Vocal Functions

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.
