Earthwatch-supported research shows that hummingbirds find attractive habitat at higher elevations, but at a cost to their maneuverability
Hummingbirds are found in a diversity of habitats, from Alaska to the southern tip of Chile, and all are known for their hovering abilities and astounding maneuverability. But research results appearing in the December 21, 2004 issue of the Proceedings of the National Academy of Sciences show a decline in flying power with altitude, not unlike that seen in athletes competing at high elevations.
"The costs of hovering flight are the same across elevations because hummingbirds compensate by having larger wings and by having a larger stroke amplitude," said lead author Dr. Douglas Altshuler, a postdoctoral fellow at California Institute of Technology. "However, that compensation doesn't come for free. They don't have as much excess power at high elevations as they do at low elevations."
What this means is that hummingbirds at higher elevations have less reserve power for the bursts of flight and other quick movements needed to chase off competitors, escape from predators, or other survival behaviors. Hummingbird mating displays, for example, often consists of repeated vertical ascents and power dives, and they engage in fast forward flight chases to kick other males off their territory.
"When you spend a lot of time watching hummingbirds, you realize they engage in an amazing suite of maneuvers, above and beyond hovering at flowers for nectar or insects," said Altshuler, former principal investigator of Earthwatch Institute's Flight of the Hummingbird project. "They are constantly using much more mechanical power to do other interesting things, such as competing with other males and courting females."
In the late 1990s Earthwatch teams helped Altshuler and his coauthor Dr. Robert Dudley (University of California, Berkeley) conduct lifting trials and flight tests with nearly 1,000 Peruvian hummingbirds, to test their flying abilities at elevations from 400 meters to 4,300 meters. Altshuler filmed the birds inside a Plexiglas chamber to obtain wing beat frequency and stroke amplitude, and filmed them lifting a chain of color-coded beads that indicated their maximum lift.
With full data from 347 of the captured hummingbirds, a sample representing 43 species, Altshuler was able to show that, whereas while the power requirements for normal hovering does not change across elevations, the power margin for other maneuvers decreases with increasing elevation.
"Higher elevations are an appealing niche, and hummingbirds have headed up there, but there are challenges," Altshuler said. Higher elevations often have more flowers and less competition from other birds and insects. Altshular notes that mountain flowers even appear to have made themselves more attractive to hummingbirds, perhaps to lure them to higher elevations.
The PNAS paper also includes the most complete family tree of hummingbirds ever produced, representing 75 of the approximately 337 known species and most of the known genera. Coauthor Dr. Jimmy McGuire, assistant professor of integrative biology at UC Berkeley, obtained DNA samples from museum specimens, and analyzed two nuclear and one mitochondrial gene from each species.
"To correlate wing characteristics with power margin at different elevations, you have to account for the phylogeny, the family tree that shows you how species are related to one another," said McGuire. "Since we see an inverse correlation between power margin and elevation, we have to make sure it's true across a wide variety of species."
In addition to supporting the paper's findings, the family tree yielded some surpises. For instance, the world's largest hummingbird, the giant hummingbird (Patagona gigas), has evidently taken is own evolutionary path away from other hummingbirds. Also, two boldly colored hummingbirds, the crimson topaz (Topaza pella) and the white-necked Jacobin (Florisuga mellivora), are apparently unrelated to other so-called "mangoes." They seem to be more primitive than even the drab hermits, regarded as the most primitive of the hummingbirds.
"If these species turn out to be the earliest, deepest lineage of hummingbirds, it would throw into question the presumed phylogeny," McGuire said.
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For more information, see "Resolution of a paradox: Hummingbird flight at high elevation does not come without a cost." Altshuler, D. L., R. Dudley, and J. A. McGuire. 2004. Proceedings of the National Academy of Sciences of the United States of America 101:17731-17736.