Groming each other.
Birds have such astounding abilities to produce sounds vocally that it is often hard to prove that one is producing a particular sound some other way. One long-standing acoustic mystery involves the high-pitched squeak that male Anna’s Hummingbirds emit during the last part of their aerial diving display. This squeak resembles part of the vocal song these hummingbirds sing when perched, but it is much louder. Since the 1940s, ornithologists have debated whether this squeak is simply a louder version of the species’ regular vocal song or a different sound produced by some unknown mechanical structure.
The mystery of the hummingbird squeak has now been solved by two students from the University of California. Using ultra-high-speed video cameras, they first filmed and recorded several wild Anna’s Hummingbirds performing their dive display. When they looked at and listened closely to their tapes, the researchers discovered that the squeak was only made during the later part of the dive, coincident with the hummingbird suddenly spreading its tail. This finding focused their attention on the hummingbird’s tail feathers.
How is the squeak sound produced by a hummingbird tail feather? When the bird opens its tail during a dive, the rear (trailing) part of the feather vane encounters high air turbulence, which causes it to flutter at its resonant frequency, thereby producing the squeak. This mechanical sound production is akin to the much lower tone produced by a flag fluttering extremely rapidly in a very strong wind, or (in a somewhat less technically analogous way) to the vibrations of a reed in a wind instrument.
Anna's hummingbirds babies. California.