Songbirds are distinguished from other perching birds by certain anatomical characteristics, especially the more complicated vocal organ, or syrinx.
Syringeal muscles are classified into two groups: extrinsic muscles, which connect the syrinx with other parts of the anatomy, and intrinsic muscles, which extend from one part of the syrinx to another.
The passerine syrinx exists in four basic types: Unspecialized: relatively little modification of the tracheobronchial region; few, if any, cartilaginous specializations, and no intrinsic muscles; found in broadbills (Eurylaimidae), pittas (Pittidae), New Zealand wrens, asities (Philepittidae), plantcutters, most cotingas, and a few manakins and tyrant flycatchers.
Important examples, in addition to those of the syrinx, are the muscle complexes controlling the tongue, the jaws, the wings and pectoral girdle, and the legs and pelvic girdle.
The presence or absence of certain thigh muscles is considered, as are the arrangement of the carotid arteries, the syrinx, and the deep flexor tendons of the toes as well as the condition of the young when hatched.
The syrinx is lacking in the New World vultures (Cathartidae), which can only hiss and grunt, but reaches great complexity in the songbirds, in which it consists of paired specialized cartilages and membranes (the inner, or medial, walls of the bronchi), controlled by as many as six pairs of minute muscles.
Differing from the mammalian larynx in both location and structure, the syrinx consists of a resonating chamber at the lower end of the windpipe (trachea), with associated membranes, cartilages, and muscles.
The syrinx is an intricately constructed organ with a firm bony framework and filmlike vibrating internal membranes over which the air, during exhalation, passes rapidly, producing all the many utterances of the bird.
Syringomyelia, chronic, progressive disease characterized principally by the development of a cyst, called a syrinx, near the spinal cord or brain stem.
