Tests hint bird tails are misunderstood

In the hotly debated matter of bird-tail aerodynamics, the first wind tunnel measurements indicate that the prevailing theory may be wrong.

The basic avian tail has the triangular shape of supersonic planes like the Concorde, explains Jeremy Rayner at the University of Leeds in England. Since the early 1990s, dozens of ornithology papers have therefore drawn on the mathematics of these delta-wing aircraft. Biologists have argued over how much of the shape of a bird’s tail comes from aerodynamics and how much from males’ need to bedazzle females.

Rayner and his colleagues recently worked out a way to test avian delta-wing theory in a wind tunnel. Their data agree with the predictions of lift from an isolated tail, says Rayner. However, with the body attached, the tail’s lift doesn’t follow the theory, Rayner and his colleagues report in the July 22 Proceedings of the Royal Society of London B.

Another specialist in avian aerodynamics, Matthew Evans of the University of Stirling in Scotland, pronounces the test results “the first nail in the coffin” for bird aeronautics based on delta-wing models. “What’s going to happen is that people who used this theory are going to rethink their interpretations of their data,” he says.

Using air speeds typical of slow flight, Rayner’s group tested plastic models of bird tails and real tails of starlings. So far, so good. To see how the body affects the results, the team looked at a frozen starling body with a tail but no wings. The theory predicts that as the tail spreads, lift increases sharply. It didn’t, however.

The study of bird tails “is very polarized,” Rayner says. One camp of theorists emphasizes the importance of ordinary physics, and the other camp points to the extraordinary tails of male peacocks and barn swallows as examples of sexy fashions overpowering sensible aerodynamics. Now, Rayner adds, “the theoretical framework of the debate is wrong.”

In the same series of wind tunnel experiments, the researchers also found what they call “a hitherto unsuspected role” for a bird’s tail. At typical flight speeds, a folded tail decreases the bird’s drag by one-quarter to one-half. Will J. Maybury, now of Westland Helicopters in Yeovil, England, and Rayner report that finding in the July 7 Proceedings of the Royal Society of London B.

That idea sounds sensible, says Evans. Engineers put structures that act as drag reducers on both cars and planes, he points out.

He suggests that bird tails may offer trade-offs among different flight styles. Streamers that he added to bank martins cut their performance on straight flight but improved maneuverability. “You would think we would understand how birds fly,” he adds, “but we don’t.”