Double-tones in Song

I must thank Jeffrey Low for referring me to an article that explains how some birds can produce 2 voices/simultaneous double tones.

The artilce can be read at:

http://www.stanford.edu/group/stanfordbirds/text/essays/Bird_Voices.html

A passage from the above article states:

Quote:

Neurobiologist Fernando Nottebohm has shown that the two sides of the syrinx are independently controlled, which explains the "two-voice" phenomenon seen in sonograms of some species: simultaneous double tones that are nonharmonically related and therefore must be derived from two independent acoustic sources.

Also at the Stanford site is an article on Vocal Development. The full article is at:

http://www.stanford.edu/group/stanfordbirds/text/essays/Vocal_Development.html

Below are extracts from the article:

Quote:

........ The learning of songs is a gradual process that takes place over a period of weeks or months. Typically, a vague, jumbled "subsong" appears first which then gradually is transformed into a more structured, but still quite variable, "plastic song." The end point of this process is the production of a stable repertoire of "crystallized" songs. Much more material may be developed than is actually needed for the eventual crystallized repertoire, leading to a process of attrition as the mature song takes shape. ...... The social bonds to the song tutor (usually the male parent) have been shown to be important in determining which vocalizations are copied by young birds. In addition, territorial males appear to also copy song characteristics of surrounding territorial males, indicating that males of some species may have the ability to expand their repertoires and replace song components each breeding season. ..............Song learning is selective, so that if offered a choice, birds will learn their own species' song. If offered only songs of other species or if reared in isolation, learning does not occur and only a simplified approximation to the normal song develops. The importance of auditory feedback is shown by birds that have been experimentally deafened after exposure to normal song during the sensitive period but prior to day 150. Such birds fail to develop anything melodic. In contrast, birds deafened after their own song had crystallized will continue to sing normally. In essence, vocal learning appears to consist of two phases: (1) exposure to and memorization of species-specific sounds by matching them to the template during a sensitive period, and (2) production of those sounds. The duration and timing of the sensitive period varies among species. Bewick's Wrens, Song and Swamp Sparrows, and meadowlarks acquire their songs during the first few months following hatching. In contrast, Northern Mockingbirds, Indigo Buntings, and Red-winged Blackbirds continue to incorporate new songs and song elements into their repertoires beyond their first breeding season. As additional field studies are conducted on individually identifiable birds over several seasons, many more species may be added to the latter category. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.