When we speak, the sound comes from vibrations of the vocal cords in the larynx. The sound produced is then modified by the tongue and the shape of the mouth. The human larynx, the voice box that we see as the Adam's apple, is situated at the point where the throat meets the windpipe.
In birds, the sound-producing chamber is lower down, where the windpipe branches into the two bronchi, the tubes that lead to the lungs. The chamber, called the syrinx after the pipes of the Greek god, Pan, contains thin, elastic membranes which vibrate as air passes over them. The tension in the membranes is altered by special muscles to regulate the nature and pitch of the sound. The structure of the syrinx varies enormously from one group of birds to another. The American turkey vulture and its relatives have no syrinx at all, and can only produce grunts and hisses.
The organ reaches its greatest complexity in the so-called 'songbirds' where it is controlled by as many as six pairs of tiny muscles. A remarkable feature of birdsong is that the two halves of the syrinx, each receiving air from one of the lungs, can act independently. This allows many species of songbird to produce two notes, and even two tunes, simultaneously Recordings of the North American brown thrasher, a member of the mockingbird family, have revealed that the bird somehow manages to produce no less than four different sounds at the same time.