Based on these metabolic measurements, Clark and Dudley predicted that long-tailed birds would show a 3 to 4 percent decrease in maximum flight speed, which is what they found: 3.4 percent, on average.
As an extra control, Clark tested the flight abilities of birds without rectrices and of birds without either rectrices or covert feathers (the short feathers that cover the bases of the rectrices, both above and below). Birds lacking both rectrices and covert feathers showed a 2 percent drop in maximum air speed, but those lacking only rectrices were unaffected.
Given that elongated tails in birds have evolved at least 26 times, if not hundreds of times, it seems as if tail feather variation is easier to live with than, for example, wing feather elongation, which would more likely affect flight and is rare in nature, Clark said. Tail feathers streamline the bird's body and reduce drag, but when folded in flight, the length of tailfeathers appears to be less important than having at least some tail feathers.
"We propose ... that sexual selection has generated enormous diversity in avian tail morphology because, by 'hiding' in the wake of the body, such modifications can be relatively cost free," they wrote in their paper.
Although most biologists think that extreme ornamentation, like long tails, arise because females select males based on that trait, there is still much debate about the evolutionary details. Some suggest that females have an innate bias that is exploited by males to win over females. Others suggest that long feathers are a good indication of a healthy male, and thus stand out like "tall, dark and handsome" in human
|Contact: Robert Sanders|
University of California - Berkeley