Hovering iridescence

By Jeff Mitton

As I walked across the meadow I heard the familiar buzz of hummingbird wings. I stopped when a female hummingbird hovered two feet in front of my face, peering at me. More precisely, she was evaluating my dusty red ball cap. Hummingbirds are most attracted to red flowers, and this bird had gotten its hopes up. She was not the first creature that I have disappointed.

Broad-tailed hummingbirds, Selasphorus platycercus, are sexually dimorphic. Both males and females wear iridescent green on their backs and heads and white on their chests. Their wings and tails are dark. A male is easily identified by his iridescent red gorget, or throat patch, in contrast to the small tan dots on a female`s throat.

Sexual dimorphism goes beyond feather color to the sound of feathers in flight. Males attract females by flying high and then plunging in a high-speed dive. At high speeds and wing-beat frequencies, males` feathers make a distinctive trill.

When the flight display has the intended effect on a female, she invites the male to mate. But immediately afterward she goes off to make a nest, lay two eggs and raise offspring while the male continues his flight display, hoping to attract another mate.

The earliest fossils that are unambiguously hummingbirds were formed in Europe 30 million years ago. Some time earlier hummingbirds diverged from swifts, their closest relatives. Approximately 320 species of hummingbirds are extant today.

Hummingbirds are the only group of birds that can hover. Unlike in other birds, hummingbird wings are attached to the shoulder with a ball and socket joint so that the wings can rotate 180 degrees, rather than just flapping up and down.

While hovering, the body is held erect and each wing carves a figure eight through the air, generating lift on both the downstroke and upstroke (or forward stroke and backstroke). The unusual wing movement also allows hummingbirds to fly straight up, straight down, and backward. They are a joy to watch.

Hovering and zipping from flower to flower is energetically expensive, for small hummingbirds beat their wings approximately 50 times per second and up to twice that fast when in courtship. Their heartbeat has been measured at more than 1,000 beats per minute.

As a consequence, hummingbirds in flight have the highest metabolic rates of any animals except insects. Broad-tailed hummingbirds are on a tight energy budget, and although they may drink their weight in nectar each day and eat all of the small insects that they find, they are usually just a few hours from starvation.

Tiny birds lose heat quickly in the cold. To reduce their energy expenditure during chilly nights, hummingbirds sink into torpor, slowing their heartbeats and allowing their body temperature to drop from 105 degrees Fahrenheit to below 60.

Hummingbirds could not survive a true winter night in the mountains. They are seasonal migrants, breeding in the Great Basin and southern Rocky Mountains during the summer and migrating to southern Mexico and Central America for the winter. Some females return to the same nest site year after year.

The initial use of my red cap to meet hummingbirds was inadvertent, but now I wear it most of the time. It attracts good company.

Jeff Mitton, mitton@colorado.edu, is chairman of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.