A big group of birds all in flight

This is both eerie, and fascinating all at the same time. Usually, whenever you see a flock of birds they are just flying in a specific formation, and they usually are just going in one direction, and that’s it. This group of birds are all flying in different formations that move around a lot. My question is WHY they are flying in this type of formation, and just over a general area (the WHY is the eerie part that needs to be explained). If anyone has any theories, I would love to hear them.