June 22, 2005

How Do Hummingbirds Hover?

WASHINGTON (Reuters) -- Hummingbirds hover by flapping their wings a bit like insects and a bit like other birds, and now a super-fast camera has made an image of the technique, scientists reported on Wednesday.

A team of U.S. researchers found that hummingbirds manage to hover in air for long periods by supporting 75 percent of their weight during their wings' down stroke and 25 percent on the up stroke.

Other birds support all of their weight on the down stroke for slow flight and short-term hovering, while insects produce equal amounts of lift on both the down and up strokes.

"We were surprised to find that the up stroke in the hovering hummingbird was much less active than the down stroke," researcher Bret Tobalske said in a statement. "This finding provides new insight into evolutionary trends that led to sustained hovering in birds."

Hummingbirds are unique in the bird world in their ability to hover for long periods, and scientists had theorized that this behavior was more like insects than other birds. It turns out that it is midway between the two, according to a study published in the current issue of the journal Nature.

Despite different ancestry, hummingbirds seem to have adapted insect flight performance using a bird-like wing that flexes, twists and arches in ways that rigid insect wings cannot.

Earlier analysis of hummingbird hovering used high-speed video, but the current study employed digital particle imaging velocimetry -- DPIV for short -- a technique often used to study the flow of liquids and gases by adding microscopic particles that can be tracked during movement.

To catch images of the hummingbird, scientists added tiny particles of olive oil to the air in a wind tunnel, then took pictures every 300 microseconds as the bird hovered at a feeder.

The wing beats made the air circulate and caused the oil particles to move. By tracking the particles' movement, the researchers were able to determine how much lift was associated with each up or down wing stroke.