know what to do, and when? Is it all about 'air-currents', or 'thermals'? Is it that they 'communicate' somehow?