Two Yale scientists have discovered how hummingbirds hum.

In collaboration with Yale biology professor Richard Prum and University of California, Berkeley professor Damian Elias, biology post-doctoral associate Chris Clark has determined that the sound produced by diving hummingbirds in mating rituals is caused by tail feather vibrations. Researchers in the field praised the project for its ingenuity, and said they believe that it could illuminate new aspects of evolutionary biology.

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“Understanding how animals use signals, how signals evolve, and the how communication develops is fundamental to understanding evolution, ecology and developing conservation strategies,” Elias said.

The study, published in the Sept. 9 edition of the journal Science, showed that vibrations of a hummingbird’s tail feathers produce a sound specific to the species, which comprise the family Trochilidae. The researchers found that different arrangements of tail feathers produce diverse sound patterns, and the noises emitted are created by the interactions between feathers.

Although the sounds were originally thought to be vocalizations, research conducted at University of California, Berkeley in 2008 by Clark and Teresa Feo GRD ’15, confirmed that the distinctive chirps produced by male hummingbirds were due to tail feather movement. Clark furthered this research in the more recent study by investigating the diversity of the sounds produced, as well as the prevalence of this phenomenon in different hummingbird mating rituals.

Ornithology experts said the implications of the Yale study are profound.

“It was beyond my personal expectations that Clark’s extended studies would reveal not only how common sound production by tail vibration is among males of different hummingbird species, but that the vibration modes themselves are quite varied,” said H. Ross Hawkins, who is the founder and executive director of The Hummingbird Society.

The research conducted by Clark, Elias and Prum also investigated an aspect of biomechanics that is often overlooked, Mount Holyoke College professor Gary Gillis said. Gillis said most people automatically think of wings when considering the flight of hummingbirds, but the study shows that other aspects of flight, such as specific types of feathers, must also be considered.

The research team collected their preliminary data at University of California, Berkeley before examining the feathers in a wind tunnel at Yale. But Clark noted that the project was not without its difficulties. Background noise had to be considered when comparing the signals produced in the wind tunnel with those produced naturally by the birds, and sometimes the wrong conditions were selected for experimentation, Clark said.

“Research is always a compromise,” Clark said.

Compromises or not, experts said that Clark, Elias and Prum’s research opens up opportunities for research into areas of study never before explored.

“Clark’s studies are trailblazing in nature and can be expected to lead others to broaden and deepen the well of knowledge that he has discovered,” Hawkins said.

Clark will travel to Ecuador in the coming months to investigate hummingbirds found only in South America.