Why do bats swarm in a circle?

they don't.not mine anyways.i watch them come out in spurts that leave the underside of a bridge.they come out like clockwork.and! what i think is cool.. they get the heck separated asap. very hard to call them a swarm after about 200 ft .. they swarm out of the sleeping area then go mad flying all over the place. so they are only a swarm for a few seconds.. then they are more like a "cloud"..