[video]http://www.youtube.com/watch?v=dASClLrWY6E[/video]
A hummingbird flying backwards, slowed by a factor of 50. (Nir Sapir)
Hummingbirds’ ability to fly backward is surprisingly efficient, even though it requires these little birds to beat their wings faster.
In a new study of Anna’s hummingbirds, Calypte anna, their metabolism was measured in association with three flight styles: forward, backward, and hovering.
Five birds were filmed in a wind tunnel feeding from a syringe of sucrose with the air flow directed at the birds so that they had to fly backward to remain stationary while feeding. Then the feeder was rotated 180 degrees so that the birds had to fly forward while feeding.
Analysis of the flight types showed distinct differences. In backward flight, the body is more upright so birds have to flex their heads more to reach the nectar. The wing beat also becomes more horizontal and faster at 43.8 Hz, instead of 39.7 Hz for forward flight.
Next, the experiments were repeated while measuring oxygen consumption.
“We expected that we would find high or intermediate values for metabolism during backward flight because the bird has an upright body position and this means that they have a higher drag,” said study co-author Nir Sapir at the University of California-Berkeley in a press release.
“Also, the birds use backward flight frequently, but not all the time, so we assumed that it would not be more efficient in terms of the flight mechanics compared with forward flight.”
However, the researchers found that backward flight is as metabolically efficient as forward flight and about 20 percent more efficient than hovering.
The results will be published in the Journal of Experimental Biology on Oct. 15, and can be accessed online here.
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