It would seem to take a lot of energy for a bird to move backward, but hummingbirds fly in reverse fairly often. Now a study reports that their backward flight is almost as efficient as their forward flight.
"When they are flying backwards they have a very upright body posture," said Nir Sapir, an avian ecologist affiliated with the Max Planck Institute for Ornithology in Germany and one of the researchers involved in the study. "We thought they might have a much higher drag, and invest much more energy in flight."
Dr. Sapir worked on the study when he was a postdoctoral researcher at the University of California, Berkeley. He and his colleague Robert Dudley, a researcher at the Smithsonian Tropical Research Institute in Panama, reported their findings in The Journal of Experimental Biology.
Dr. Sapir captured five Anna's hummingbirds outside his laboratory and trained them to fly in a wind tunnel and feed on a syringe of sucrose disguised as a flower.
As they fed, Dr. Sapir turned on the air in the tunnel so that the birds had to fly backward to remain at the flower. He repeated the experiment with the feeder rotated 180 degrees so that the birds had to fly forward to keep feeding.
When flying forward, the birds beat their wings about 39.7 hertz. When they flew backward, they beat their wings only slightly faster, at 43.8 hertz. The rate of oxygen consumption was also similar, Dr. Sapir said.
The researchers also found that the birds were unable to fly backward at more than 10 miles per hour.
Correction: October 2, 2012, Tuesday
This article has been revised to reflect the following correction: An earlier version of this article included a photo with an incorrect orientation. It should have been rotated 90 degrees, so the bird was shown feeding with its beak parallel to the ground, not facing downward.
This article originally appeared in The New York Times.