Birds have been kept captive for thousands of years because of the uplifting and musical sounds that they make. Of course, for every bird with an attractive song there are other species that make a raucous noise, and in this section we will look at the reasons that birds 'sing'. The songs and calls of birds are produced by air being forced across membranes in the walls of the syrinx or voice box, which is situated at the junction of the trachea and the bronchi which branch off to each lung. The membranes vibrate, and can be varied in tension to give notes of different pitch. Songs and calls are two different types of sound made by birds.
Songs are more complex and are mostly uttered by males.
Some birds, like jackdaws, crows and jays use song to transmit information to others of the same species. Others, for example the parrots and mockingbirds, are able to mimic a variety of sounds, from the ring of a telephone to the human voice. Birds that live in dense forest habitat, like the parrots, seem to have lower voices than birds that live in more open environments. Possibly the lower frequencies help the sound to carry through the vegetation. Migration One of the marvels of the world of birds is the phenomenon of migration.