Anatomy of the Syrinx
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The syrinx is the y-shaped dual voicebox of songbirds. Song control involves both theĀ intrinsic and extrinsic muscles of the syrinx. Some songbirds can control each branch of the syrinx independently.

Chris Tenney recorded the Swainson's thrush near Carmel River, and I used his dawn chorus recording from a nearby location.

I highly recommend watching one or more of the online multimedia animations of the workings of the syrinx: An older animation is from the Suthers Lab Animations of the Syrinx. A newer set of animations and explanations of the syrinx comes from the Cornell Ornithology Lab's "All About Bird Song".