Hermit crabs

Researchers had been divided over whether the sound was vocal or not, but the latest study, published online in the Royal Society Proceedings B shows unequivocally that the tail feathers were the source of the sound. When the researchers cut back the tail feathers — which grow back in five weeks and do not affect the birds ability to fly — the males were unable to make the noise. Tests in wind tunnels confirmed that the vanes of the feathers vibrated at the right frequency to make the sound when exposed to air currents of 80 km/h. Research will now turn to other species that have tail feathers with tapered or narrow tips and that have mating dives and make different sounds. “It is possible that sexual preference by females has caused the shape of the tail feathers and, thus, the sound, to diverge, thereby driving the evolution of new species,” says Clark. Non-vocal mechanisms, such as this tail feather mechanism, “enhance the diversity of sounds birds can produce,” the authors say. “Small birds may be limited in their ability to produce loud vocal sounds by the size of their syrinx. The dive sound of the Anna’s hummingbird is much louder than its song,” they write. “This suggests that switching to feather sonations has allowed it to escape the intrinsic constraints on vocal sound volume. In the bee hummingbird clade many related species perform dives, have specific tail morphologies and make a diverse array of sounds,” the authors write. They point out that this is also true of snipes. “We predict that in these clades, dive behaviours and tail morphology have coevolved to produce a diversity of mechanical sounds.”