The largest urban bat colony in North America lives under the Congress Ave. Bridge in Austin, Texas. When they emerge in the evening during “bat season” they stream like clouds. I've shot them many times. In this instance, I positioned my drone below the bat cloud, however they changed course as they are like to do and quickly the drone was in the swarm. The bats use their echolocation to avoid the drone so the best a drone pilot can do is not move the machine. But still, this is unnerving to watch unfold on your as they pass, coming directly at the drone.
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