Credit: Arturo Naum

The giant hummingbird, Patagona gigas, native to the Andes, is an oversized cousin of the Anna’s hummingbird beloved in North American backyards. It weighs about 20 grams, as much as a toothbrush, which is twice as much as the next largest of more than 300 hummingbird species. Dr. Robert Dudley and María José Fernández of UC Berkeley’s integrative biology department analyzed the flight mechanics and metabolism of Patagona gigas in search of an explanation for its extreme body size. “Muscle efficiency in general tends to be greater for larger things,” said Dudley, who wondered whether Patagona gigas evolved to take advantage of more effective energy use in the high elevation of the Chilean Altiplano. However, the giant hummingbird’s metabolic rate relative to its size correlates with data Dudley’s group collected for a number of smaller species. So if the giant hummingbird doesn’t expend more energy to sustain itself in flight, why haven’t hummingbirds evolved to grow even larger? Dudley cites the example of the nectar feeding bat, another vertebrate species that hovers, and can weigh up to 40 grams. “The wing design is very different,” says Dudley. “Bat wings connect to the hind legs, so they’ve got about twice the wing area. Depending on the aerodynamic mechanisms involved, the motor may be irrelevant: it’s your ability to convert that to aerodynamic force.” He speculates that the counterbalance between increasing wingspan and decreasing wingbeat frequency may place an upper limit on body size, which is represented by Patagona gigas. Though the results show that the hummingbird’s characteristically high metabolism is unaffected by size, it’ll be a while on the evolutionary timeline before we see birds as big as falcons hovering at our feeders.

This article is part of the Fall 2011 issue.