A large group of flying drones is a swarm.
Together, the drones exhibit emergent behavior which surpasses the simple operations by any single unit.
Swarm attacks generally follow a circling pattern with sudden vectoring towards the target.
Stigmergy is a mechanism of indirect coordination between agents or actions. The principle is that the trace left in the environment by an action stimulates the performance of a next action, by the same or a different agent. In that way, subsequent actions tend to reinforce and build on each other, leading to the spontaneous emergence of coherent, apparently systematic activity.