We think of birdcalls as whistling, and when we whistle, the sound is made in our mouths. But birds don't whistle, they sing, and the sound is produced deep in the bird's body in an organ only birds possess, the syrinx.
The syrinx puts our voice box - or larynx - to shame. Both the syrinx and the larynx contain elastic membranes, we call them vocal cords, controlled by muscles. But the syrinx is a far more efficient sound-producer. When we speak, we only use about ten percent of the air passing our vocal cords. The syrinx uses almost a hundred percent of the air passing through it, so even a small songbird can make a big sound.
Our larynx sits at the top of the trachea, in our throats. The trachea forks at the bottom like an upside-down Y, with a branch going to each lung. That fork is where the bird's syrinx is located. The syrinx has two sets of membranes and muscles, and the two sides are independently controlled, allowing for complex vocalizations. This is almost like having two voice boxes in one throat, and enables birds like the varied thrush to sing two different notes at the same time.
Turkey vultures and ostriches have no syringeal muscles and can only hiss. But versatile vocalists like songbirds have as many as nine pairs of syringeal muscles, which enables a small bird like the winter wren to sing complex songs, packing hundreds of notes into just a few seconds.