The syrinx is the voice box for birds. It is a muscle-wrapped chamber in the windpipe, close to where it branches to the lungs on each side of the body. Sounds are produced when air passes from the lungs and vibrates membranes in the syrinx. The physical structure of a bird constrains the types of sounds which it can make. For example, small birds usually make high pitched sounds such as the "peeps" of chicks and ducklings.
Others like swans and cranes have long windpipes which resonate like bugles.
In humans, our larynx is the key sound producing voice box, and it is located relatively higher in the windpipe.
Some birds have no syrinx at all. One example is the Turkey Vulture, which is only able to utter hisses and grunts.
One evolutionary branch perfected the avian vocal apparatus to yield the master singers, the song birds. The pathway was so successful that nearly half of the living species of birds are in this sub-order of perching birds called Oscines, from Latin for "birds used in augury" (fortune telling).
Song birds achieve their vocal talents with a syrinx surrounded by 5 to 9 pairs of muscles which allow them to finely modulate their tones. In many species, the syrinx is low in the wind pipe and forms two chambers, one piped to each lung. Such birds are able to produce different pitches simultaneously or in rapid alternations.
Based on the development of fine controls over sound production, song birds evolved into thousands of species over the past thirty million years. Some songbirds have a large repertoire of songs, and others sing just one song. Some species are mimics and incorporate new sounds throughout their life times. Each innovation seems to be an indicator of a singer's health and knowledge of its environment, and females through their choice in mates select the best available male. Fundamentally, singing advertises territories, attracts mates, and strengthens pair bonds. But, birds also sing to practice and quite possibly as a release, a form of avian play.