June 6, 2013 2:03 pm
When you think of bird songs, you probably think of songs that come from their mouths. But hummingbirds have a whole different kind of music—one that comes from their feathers. As they fly, they spread their tail feathers, and the air passes through them, causing them to flutter. And that flutter, like a violin string, creates sound.
Different species of hummingbirds have differently shaped tail feathers, and thus different sounding songs. Here, researcher Christopher Clark explains how the vibrations work, and you can listen to some of the songs:
Clark has published several papers documenting hummingbird tail resonance. He has also suggested in past papers that hummingbirds might have even evolved this form of singing before they found their voices.
In his most recent paper, Clark wanted to understand a little better just how these feathers were singing. He already knew that these feathers made sounds. And he knew that they only made those sounds at particular wind speeds. What he wanted to know now was what kind of sounds did these feathers make. More specifically, do the feathers flutter to make resonant frequencies or not? In other words, does the feather act like a violin string, or is it hitting just a random frequency?
Simply put, resonant frequencies are the frequencies at which the object vibrates most easily. So, in our violin example, each string has a set of resonant frequencies that they vibrate at. (As does the body of the instrument and the air inside it. But we’ll focus on the strings.) Combined, you get certain resonant frequencies of the violin, and at those frequencies the violin will sound louder.
In this new study, Clark once again put feathers into a wind tunnel. But this time, he and his colleagues predicted the intrinsic resonant frequency of the feather, based on its shape.
Now, the feathers do have an intrinsic resonant frequency. Everything does. But Clark and his team wanted to know whether they could reach those resonant frequencies in the wind tunnel. Did the structural resonant frequencies they had predicted match the actual way the feathers fluttered?
And they did, just like a guitar or violin. In the researchers’ own words, “We conclude flutter occurs when airflow excites one or more structural resonance frequencies of a feather, most akin to a vibrating violin string.”
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