In his field laboratory, Althshuler put the hummers through their paces. After weighing and measuring wing size, he filmed them hovering inside a Plexiglas cube to obtain wing beat frequency and stroke amplitude, from which he could calculate the power exerted in normal hovering.

The second experiment, however, measured the maximum power the birds' muscles were capable of. In a setup originally developed by Dudley and former post-doc Peng Chair, he attached a string of evenly spaced, color-coded beads to the bird's body and filmed the bird as it tried to lift the string off the floor of the cage.

"When you release them from the floor of the chamber, they fly up to escape, lifting progressively more weight," Altshuler explained. "This gives us in a single test their maximum lift, because they go as high as they can trying to escape. And then they tend to hover briefly, a second or less, and at that moment, we get a snapshot of them with their muscles working as hard as they possible can."

With full data from 347 of nearly 1,000 captured hummingbirds, a sample representing 43 species, Altshuler was able to show that, whereas body mass and power output increase with elevation in hummingbirds, the power margin goes down.

"Higher elevations are an appealing niche, and hummingbirds have headed up there, but there are challenges," Altshuler said.

The other important aspect of the paper, a hummingbird family tree encompassing three times the number of species as earlier genealogies, showed a few surprises, too, said McGuire, a specialist in phylogenetic analysis of animal groups. While much of the tree confirms earlier results, the world's largest hummingbird, the giant hummingbird (Patagona gigas), stood out on its own, perhaps indicating that it has taken its own evolutionary path away from the other hummingbirds. It grows as large as 26
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Contact: Robert Sanders
rsanders@berkeley.edu
510-643-6998
University of California - Berkeley
16-Dec-2004