Nearly all birds produce sound through an organ unique to birds, the syrinx. In many songbirds, the syrinx is not much bigger than a raindrop. Extremely efficient, it uses nearly all the air that passes through it. By contrast, a human creates sound using only 2% of the air exhaled through the larynx. Birds whose syrinx is controlled by only one set of muscles have a limited vocal range. This Song Sparrow, using several pairs, can put forth a cascade of trills and notes.

More info and transcript at BirdNote.org. 

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