Monday, April 2, 2007
More aerial dogfights
The aerial dog-fights of spring are still going on, and I finally got a picture of it. Rufous hummer, the aggressor here, pilots the smaller aircraft. He is a barnstorming stunt pilot. The sight of him whizzing about has the Anna's hummers on guard.
At the feeder he just points his nose-cone and intruders back off.
For a long time the flight of hummingbirds remained a mystery, but high-speed stroboscopic photography opened the door to understanding. We have Harold Edgerton (1903-1990) to thank for that. He invented the electronic flash and stroboscope. Doc Edgerton was a small town boy from Nebraska whose interest in photography and electronics led to a distinguished career at MIT. He captured stunning images of things the human eye would never otherwise see. Why Edgerton never tackled the question of hummingbird flight will remain a mystery. He certainly wasn't opposed to applying the technology to biological problems, as he did with Jacques Cousteau. Anyway, his invention held the key to seeing the magical wings at work.
It was a contemporary of Edgerton's, the CEO of the Du Pont Company, Crawford H. Greenewalt (1902-1993) who decided to study hummingbird flight with the use of stroboscopic photography. How refreshing to learn of a CEO who was not only willing to support science, but became personally involved. With a hummingbird trapper, camera equipment, and wind tunnel in tow, he photographed 55 species of hummingbirds from the US to Brazil. The output, a coffee-table book titled simply "Hummingbirds" was published in 1960. It was a labor of love. Nowadays, original editions command as much as $225.
Greenewalt's treatment of hummingbird biology wasn't superficial, but he wanted to translate science for the lay audience. A reviewer's copy of the book landed on the desk of the late ornithologist Alden Miller at UC Berkeley. (His father, Loye Miller was among the first to suggest that the Anna hummingbird's dive-noise resulted from the braking sound produced by the outer tail feathers.)
But Greenewalt failed to include a bibliography in his book. Miller commented: "I think that neither the author nor the American Museum should feel compelled to try so hard to meet the untrained reader as to fail to provide avenue of access to source articles should his interest become aroused. He should not be so assiduously walled off from the pursuit of scholarship." (The codger couldn’t agree more.)
More recently a team of scientists led by Doug Warrick made a breakthrough in the aerodynamics of hummingbird flight. Previous researchers believed that hummingbirds fly more like insects--that is, the wings' downstrokes and upstrokes generated lift equally. Warrick and colleagues used a new method called digital particle imaging velocimetry (DPIV). Basically you put your hummer in a wind tunnel, atomize olive oil in the jetstream, and let the little guy do his thing. Then you use a pulsing laser and digital videography to measure the effects of the wing movements. They found that hummingbirds support three quarters of their airborne weight during the wings' downstroke, and only a quarter of it during the upstroke.
So, we are still learning about the flight of hummingbirds. As for the Anna's hummingbirds dive-noise, the late and great Luis Baptista and Margaret Matsui questioned the conventional wisdom after observing the behavior and recording the sounds. They submitted "that the dive noise. . . is mostly, if not entirely, vocal in origin".
Baptista, L.F. and M. Matsui. 1979. The source of the dive nosie of the Anna's hummingbird. Condor 81:87-89.
Greenewalt, C.H. 1960. Hummingbirds. American Museum of Natural History and Doubleday and Company, N.Y. 250 pp.
Miller, L.H. 1940. Sound produced in the nuptial dive of young Anna Hummingbirds. Condor 42:305-306.
Miller, A.H. 1960. Review of "Hummingbirds". Condor, 61:100. (http://elibrary.unm.edu/sora/Condor/issues/v063n01/index.php)
Warrick, D.R., B.W.Tobalske, D.R.Powers. 2005. The aerodynamics of hummingbird flight. Nature 453:1094-1096.