Ten Thousand Geese Fly Off Together

These kinds of things have always fascinated me. If You have ever noticed birds will move together and all of them will suddenly change direction in an instant together. How do they do that how do they communicate how do they suddenly all go in that direction?  I have to assume through sound but it's amazing to watch them swoop and roll together. I have also noticed that birds in my backyard will sing loudly all together and then suddenly stop at once all at the same time. This and the bats flying out of Carlsbad Cavern is amazing to me. Nothing but a big cloud of bats going out at once. In fact a farmer found the cave after seeing it. He thought it was smoke from a fire and went to investigate to find a massive cave. Nice! Got a video of that too down below Great Stuff in nature but the bat population has dwindled considerably now sorry to say.