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Wood thrush

But birds do not have lips to form sounds, like humans do. Watch a bird sing and its beak will open but not move like our mouths do! How are the songs created? In a bird, below the larynx or windpipe is the syrinx. This part of their anatomy vibrates to create songs.. Changes in tones and melodies are created from muscles contracting and relaxing, forcing the air through the passage differently to form multiple sounds and tones, creating the beautiful melodies that we all love. Different species of birds may have varying numbers of these sound-creating muscles, with the birds having the highest number being able to produce the most varied sounds.

Donald Kroodsma is a renowned specialist in the interpretation of bird songs. His new book, The Singing Life of Birds: The Art and Science of Listening to Birdsong, describes how birds communicate and why. But Kroodsma is also the subject of another book -- about those who listen to birds. Kroodsma says song birds sing two song at the same time one on the left and one on the right then harmonize the two into song. Super Cool. He had recording of the two different tunes and then both together

Birdsong, by Don Kroodsma, details the work and passions of people who analyze the sounds of birds.
Listened to this on NPR today, amazing stuff.
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