Many
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." SEE: Vocal
Dialects;
Vocal
Functions;
Territoriality;
Sexual
Selection. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.