This gentlebird has developed a very neat ability. He can produce a high pitched chirp through the vibrations of its tail-feathers in a high speed dive. Christopher James Clark and Teresa J. Feo at Berkeley did some careful analysis to show that this was in fact not a vocalization and they speculate that this act of sonic production is most likely useful for attracting females1.
Generally, I think of evolution as being driven by fitness, but this case reminded me of how critically important sexual selection is in determining special stability and the retention of genetic traits. Peacocks present a more well-known example, their elaborate tail feathers being of a similarly minimal utility in any other (non-reproductive) aspect of their lives.
In sexual creatures, its actually fairly obvious that this would be the case, since maximal reproduction leads to maximal offspring. It is thus important to remember that what drives evolution is not simply survival, but survival and reproduction.
References
1. Clark CJ, Feo TJ. (2008) The Anna's hummingbird chirps with its tail: a new mechanism of sonation in birds. Proc Biol Sci. 22;275(1637):955-62.
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