Animal Behavior/Bird Song
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[edit] Bird Song
Birds communicate information about danger, food, sex, group movements and many other purposes via acoutic signals. A subset of these have been termed song, as they frequently feature with extended, tonal, melodic characteristics. The Zebra Finch's song, for instance, includes several introductory notes followed by a string of syllables within an extended melodious pattern. Sonograms (i.e., a plot of the intensity of pitch against time) are commonly used as a primary tool for studying and comparing bird songs.
Respiratory muscles force streams of air from large air sacs through the bronchi. Membranes in the syrinx vibrate as air expressed from bronchi passes over them. Syrinx muscles for left and right sound producing structures can act independently, and many birds are able to sing harmonies with themselves. Song appears to play a role in advertising for sex and species recognition as song complexity frequently coincides with the presence of ornate plumage. It also stimulates and synchronizes courtship behavior, stimulates reproductive readiness in females, and contributes to pair bond maintenance. Local song dialects exist in many species.
Successful song in most adult male songbirds depends on memorizing the calls of a conspecific tutor during an earlier, sensitive phase in life. The appropriate song repertoire is acquired in a series of distinct stages. Young birds, during an early Sensory Phase, listen to a conspecific tutor and thereby obtain information about the characteristics of its own song. Only a very specific subset of surrounding songs is actually accepted as suitable, suggesting the presence of an in-built song template. Following this sensory phase, young birds actively begin to vocalize themselves. Their Subsong is an atonal, noisy, meaningless repetition of sounds, which lacks recognizable syllables. Akin to human Babbling birds practice coordinated movements of the respiratory system, sound producing organs, and related structures (e.g., tongue). During Sensory-motor Phase, young birds spontaneously produce Plastic Song, consisting of vocalizations with distinct syllables and recognizable elements. Such "work in progress" will include elements from the song of tutors and elaborate them into a variety of syllables and phrases that even exceed what eventually will be used in its adult song. The ability to hear its own vocalizations are critical for normal development. In transition to the Mature phase, birds adopt a Crystallized Song with syllables and syntax structure that is characteristic of its species. Once established, these song patterns remain fixed in many species, are no more disrupted by deafening, and are presented intact during each subsequent breeding season. In contrast open-ended learners (e.g. starlings and canaries) retain the capacity to adjust or alter their song throughout life.
Song production is under the control of multiple hormonal systems from embryonic gonads. Injections of testosterone induce adult males to sing, even out of season, while similar injections in females have no such effect. The presence of estrogen during male development appears to be essential. When estrogen is blocked experimentally in developing males, testosterone injections fail to elicit song. However, when estrogen had been delivered to developing females, injection of testosterone elicited song in them (Konishi).
Several neural centers with a role in song have been identified. The Higher Vocal Center (HVc) is a group of neurons in the forebrain that is larger in (singing) males than in (non-singing) females. Damage to it blocks song production in adults. The nucleus of the archistriatum (RA) in males is larger than in females and its neurons increase in size and dendritic arborization during song learning. Damage to this area blocks song production in adults. The lateral magnocellular nucleus of the anterior neostriatum (LMAN) is neither sexually dimorphic nor shows seasonal change in neuron size or number. Its ablation in young birds interferes with song acquisition but its ablation in adults brings about few deficits as long as song had already been been learned prior to damage. Area X of the paraolfactory lobe (Area X) is sexually dimorphic and new neurons are added in song learning. Damage to it interferes with song acquisition in young birds but not in adults.
[edit] Zebra Finch Song and Tinbergen's Four Aims
As in every behavioral system, a series of independent questions can be addressed for song behavior in Zebra finches (Taeniopygia guttata).
Proximate Causation: Zebra finch song production requires the flow of air through semi-independent vibrators in syrinx and vocal tract. The presence of song, and song repertoire size are reflected in sexual dimorphism of its controlling brain areas and nuclei. A learning pathway esists separate of a motor pathway. Singing, which is largely restricted to males, is under the control of androgens.
Ultimate Causation: Song in Zebra finches is a learned vocalization used during courtship and defense of a territory. Advertising the individual's presence it serves to elicit mating opportunities from females and to stimulate the partner's reproductive behavior and physiology. Moreover, it functions to claim a territory and to repell competitors from it.
Phylogeny: Virtually all 9000 species of birds have the ability to vocalize, including crows, turkeys, owls or nightingales. A large subset of them, including the zebra finch, are characterized by complex vocal organs, distinctive brain circuitry for song, and acquisition of species-characteristic vocalizations through learning. Taxonomically these are all restricted to a single order - the Passeriformes.
Ontogeny: The emergence of adult zebra finch song illustrates the interactions of genetic and environmental factors in behavioral development. After periods of listening to the songs of tutors, starting its own partial vocalizations, rehearsing and adaptating its own song, the species-specific adult version slowly emerges. Song circuits exhibit extensive plasticity even in adults with ongoing neurogenesis and seasonal changes in neuronal morphology.
[edit] White-crowned Sparrows
White-crowned sparrows (Zonotrichia leucophrys nuttalli) males sing a single song that shows considerable geographic variation in the form of stable dialects. Bilingual and blended strategies exist at the boundaries. The distinctiveness of the song depends on patterns of natal dispersal and the timing of learning. Subject to reinforcement by the song of neighbors, the system is highly dependent on auditory feedback. The work by Peter Marler, Doug Nelson and others for over 30 years illustrates how genetic and environmental factors interact during the development of a complex communication system. Males generally establish territories during late plastic song with a repertoire that consists of ~4 different songs. Improvisations yield individual-specific songs which closely match that of the nearest rival.
Cross-fostering experiments illustrate the role of auditory templates in song learning. Young birds reared in the presence of taped song will learn and present that song, even if the tape came from another species. A Song sparrow raised with a swamp sparrow tape will experience little difficulty to learn the swamp sparrow song. Birds in Isolation experiments are raised without access to intact adult song (i.e., no template) and will subsequently show deficiencies in their own song upon maturation. The song does nonetheless contain valid elements of intact adult song. Moreover, Deafening experiments, which deafen birds at hatching, results in song that still contains some valid elements but is an even cruder version than those of isolated birds. When Song preference experiments present young birds with a wide range of conspecific and heterospecific songs, they recognize and preferentially learn the song of its own species. Birds raised in Mixed syllables experiments in the presence of a mixture of swamp- and song sparrow syllables, will acurately produce these syllables in their song but lack the normal adult syntax.
Castration Experiments have shed light on the roles of hormones in song learning. Swamp sparrows that are castrated early in development have low testosterone levels compared to their male siblings. They acquire song but progress to plastic phase only. Treating such birds with injections of testosterone (Enhanced Testosterone Experiment) immediately crystallizes the song. Interfering with testosterone function in adult birds (Reduced Testosterone Experiment) degrades previously crystallized song back to plastic.
[edit] Brown-headed Cowbirds
Brown-headed cowbirds (Molothrus ater) are gregarious birds that follows cattle herds. Brood parasites that are raised by parents of different species, no consistent, conspecific tutor available. So, how do they learn their own conspecific song? One of the strongest stimuli is the bird's own crystallized song and feedback from females is important (i.e., action-based learning).
This article is a stub. You can help Wikibooks by expanding this section of the Animal Behavior Book. In the meantime please consult Wikipedia in this subject.[edit] References
Konishi M. 1989. Birdsong for Neurobiologists. Neuron 3: 541-549.
