I was straight overhead, holding in a hover at about 300 ft. when all of a sudden, a falcon of some species buzzed by the drone, passing within probably 12 inches of it at very high speed. I only saw him literally as he passed the drone. He climbed, turned and buzzed it again. No contact was made thankfully. He landed in the top of a nearby cottonwood tree. It was all over in 5 seconds. I couldn't get out of the air fast enough! It's a bit unnerving to know we can be doing everything right and still lose a drone to an unseen raptor at any moment. It's pretty much like lightning striking!