What a fascinating view. I don't pretend to know what triggers a bird's decision to take flight or what makes an entire flock want to take off all at once but man, is it cool to see. This video shows three recordings of starlings taking off from a power line: in real time, at half speed and at 300 frames per second.
Dennis Hlynsky explains what's going on in his video:
So, I am focusing my energy on those moments when the birds take off. Here are three renderings of the same recording The first approximates "real time", the second is half speed, the third is processed and is the 600 FPS footage played back at 300 FPS.
I am looking at the way the birds leave the line. Often they move from one line to another ... taking off when a second group arrives. Sometimes it looks like a row of dominoes ... one flight triggering the bird next in line. Here it is clear to sea the original take off is not sequential but appears to be crowd reactive.