Study says hummingbirds fly more like birds than insects

PORTLAND — Not much bigger than a honey bee, the tiny hummingbird is able to hover gently over a flower because it uses some of the same wing motion as an insect — but not as much as previously thought, a new study says.

The hummingbird neatly splits the difference between birds, which get all their lift from the downstroke of their wings, and bugs, which get equal amounts of lift from both downstroke and upstroke.

Researchers used sophisticated technology originally developed for engineering design to analyze the movement of air around a hummingbird's wings and provide details about its flight that had been limited to mere guesswork for decades.

Hummingbirds get about 75 percent of their lift from the downstroke, with the remaining 25 percent provided by the upstroke, according to University of Portland and Oregon State University researchers.

For all other birds, the lift is 100 percent downstroke, while bugs are 50-50, up and down.

The study demonstrates a strong example of biological convergence — unrelated species evolving similar characteristics in order to exploit their niche, said Douglas Warrick, an Oregon State zoology professor who led the research published this week in the journal Nature.

"This is probably the result of how far natural selection can take an avian body plan toward looking like an insect and functioning like an insect, in terms of flight," Warrick said.

Warrick and University of Portland biologist Bret Tobalske put bird feeders in a specially designed wind tunnel equipped with digital particle imaging velocimetry equipment — a laser device that is linked to a computer to measure the movement of tiny droplets of olive oil swirling in the air.

The device allowed researchers to take "snapshots" of hummingbird wing motion only 250 microseconds — millionths of a second — apart.

The results challenged previous assumptions about hummingbird wings providing equal lift using two strokes, like insect wings, simply because the bird and bugs move their wings in a similar pattern, said Michael Dickinson, a Caltech bioengineering researcher who studies both insects and hummingbirds.

"But it's not surprising they can exploit the same characteristics of the physical world," Dickinson said. "This really is an example of convergence."

Warrick noted that bird wings are different from insect wings, with a bony structure that is more like a human arm than an insect wing, along with feathers that help form a contoured front edge. Flying insects, in contrast, have wings that are almost flat, like paddles, allowing bugs to gain lift with two mirror-image halfstrokes as the wing moves in a figure-eight pattern.

"They look so similar and function so similarly, and they're about the same size," Warrick said of the differing wing structures, "and those similarities always led people to believe that hummingbirds must fly like insects. But a hummingbird will never be anything but a bird."

Tobalske said the images showed the hummingbird bobbed while hovering, moving up and down a little as it switched from upstroke to downstroke.