Ask a songbird: how does it change tunes?

Songbirds may change tunes with ups and downs, and cresendos and decresendos to make a elegant sound to attract mates. But how do they do it? First off, the sound is produced from the syrinx, which act like human vocal cords. The sound is produced when air passes through the syrinx, folds of tissue resonates, making the beutiful sound you hear when you take a walk. Before, they thought that a bird's upper vocal tract was like a stiff tube; the sound depends on how wide the beak is opened. X-ray movies show how a northern cardinal change sound by rapidly changing the shape of the vocal tract from a drinking straw like shape to a flower pot like shape. The point of the vocal tract is to shape the monotone call from the syrinx and add some color to the call. So when you go out for a walk in the park, the songbird may have a vocal tract changing to make the song for you!