Ultrafast Camera Captures how Hummingbirds Hover

CORVALIS, Ore., June 23 -- Hummingbirds are masters of the air, unique among birds for their ability to hover for long periods. Using a sophisticated digital imaging technique, scientists have determined the aerodynamics of hummingbird flight. These latest data disprove conclusions from numerous earlier studies that hummingbirds hover like insects, despite their profound muscle and skeletal differences.

Researchers discovered that hummingbirds differ from both birds and insects in how they hover. Hummingbirds support 75 percent of their body weight on the downstroke, which is actually a forward motion in the nearly vertical hovering birds. The new finding will provide engineers with a refined model for developing miniature autonomous flying vehicles. (Photo courtesy Nicolle Rager Fuller, National Science Foundation)

The team found that hummingbirds support 75 percent of their weight during the wing's downstroke and 25 percent on the upstroke -- in contrast to insects, which produce equal amounts of lift during their down and up strokes.

Researchers from Oregon State University, University of Portland and George Fox University published the new findings in the journal Nature this week.

Co-author Bret Tobalske said, "We were surprised to find that the upstroke in the hovering hummingbird was much less active than the downstroke. This finding provides new insight into evolutionary trends that led to sustained hovering in birds."

Scientists used computer-aided digital imagery to analyze the aerodynamics of the rufous hummingbird hovering. (Image: Dean E. Briggins, US Fish and Wildlife Service; courtesy National Science Foundation)

This allocation of wing workload differs from that of other birds, which use the downstroke to support 100 percent of their weight during slow flight and short-term hovering.

Insects support 50 percent of their weight with each stroke. Tobalske pointed out that, despite different ancestries, hummingbirds seem to have adapted insect flight performance using a birdlike wing that flexes, twists and arches in ways that the rigid insect wing cannot.

Previous research to determine how hummingbirds stay aloft employed high-speed video, but motion analysis alone was not sufficient to fully reveal the underlying aerodynamics.

In this study, the researchers applied "digital particle imaging velocimetry" (DPIV) to follow the flapping wings. DPIV is used in various applications to study flow characteristics of liquids and gases. By taking pictures with a special computer-coupled camera lighted with a laser, the distance traveled by individual particles seeded in a liquid or gas can be tracked through successive images. Hence, DPIV allows the researchers to follow the particles' movement image by image, like looking through the pages of a high-tech flipbook.

To observe the hummingbird in flight, the air in a wind tunnel was seeded with microscopic particles of olive oil, and digital images were captured every 300 microseconds as the bird hovered at a feeder. The wing beats caused the air to circulate, which in turn caused the floating oil particles to move. Computer-aided image analysis of each oil particle's position in consecutive frames allowed the scientists to reconstruct the lift and characteristics associated with each up and down wing movement.

It is said that Igor Sikorsky -- a name synonymous with the invention of the helicopter -- considered the flight of hummingbirds while going through numerous design modifications. So, according to Tobalske, it is fitting that this new description of hummingbird aerodynamics will provide engineers with a refined model for developing future miniature autonomous flying vehicles.

Hummingbirds seem to garner a universal appeal, something that was largely spurred by former DuPont company president, Crawford Greenewalt, who, in the 1960s, used novel strobe-flash technology to capture the birds in color photographs, leading to popular theories about their extreme hovering ability and numerous National Geographic articles.

"You would be hard-pressed to find someone who isn't amazed by hummingbirds," said H. Ross Hawkins, founder and executive director of The Hummingbird Society. "Perhaps it's their iridescent coloration and miniature size, or their ability to drop their heart rate from 500 beats per minute during the day to 40 beats per minute at night."

Hawkins said that it was logical, but perhaps naive, for scientists and bird-lovers to postulate that hummingbirds fly like insects of similar size.

"Fascinating!" Hawkins said of the findings.

The National Science Foundation's division of Integrative Organismal Biology supported this research.

For more information, visit: www.oregonstate.edu