The syrinx (from Ancient Greek σῦριγξ (súrinx) 'pan pipes') is the vocal organ of birds. Located at the base of a bird's trachea, it produces sounds without the vocal folds of mammals. The sound is produced by vibrations of some or all of the membrana tympaniformis (the walls of the syrinx) and the pessulus, caused by air flowing through the syrinx. This sets up a self-oscillating system that modulates the airflow creating the sound. The muscles modulate the sound shape by changing the tension of the membranes and the bronchial openings. The syrinx enables some species of birds (such as parrots, crows, and mynas) to mimic human speech. Unlike the larynx in mammals, the syrinx is located where the trachea forks into the lungs. Thus, lateralization is possible, with muscles on the left and right branch modulating vibrations independently so that some songbirds can produce more than one sound at a time. Some species of birds, such as New World vultures, lack a syrinx and communicate through throaty hisses. Birds do have a larynx, but unlike in mammals, it does not vocalize. The position of the syrinx, structure and musculature varies widely across bird groups. In some groups the syrinx covers the lower end of the trachea and the upper parts of the bronchi in which case the syrinx is said to be tracheobronchial, the most frequent form and the one found in all songbirds. The syrinx may be restricted to the bronchi as in some non-passerines, notably the owls, cuckoos and nightjars. The syrinx may also be restricted to the trachea and this is found in a very small number of bird groups that are sometimes known as Tracheophonae, a subset of the suboscine passeriformes that includes Furnariidae (ovenbirds), Dendrocolaptidae (woodcreepers), Formicariidae (ground antbirds), Thamnophilidae (typical antbirds), Rhinocryptidae (tapaculos), and Conopophagidae (gnateaters). The trachea are covered in partly ossified rings known as tracheal rings. Tracheal rings tend to be complete, while the bronchial rings are C-shaped and the unossified part has smooth muscles running along them. The trachea are usual circular or oval in cross section in most birds but are flattened in ibises. The trachea is simple and tubular in ducks. The last few tracheal rings and the first few bronchial rings may fuse to form what is called the tympanic box. At the base of the trachea and at the joint of the bronchi a median dorsoventral structure, the pessulus, may be developed to varying extents. The pessulus is bony in passerines and provides attachment to membranes, anteriorly to the semilunar membranes. The membrane that forms part of the first three bronchial rings is responsible for vibrating and producing the sound in most passerines. These membranes may also be attached to the pessulus. In some species like the hill-myna, Gracula religiosa, there is wide gap between the second and third bronchial semirings where large muscles are attached, allowing the inner diameter to be varied widely. Other muscles are also involved in syringeal control, these can be intrinsic or extrinsic depending on whether they are within the syrinx or attached externally. The extrinsic muscles include the sternotrachealis from the sternum.
Evolution of the syrinx
An evolutionary timeline Within the avian stem lineage, the transition from a larynx-based sound source to a tracheobronchial syrinx occurred within Dinosauria, at or before the origin of Aves. The earliest fossilized record of syringeal remains is from a single specimen of Vegavis iaai from the same epoch. Before this discovery, syringeal components were thought to enter the fossil record infrequently, making it difficult to determine when the shift in vocal organs occurred. An intact specimen from the late Cretaceous, however, highlights the fossilization potential of the ancestral structure and may indicate that the syrinx is a late-arising feature in avian evolution. There is uncertainty about the relationship between the larynx and syrinx during this morphological shift, but there are two predominant evolutionary possibilities: regimes unrelated to sound production could have led to the loss in vocal function of the larynx. A new structure, the syrinx, then arose after selection for acoustic function. Conversely, the larynx could have retained some vocal capabilities, though at a diminished capacity. The syrinx then evolved to supplement sound production, which would have been followed by the loss of the larynx as a sound source. The former scenario would have led to a "silent" period in the evolution of avian ancestors. The current fossil record does not provide definitive evidence for whether the function of the larynx was lost before the syrinx was gained. The fossil record does, however, provide clues for the evolutionary timeline of some syringeal elements. For example, increased mineralization at the tracheobronchial juncture is likely a late-arising feature in avian evolution. Despite new discoveries of preserved avian tracheobronchial rings from the Cenozoic, these structures have not been recovered from Mesozoic archosaurs. This might be a product of weak mineralization in the bronchi and trachea of Mesozoic archosaurs, a condition which would inhibit preservation potential. Thus, a shift towards a mineralized structure may have been preceded by many key avian adaptations, including respiratory shifts, increases in metabolic rates, and feather ornamentation.
Evolutionary causation The archosaurian shift from larynx to syrinx must have conferred a selective advantage for crown birds, but the causes for this shift remain unknown. To complicate matters, the syrinx falls into an unusual category of functional evolution: arising from ancestors with a larynx-based sound source, the syrinx contains significant functional overlap with the structure it replaced. In fact, there is no evidence that an original, simplified syrinx could produce calls with a larger frequency range or longer or louder calls than an alligator-like larynx, which would have potentially increased fitness.
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