Not a bird. Not a plane. It's a mildly venomous snake.
If you visit Thailand, Indonesia, and other forested parts of southeast Asia, and look up, you might catch a glimpse of a snake careening through the air. The Chrysopelea paradisi doesn't fly, but it does have a pretty good handle on the controlled glide. The snakes launch themselves off of trees and undulate in midair. From a 33-foot-high jump the snakes can cover another 33 feet of distance. Scientists recently figured out how changes the snake makes to its body shape contributes to this ability. This video from Science News explains.
Meanwhile, I'm imagining slapstick comedy routines where unsuspecting people get slapped in the face by a flying snake. I can only assume this happens all the time in Chrysopelea paradisi's native habitat.