The sound is a male Anna's hummingbird (Calypte anna) displaying his flying skills for a nearby female. During mating season, males perform high-speed dive bombing runs, in which they climb high into the sky (100 feet?), hover for a few seconds, then fly almost straight down at high speed, eventually pulling up to form a J-shaped trajectory, and making a loud 'chirp' sound at the bottom of the bombing run. It's quite an amazing experience to watch these tiny acrobats (This 47 kB WAV file contains the chirp).
Until last year, no one knew how such tiny birds could create such a loud sound. Two U.C. Berkeley researchers spent months in the field (the Albany Bulb) and the laboratory studying the birds, eventually discovering that the tail feathers are responsible for the sound: air passing over tail feathers cause them to vibrate like the reed in a woodwind instrument. The researchers confirmed this by capturing a hummingbird and trimming his tail — the manipulated feathers did not make a chirp.
The results were published in the Proceedings of the Royal Society B (the full article is subscription only, but supplementary video and audio are available for free). Good overviews of the research can be found in UC Berkeley News and the San Francisco Chronicle.For some visual pleasure, check out stunning photos of hummingbirds at National Geographic by Luis A. Mazariegos.
Image credits: Photo of single hummingbird from ingridtaylar's flickr collection, subject to a Creative Commons License. Photo of two hummingbirds from Southernpixel's flickr collection, subject to a Creative Commons License.
Random link from the archive: Beans and ice for dessert: Ais Kachang in Singapore