The Flight of the Hummingbird

Contact Stephanie Lam at [email protected]

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The hummingbird's common name is derived from the fact that in flight, the strong wing beat is so rapid it produces the sound of a hum. As researchers from both UC Berkeley and Caltech have discovered, this hovering ability of hummingbirds is altered in accordance with changes in elevation, lending insight both into power reserve in the birds during flight, as well as a more comprehensive tree of evolutionary relationships among species.

The study, combining the efforts of Douglas Altshuler, a postdoctoral fellow at Caltech, and professor Robert Dudley and assistant professor Jim McGuire of the Department of Integrative Biology at UC Berkeley, discovered that at higher elevations, hummingbirds exhibit a decreased "power margin," or the ratio of maximum flight performance relative to normal hovering, which indicates the excess reserve power available.

"The motivation for the study was the observation that at higher elevations, air density is lower, making it harder to fly, and there is also less oxygen so it is harder to breathe. We wanted to know how birds compensated for these effects," Dudley said.

In the study led by Altshuler, almost 1,000 hummingbirds representing 43 different species were tested through lifting trials and flight tests.

The experiment was two-fold. The birds were first videotaped as they hovered normally to get data on their normal wing motions, or wing amplitude, in flight and number of wing beats per second. The hummingbirds were then put through a lifting test to see the maximum amount of power their muscles could produce. This was measured by first attaching around the bird's neck a harness of colored beads spaced at constant intervals, and then allowing the bird to fly vertically upwards to see how much weight the bird could lift.

The researchers discovered that birds at higher elevations tend to have longer wings yielding a greater wing area, and that the birds also flap their wings through greater angles, up to 180 degrees on either side.

"One might guess that the birds confronted with lower density air, such as that encountered at high elevation would flap faster, but that wasn't really the primary response. What they did instead was to increase the wing-beat stroke amplitude, further and further, until the wing amplitude reached 180 degrees-essentially wing-tip to wing-tip-at which point they could no longer hover," McGuire said.

Contrary to the researchers' expectations, however, was the discovery that body size also increased with an increase in elevation.

"Size always plays a role. Basically a larger size presents a cost for locomotion. Though you might expect body size to decrease, it actually increases because there is a competing cost-as they go to higher elevations, the temperature decreases," McGuire said.

However, as McGuire noted, since the relative size of the wing actually increases at a faster rate than body mass, wing-loading-the amount of weight the wing must support in flight-decreases, so there is no increased cost in hovering at higher elevations. But this was an enigma in itself-at higher elevations, the birds had fewer predators, and less competition for food, so why didn't all the birds simply move to higher elevations? Ultimately, the researchers discovered that the cost came in the form of a decreasing power margin.

Researchers found that in maximum hovering, all birds, independent of elevation, reach 180 degrees of wing motion on either side. Thus birds at higher elevations are closer to the limit of their wing motions and have less of an excess range through which they can beat their wings, thereby reducing the power reserve, according to Dudley.

This would mean that hummingbirds at higher altitudes have less excess power, limiting their maneuverability and agility in escaping predators, chasing other hummingbirds of lower elevations, and completing mating rituals.

"Other more costly forms of locomotion like burst flight (a rapid burst of speed used to fly straight vertically up) are more clearly limited at higher elevations than is hovering. A hummingbird confronted with a predator or competitor at high elevation has less excess locomotor capacity for escape or rapid maneuvering," McGuire said.

The experiment also allowed the researchers to compile a more comprehensive and detailed phylogeny or family tree for hummingbirds based on a large DNA sequence data set, and demonstrated how hummingbirds gradually colonized higher elevations in the Andes.

"It is a very good physiological analysis of how hummingbirds radiated into a greater number of species during higher altitudinal colonization. Such diversification gives insight into the morphological and physiological consequences of high-elevation residence by birds," Dudley said.

Future plans for study include increasing the scope of the study to include more of the 330 different species of living hummingbirds.


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