All birds make sound in flight. Noise arises from the drag of air over a bird’s body, from the vortices, or turbulent whirls of wind in a bird’s wake that generate sound waves, from air squeezing through the slits in feathers, and from feathers rubbing together. With every flap, bird wings rustle and flutter, whistle, hum, and whish. They drum and snap and clap. But the sounds that many owls make when they fly are so faint that they’re below the threshold of human hearing.
