Can an ultrasonic sensor mounted on a drone pick up another drone strongly enough to sense and avoid?
The range of the ultrasonic sensors ends up at somewhere 5 meters (here is the pre-configured search). However a simple sensor would only measure the distance, by the signal travel time. It does not provide the angular co-ordinates or exact location. Same problem as with the simple radar. 

May make more sense to fit the drones with transponders so that they could actively broadcast they own location, speed, orientation and intentions to other members of the flock.