The
advent of the breeding season in spring is heralded each
dawn by a chorus of bird song that continues intermittently
throughout the day. With practice, we can identify a species
by its songs and calls even without seeing the vocalizer,
and we infer that birds can similarly distinguish between
members of their own and other species by voice alone. This
assumption has been verified experimentally for numerous
passerine species by playing tapes of vocalizations in the
field carefully observing responses of individual listeners.
By altering the tempo, frequency characteristics, length, or
other features of tape-recorded songs, and then observing
birds' responses to them, the actual components of songs
used for species recognition have been identified in several
instances. For example, the duration of intervals between
elements within songs is important for species recognition
in Common Yellowthroat, Rufous-sided Towhee, and Field,
Song, and White-throated Sparrows. Other species, such as
Winter Wren and Brown Thrasher, encode identification mainly
in the syntax (sequence of elements) of their
songs. Ornithologists
differentiate, somewhat arbitrarily, between a call and a
song by the length and complexity of the vocalization. Calls
tend to serve specific functions and are generally innate
rather than learned. For example, alarm calls serve to alert
all within earshot that danger is present; they tend to be
rather similar among groups of birds and often communicate
their message across species. Contact calls are used among
members of a flock or between mates to indicate the location
of the caller. Many species in groups that lack song (such
as gulls and parrots) have complex repertoires of calls that
serve varied functions. Song is a well-developed
feature primarily of oscine passerines (hence, they are
referred to as "songbirds"), and generally must be partly or
entirely learned. Songs identify the species of the singer.
In addition, the territorial or advertising song of males
serves the dual function of territorial proclamation
directed at other males and of mate attraction directed
toward females. Thus the song warns the former to keep out
of the defended territory and invites the latter to join the
singer. There are other, more subtle functions and messages,
as well. The motivation of the singer can be conveyed by the
amount that he sings; in order to attract a mate, unpaired
males devote more time to singing than do paired males. When
excited, such as during and immediately following a
territorial encounter with a rival male, the rapidity of
singing often increases. Song length also may increase or
decrease when the bird is agitated. Males of approximately
three-quarters of all songbirds sing two or more different
songs and are said to possess "song repertoires." Each song
having a particular configuration of syllables and phrases
repeated in a stereotyped fashion is referred to as a
"song-type." At the extreme end of the range in repertoire
size is the male Brown Thrasher, estimated to sing in excess
of 3,000 song-types. The evolution of elaborate song
repertoires is presumed to be the result of sexual selection
arising from competition between males for females. In
selecting a male with which to pair, females may use size
and complexity of the song repertoire to assess a male's
overall potential "fitness" as a partner. In some species,
these characteristics are known to increase with age, and
may serve as an indirect gauge of breeding experience and
health. Increased complexity and size of repertoire also
have been shown to correlate with measures of territory
quality in some species, thus providing further information
to a female about to invest her immediate reproductive
future on the basis of what she hears. There is some experimental
evidence that song is important in coordinating the
reproductive cycle between mates in addition to its presumed
role in maintaining the pair bond. Male song is known to
stimulate ovarian development and egg laying in Budgerigars
and to accelerate nest-building activity in female Canaries.
In fact, ornithologist Don Kroodsma has shown that female
Canaries exposed to large repertoires are more stimulated to
build nests than are females exposed to impoverished
repertoires. Laboratory experiments with female Song
Sparrows demonstrate that larger song repertoires elicit
more copulation-soliciting displays; females prefer
repertoires of 4 song-types compared to 1, 8 compared to 4,
and 16 compared to 8. Observations of pairing in the field,
however, revealed no relationship between repertoire size
and either date of initial pair formation or the speed with
which a second mate was acquired following removal of the
first female. Thus, although larger song repertoires appear
to serve as stronger stimuli in sexual and nesting behavior,
there is little field evidence that they influence female
choice of mates in species where song repertoire size is not
correlated with male age (as in the Song
Sparrow). Individual males often can
be identified by characteristic features of their songs, and
birds of many species have been tested in the field for
their ability to discriminate between the songs of neighbors
(males that are already established on territories and pose
no real threat) and strangers (males that are searching for
a territory on which to establish themselves). The ability
to distinguish between neighbors and strangers without being
able to see them should afford considerable energy savings
to a territory holder. The song of an established neighbor
can be answered with a song or can be ignored; the song of a
stranger, however, necessitates a vigorous physical as well
as vocal rebuff. Territorial males identify each other using
both song features and location from which the song
originates. Interestingly, species with large repertoires
show a somewhat reduced ability to discriminate between
neighbors and strangers than do species with limited
repertoires. For example, Song Sparrows, with repertoires of
8 to 10 songs, show weaker discrimination than closely
related Swamp Sparrows, with repertoires of 4 to 5
songs. The ability to recognize
neighbors by vocalizations alone has not been found in
species that nest in dense colonies. This is true even for
species (such as the Northern Gannet, Laughing Gull, and
Black-legged Kittiwake) that can recognize their young or
their mates by vocalizations. Presumably under conditions of
unobstructed visual contact, natural selection has not
favored vocal recognition of neighbors. SEE: Vocal
Development;
Vocal
Copying;
Visual
Displays;
Sexual
Selection. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.