[[File:Syrinx.jpg|right|framed|Schematic drawing of an avian syrinx<br />
1: last free cartilaginous tracheal ring,
2: Trachea
3: first group of syringeal rings,
4: pessulus,
5: membrana tympaniformis lateralis,
6: membrana tympaniformis medialis,
7: second group of syringeal rings,
8: main bronchus,
9: bronchial cartilage]]
thumb|220px|Syrinx (serial 5) seen just below the [[Crop (anatomy)|crop]]
The syrinx () 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.
<gallery>
File:Syrinx Bucerotidae.jpg|The syrinx of hornbills
File:Leptosomus syrinx.jpg|The cuckoo roller
File:Struthio syrinx.jpg|The ostrich
File:Syrinx forms.jpg|Suboscines and a shoebill
</gallery>
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.
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. In bird-lineage archosaurs with bifurcated airways, the evolution of an increased metabolic rate and continuous breathing exposed airway walls to altered amounts of wall shear stress, a measure of friction between a fluid and a vessel wall. In continuous breathers, such as birds and mammals, the trachea is exposed to fluctuations of wall shear stress during inspiration and expiration. In simulations with a simplified airway conducted by Kingsley et al. (2018), fluctuations in flow patterns led to localized wall shear stress, with the highest stress during exhalation at the tracheobronchial juncture. Localized stress may have provided selective pressure for an airway support located at the tracheobronchial juncture to maintain airway patency. Understanding whether these forces would have favored the evolution of soft tissue or cartilage requires further experimentation. Coupled with respiratory shifts, these characteristics may have favored syrinx evolution in birds. Distinct airway geometries in Mammalia and Archosauria may have also impacted syrinx evolution: the bronchi in crocodiles and humans, for example, diverge at different angles. Because of this, vibratory tissue precursors must have, at most, briefly predated the attachment of the first muscles to the trachea to clear the airway for respiratory function.
Further fossil data and taxonomical comparisons will be necessary to determine whether structural modifications of the syrinx unrelated to sound, such as respiratory support during continuous breathing or in flight, were exapted in the development of a vocal organ. Using physical and computational models, Riede et al. discovered that because of the dynamics between inertance and tracheal length, a structure in the syringeal position can be significantly more efficient than a structure in the laryngeal position. It is possible that during these changes, certain co Efficiency permits more "dead space" within the avian trachea, allowing the trachea to lengthen without a subsequent decrease in tracheal diameter. With a longer trachea, the avian vocal system shifted to a range in which an overlap between fundamental frequency and first tracheal resonance was possible. Without the critical tracheal length, mammals were unable to achieve an ideal length-frequency tracheal combination. At this point in avian evolution, it may have become advantageous to move the vocal structure upstream to the syringeal position, near the tracheobronchial juncture.
Selection for long necks, while highly variable, is often driven by beneficial feeding adaptations. Specifically, long necks facilitate underwater predation, evident in, for example, swans and cormorants. Longer necks likely predisposed Aves for syrinx evolution. Because of the correlation between neck length and tracheal length, birds are considered to have an "acoustically long trachea." Technically, this refers to a tube where the lowest resonant frequency of a vibrating object (i.e. the syrinx) is four times longer than the length of the tube. A shorter tube would be less efficient; a longer tube would cause wave-form skewing. In most mammalian species and their therapsid ancestors, tracheal length was not sufficient to facilitate a boost in vocal efficiency. For example, polygynous birds with leklike mating systems have evolved to use louder sounds and a wider range of frequencies during displays; wood warblers with higher trill performance have higher fitness. While the specific acoustic advantage of the ancestral syrinx remains speculative, it is evident from modern avian diversification that sexual selection often drives vocal evolution.
Sexual dimorphism
Sexual dimorphism leads to different syrinxes in birds, and the degree of differences varies. Some species do not present differences between sexes while others, like the mallard (Anas platyrhynchos), have distinctly different syrinxes between males and females. This difference is significant given that sexing birds is difficult at younger stages. Birds that exhibit sexual dimorphism in the syrinx can present itself at around 10 days in Pekin ducks (Anas platyrhynchos domestica). The membranes in males are thick and nontransparent, but the females have thinner, sheer membranes.