Just
as our speech patterns vary regionally, the songs of many
avian species also show geographic variation. For example,
in states and provinces east of the Mississippi, the songs
of Rufous-sided Towhees consist of two introductory notes
followed by a buzzy trill. Songs in the Rocky Mountain
states begin with a single introductory note followed by the
trill, and West Coast populations have dropped the
introductory notes entirely -- their songs are composed of
just the buzzy trill. Although such geographic variation is
quite evident to the human ear, we still recognize the
singer as a Rufous-sided Towhee. The songs of populations
often differ markedly on a much smaller geographic scale.
Local variants are called dialects. They are commonly found
in songbirds with populations restricted to particular
habitats and separated from other populations by unsuitable
terrain. The separation can be on the order of a mile or so,
but in some species it can be much less. Among the
White-crowned Sparrow populations of coastal California,
distinct dialects may be separated by as little as a few
yards in what appears to be essentially continuous
habitat! Vocal dialects appear to be
learned. Young birds hear the songs sung around their natal
territories by their fathers and neighboring males, and
acquire the peculiarities of these renditions. Factors that
determine the geographic pattern of dialects include the
accuracy with which the pitch and temporal characteristics
of individual song components are learned, the distance
young males disperse from where they hatch to where they
breed, and the timing of dispersal relative to the sensitive
period for learning. The "why" of dialects, their
functional significance, has proven more elusive than the
question of how they arise. Many ornithologists have assumed
that dialects serve as indicators of genetic adaptation to
local conditions. The dialects thus enable females to choose
males from their own birth area, who presumably are carrying
genes closely adapted to the specific environment in which
breeding occurs. In other words, dialects function to
promote "positive assortative mating" -- the breeding
together of similar individuals. Experimental work with
several species has shown that females are more responsive
to their own song dialects than to more distant song
dialects. Genetic studies of White-crowned Sparrows along
the coast of northern California have shown genetic
differences between birds of different dialects. But nature is rarely that
straightforward. Additional work has shown that females of
some species are more responsive to songs of males with
dialects other than their own, so that the assumption that a
female breeds within the dialect region in which she was
hatched is not always warranted. In a series of experiments,
paired female White-crowned Sparrows were captured on their
territories and injected with the male hormone testosterone
(to induce singing). Surprisingly, they sang the song of
nearby dialects, rather than their own local dialect.
Presumably this indicates that they had chosen to mate with
males from "foreign" dialects and had dispersed out of their
native dialect region. In addition, reanalysis of the
genetic data from the White-crowned Sparrow studies has
demonstrated that the results may be attributable to other
factors. The data could be interpreted as indicating that
environments are as heterogeneous within a dialect region as
they are between dialect areas. If so, one might expect to
find as much genetic differentiation between groups living
in dramatically different habitats within a dialect region
as between different dialects. Thus, the adaptive
significance of dialects remains to be demonstrated. It may
be that no selective force has acted to promote dialects.
Instead, dialects may simply be epiphenomena -- that have
arisen simply as a consequence of vocal learning -- and have
no evolutionary significance. SEE: Vocal
Development;
Vocal
Functions;
Vocal
Copying;
Hybridization. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.