When it comes to finding and identifying birds, sound can be as important as appearance. Learning to recognize bird sounds will increase your enjoyment of birding enormously. Birds make a variety of sounds with a variety of purposes. All bird vocalizations are produced by the syrinx, an organ located in the trachea just above the branching of the bronchi leading to the lungs. As a rule, the more developed the syrinx, the more complex the sounds it can produce.
Songbirds generally have highly developed syrinxes and thus produce the most complex song.
Interestingly, because the song is produced entirely within the syrinx, many birds can sing with their mouths full or even closed. Every songbird species has a distinctly different song, full of rich harmonies. Birds outside the songbird family also produce distinctly different calls and other sounds that are an aid to identifying the species and to determining where the bird is and what it is doing. For example, it is visually difficult to tell the difference between a lesser and greater yellowlegs unless two birds are near each other for a size comparison. However, the voices of the two species are quite different and easily distinguished.