To figure out the source of a hummingbird’s hum, scientists built a special rig to measure air pressure, twelve high-speed cameras, and over 2,000 microphones to observe hummingbirds fluttering in place at an artificial flower. The researchers found that the hum derives from the difference in air pressure between the top and bottom of the wings, which alternates forty times a second as the hummingbird flaps. The rapidly shifting air pressure produces a harmonic set of sounds, from low to high, creating that iconic, musical hum.

More info and transcript at BirdNote.org. 

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