photographs of a Northern Mockingbird (top) and a European Starling (bottom by Rohan Kamath
species of songbirds learn the specific song elements of
their repertoires from one or more adult tutors, most often
from the male parent. Such learning, for at least some
species, is not confined to the period prior to sexual
maturity. For example, territorial male Swamp Sparrows
listen to songs from adjacent territorial males and
incorporate those songs into their own repertoire.
Generally, this type of vocal copying, where the individual
copied (the model) is a member of the same species, is
referred to as "vocal imitation" and serves as the basic
mechanism underlying the evolution of dialect systems --
variation in songs among local populations.
There are, however, many examples of vocalizations characteristic of one species being copied by a second species. Such "vocal mimicry" is well known in the Northern Mockingbird and European Starling. The function(s) of acquired alien sounds is still debated. Even the term "vocal mimicry" is a source of dispute among ornithologists. In biology, mimicry generally connotes deception by the mimic directed toward some signal-receiver, generally a predator or competitor. With most mimicked bird vocalizations, the true identity of the singer is quite clear because the mimic imparts some characteristic tonal quality, temporal pattern, or context of use that serves to differentiate it from the model's vocalizations. The human ear can detect these differences, and the model's more sensitive avian ear would certainly be expected to detect the rendition of a mimic. In short, in the vast majority of examples it is unlikely that anyone is fooled by vocal mimicry.
|Why, then, are sounds of other species (as well as nonavian sounds such as the barking of dogs, screeching of machinery, or human whistling) sometimes incorporated into a bird's repertoire? The answer seems to be that selection has favored a large and diverse repertoire in some species and that one way of increasing repertoire size and diversity is to incorporate sounds from the surrounding acoustic environment, even sounds that do not belong to the bird's own species. Evidence from several studies indicates that an expanded repertoire may improve ability to attract a mate, intimidate rivals, and stimulate females. Thus the effects of sexual selection tend to favor an increasingly large and diverse song repertoire within the limits imposed by the need for species recognition and by the capacity of the singer to memorize sounds. The common, nondeceptive use of such vocalizations has been termed "vocal appropriation" to eliminate the connotation of deceit implicit in the biological use of the term "mimicry."|
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.