Birds' Long Tails No Hindrance to Flight

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By adding feather extensions to hummingbirds, UC Berkeley scientists found that long, alluring tails on some species do not do much to slow the birds down.

Scientists had originally thought the tails, which can grow to more than twice the length of the birds' bodies, would weigh birds down and require them to use more energy when they are flying. Only male hummingbirds grow the tails, and birds with longer tail feathers are more attractive to female birds in the wild.

Instead, the study, published in last week's online edition of Proceedings of the Royal Society B, showed that hummingbirds with longer tails do not exert much more energy in flight than their shorter-tailed counterparts.

"In general, female birds prefer males with a longer tail, but also, it has been assumed that there is a larger cost for the long tail, and eventually the cost will outweigh the benefits," said Robert Dudley, a UC Berkeley professor of integrative biology who co-authored the study. "This is the first study to measure the cost directly."

To measure the energy cost, scientists attached the long tail feathers of red-billed streamertail hummingbirds onto short-tailed campus hummingbirds and put them in a wind tunnel to see how their breathing rates changed.

Researchers found that the hummingbirds' metabolic rates decreased only slightly, indicating a marginally higher energy expenditure.

In another experiment, researchers found that the hummingbirds' long tails only reduced their maximum flight speed by about three percent.

"The wind tunnel is sort of like a treadmill," said Chris Clark, a UC Berkeley graduate student of integrative biology who co-authored the study. "The bird has to fly to stay at the same place. So you can imagine running on the treadmill and having the speed of the treadmill slowly increase, but in this case, the bird flies faster and faster."

Scientists found the birds fold their tails behind them when flying at high speeds, minimizing wind resistance.

"The tail is hiding in the wake of the body," Dudley said. "The presence of the body obstructs the presence of the tail, so the aerodynamic forces on it are much reduced."

The tails of hummingbirds have required a great deal of fine-tuning over time. Scientists said that hummingbirds' tails have evolved about 26 times in the history of the species, but not much had been known about the effects of having longer tails prior to the experiments.

"Tails are probably the easiest part of the morphology for birds to change. This is a step toward thinking about these characteristics as being not very costly," Clark said. "People used to think flying with a tail like this would be like running around with a giant parachute behind you-it would slow you down a lot. I think it is acting like a parachute, but a really small one."


Christine Chen covers research and ideas. Contact her at [email protected]

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