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Data source: NASA Earth Observatory and Xeno-Canto
While humans appreciate their songs as enjoyable sonic flourishes, birds sing to accomplish some vital goals, like finding mates and asserting claims over their home territory. So it is no surprise that they’re good at figuring out how to tune their songs so they have maximum effect. After all, when sex and survival are on the line, you want to make sure your voice is heard.
Clinton Francis, a biologist at California Polytechnic State University, studies how birds vary their songs. He says the “acoustic adaptation hypothesis” explains how animals who use vocalizations over long distances, like birds and frogs, alter their calls to maximize reproductive success. In other words, they sing the song that potential mates can hear best.
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Francis says that in addition to all this racket, another likely factor affecting song pitch is vegetation density. Low-pitch, or low-frequency, sounds travel much better through dense trees and plants than do high-pitch sounds. High sounds tend to scatter when they hit tree leaves or needles, and they end up going unheard.
On the U.S. map above, the darker areas indicate denser, taller, and more robust tree coverage. “In deciduous forests of the East,” says Francis, “some species may have lower-pitched vocalizations compared to those in the more open spaces of the West.”
Take a listen for yourself.