University of California-Berkeley graduate student Christopher Clark and former grad student Teresa Feo used a high-speed camera to record the "dive-bomber" characteristic of the Anna's hummingbird (Calypte anna).
The researchers discovered the chirp a male bird makes at the bottom of his dive coincides with a 60 millisecond -- faster than the blink of an eye -- spreading of his tail feathers. Wind tunnel tests confirmed the split-second tail spread produces a loud, brief burst that sounds like a chirp or beep.
"This is a new mechanism for sound production in birds," said Clark. "The Anna's hummingbird is the only hummingbird for which we know all the details but there are a number of other species with similarly shaped tail feathers that may use their tail morphology in producing sounds."
Clark's adviser, Professor Robert Dudley, said the phenomenon illustrates the strength of the evolutionary process, and sexual selection in particular, to derive novel functions from pre-existing structures.
The research is reported in the online edition of the Proceedings of the Royal Society B.