Scaling down the size and mass of micro aerial vehicles (MAVs) increases their agility and their ability to operate in tight formations. In addition, smaller robots are safer and, as we will show in this paper, more robust to collisions. This paper addresses the development of a pico quadrotor measuring 11 cm from tip to tip, with a mass of 25g. To increase the robustness of the robot to collisions, the vehicle is equipped with a 2 gram carbon fiber cage that protects it from impact velocities in excess of 4 m/s and also permits recovery after collisions. We present the design of the electrical, mechanical and computational elements, as well as experimental results demonstrating trajectory following with feedback from an external motion camera system, recovery from collisions with walls and other robots, and formation flight.