Swarming birds

Here's a local video of migrating (?) birds. Seemed like a million birds converged in the yard. They swooped down almost as one to cover the neighbors yard, then would move back to the trees. There were always some that chose to sit out the movement, maybe they were the sentinels watching for trouble. Sometimes one group would clear the trees and move over to another section. At times it reminded me of a chorus group doing rounds. This section would move, then the other would move.

A couple of times they all stopped chirping completely. Just dead silence. I filmed for over five minutes, but it's not the kind of video you sit and watch for 6 or 7 minutes unless you're someone who studies bird habits... This is a shortened version obviously!