This is one of the most impressive SWARM (Scalable sWarms of Autonomous Robots and Mobile Sensors) fleet that I have seen. With so many of them, it can fly in formation (check out the figure of 8 formation) and avoid obstacles while flying in formation.
These Nano Quadroter experiments by GRASP lab shows the potential of future unmanned aircraft. However, I guess it will be some time before we can see them leaving the lab. I am sure that at this moment, the precision flying is with the help of many cameras and sensors around the area.