Hummingbirds All A-flutter During Courtship

Though famous for their mid-air hovering during hunting, tiny hummingbirds have another trait that is literally telltale: males of some hummingbird species generate loud sounds with their tail feathers while courting females. Now, for the first time, the cause of these sounds has been identified: a paper published in the Sept. 9, 2011 issue of Science by Christopher Clark of Yale University reveals that air flowing past the tail feathers of a male hummingbird makes his tail feathers flutter and thereby generate fluttering sounds. Male hummingbirds only produce fluttering sounds during their elaborate courtship rituals. Typically, during such a display, a male hummingbird will climb into the air five to 40 meters, and then quickly dive-bomb down past a perched female; when the courting male bird reaches the lowest point of his dive, he rapidly spreads and then closes his tail feathers. This spreading exposes the tail features to air, which causes them to flutter and generate sound. Clark's research shows that the males of each hummingbird species have their own signature sound--largely determined by whether and how the fluttering frequencies of its different tail feathers interact with one another and blend together. For example, the fluttering of one of a hummingbird's tail feathers may cause a neighboring feather to flutter, similar to the way that the vibrations of one tuning fork may cause another nearby tuning fork to similarly vibrate.

When interacting in this way, a hummingbird's neighboring fluttering tail feathers produce a sound that is louder--usually about 12 decibels louder--than would the two tail feathers fluttering independently of one another. What's more, neighboring feathers that flutter at the same frequency produce a different tonal sound than neighboring tail feathers that flutter at different frequencies.

Other factors, such as the size, shape, mass and stiffness of the hummingbird's feathers, also help determine the tone of each species' particular sound. "The sounds that hummingbird feathers can make are more varied than I expected," said Clark.

When diving during their courtship dances, male hummingbirds exceed their usual flight speeds--with birds of at least some hummingbird species reaching their possible maximum speeds during dives. Also, the faster a male hummingbird dives, the louder his fluttering sounds become.