photographs of White-crowned Sparrow (top), Bewick's Wren middle) and Red-winged Blackbird (bottom) by Rohan Kamath
practice, most birders learn to identify many species by
their characteristic calls and songs. What are the
underlying mechanisms that lead to "standardized"
repertoires for each species? Are these characteristic
sounds learned or are they genetically determined and fixed
without learning (innate)? Where learning is involved, when
does it take place? And how does each species "know" which
sounds are appropriate and should be learned? The answers to
these questions are as varied as birds themselves and have
long served as a focus of research by ornithologists,
ethologists, and neurobiologists.
Most of this research has been concerned with song development in species of songbirds, but relatively few species have been examined in detail. Most songbirds must learn at least part of their song repertoire. What little we know of vocal development in nonpasserines indicates that calls of those species (Mallard, American Coot, Ring-billed and Franklin's Gulls, domestic chicken, and Ringed Turtledove) are innate rather than learned, and that precocial young tend to have larger, better-developed repertoires of calls than do altricial young. Two exceptions among nonpasserines are hummingbirds and parrots, for which there is some evidence of vocal learning (there also is suggestive evidence for Greater Prairie-Chicken and Sharp-tailed Grouse). Studies of many groups have yet to be done.
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. Swamp Sparrows, for example, generate four to five times more song material during development than they eventually retain in the adult repertoire.
The most thorough studies of song development, pioneered by ethologists W H. Thorpe and
involve detailed experimental procedures using
birds that have been isolated in soundproof chambers as
hatchings or nestlings. The development of songs and calls
can then be followed in these birds, which have been
deprived of any chance to hear the normal song of their
species. In most species that have been examined, the
resulting songs bear only a slight resemblance to normal
songs, and are not recognized by others of the same species.
Isolated birds allowed to hear the singing attempts of other
isolates form better, but still imperfect, songs, whereas
isolates allowed to hear normal song during their "sensitive
period" (the limited period of time during which song
learning can take place) develop normal songs.
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.
Studies of song learning have led to the "auditory template hypothesis" -- the idea that each species is born with a neurological model of what its song should sound like and it develops that song by matching sounds that it hears with the template in its brain. This process enables a young bird to filter out inappropriate sounds and to produce sounds matching the template, which are then stored for future use when breeding the following spring. Swamp Sparrows are able to store and precisely reproduce song syllables without rehearsal after as long as nine months.
The template model is exemplified by studies of young White-crowned Sparrows, which manifest a sensitive period for song acquisition roughly from day 10 through day 50 after hatching. Around day 150 they begin to practice what they have learned, and by day 200 they have developed a stable "crystallized" song that matches parts of the song heard during their sensitive period. 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.