Songbird Signalling Behavior: Duetting
Biology 342 Fall 2010
Avian vocalization is a complex of many anatomical structures that must function in tandem with one another to effectively and efficiently foster birdsong.
(Image courtesy of http://speakbeakblog.blogspot.com/2008/08/remarkable-sounds-of-birds.html)
Although the exact mechanism of avian vocalization is heavily debated, vocalization is essentially rooted in the flow of air past the elastic membrane of the syrinx and rapid changes in tension of the membrane determines pitch(Nottebohm). The syrinx (displayed above) itself is a bilateral structure surround by specialized muscles located at the junction of the bronchi and trachea (Doupe and Kuhl). The most widely accepted model of sound production is vibration in the syrigeal structures. The vibrations are caused by partial constriction of syrigeal lumen which leads to high velocity air flow, which generates suction in tandem with the opposing force of the viscoelastic forces (Larsen and Galler). This model, although widely accepted, does not account for the varying tones found in Passeriformes (song birds). Another theory proposed about the underlying mechanism of the syrinx is the whistle hypothesis. It states that the syrinx operates like a hole tone whistle, which means that it forms a rigid boundary, and sound is generated by self-sustained vortex from constriction (Larsen and Galler). After endoscopic analysis, however, the presence of vibrations debunked this hypothesis. With endoscopic analysis, researchers found during vocalization the flexible structures are adducted (medial and lateral labia in song birds) into the middle if the bronchial or tracheal lumen (Larsen and Galler). Essentially this means without the jargon) that during vocalization flexible pieces of epithelium in the throat are pulled toward the mouth while vibrating with different frequencies depending on which syllable is being produced.
(Image courtesy of wikipedia)
Like humans, songbirds have specialized area of the brain that controls vocalization. The structures responsible are found in the forebrain which uses a motor pathway consisting of a chain of nuclei including parts of the HVc (Doupe and Kuhl). This interaction can be seen in the image displayed above. The HVc or High Vocal Center is an integral structure in the learning and production of song in Passeriformes. It is located in the lateral caudal nidopallium and has direct connections to both the direct and the anterior forebrain pathways The HVc relays information to the RA or the nucleus robustus which in turn connects directly to all the nuclei involved with motor pathway (Doupe and Kuhl). The RA is located in the back of the brain or the area correlating with the occipital lobe in humans. Electrical stimulation of the HVc or RA in a silent bird will elicit vocalization. If the bird is already singing electrical simulation to the HVc will result in arrest of singing and a restart in the pattern and stimulation of the RA will result in disruption in syllable produced without altering ongoing temporal pattern (Doupe and Kuhl). Lesions to any part of the HVc or RA result in the loss of vocalization. However lesions to the left half of the HVc result in a greater loss than those of the right indicating a peripheral unilateral dominance of syringeal function (Doupe and Kuhl). After the RA has been activated the message is sent to the hypoglossus nerve. This nerve provides motor input to the syrinx and if the connections are cut in an adult with fully formed song there is a loss of song (Nottenbohm). Thus the hypoglossus nerve is responsible for muscle output of neural input.
Duetting by and large has been merely studied in terms of the organism's behavior and has not been heavily studied in terms of neurophysiology. Consequently there exist minimal research that makes any assertions about the underline mechanism involved in this behavior. Although the underline mechanistic factors contributing to bird song duetting have not been fully research, the underline mechanism may be attributed to the HVc and mirror neuron. Mirror neurons area found in the premotor cortex and are responsible and active for behaviors that require performance and observation aspects in tandem (Rizzolatti et al). This has been most notably researched in respect to motor functions but it has been shown that these neurons are also active in both motor and sensory performance. Thus they may play an integral part in duetting since a bird must both listen to the vocalizations from its mate and respond accordingly. The HVc as already stated is responsible for vocalization in respect to learning and production thus this must also play a pivotal role in duetting. What is know that species that engage in duet