This video says about itself:
Drone Captures Millions Of Swarming Bats
4 March 2016
Forget Gotham City, this flying formation turned the sky above Khao no-Khao Kaeo, Thailand, dark as night.
As the video begins, the river of bats starts to appear above the lush forest below.
With the creatures corkscrewing around in unison, the drone – being operated by Charlie Onians – is swiftly surrounded by flapping wings.