Permissiveness in the Learning and Development of Song Syntax in Swamp Sparrows
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ANIMAL BEHAVIOUR, 1999, 58, 93–103 Article No. anbe.1999.1140, available online at http://www.idealibrary.com on Permissiveness in the learning and development of song syntax in swamp sparrows JEFFREY PODOS*, STEPHEN NOWICKI† & SUSAN PETERS† *Department of Ecology and Evolutionary Biology, University of Arizona †Evolution, Ecology and Organismal Biology Group, Department of Zoology, Duke University (Received 5 February 1998; initial acceptance 24 February 1999; final acceptance 24 March 1999; MS. number: A8204R) Vocal learning in swamp sparrows, Melospiza georgiana, is subject to a host of sensory and motor limitations. One such limitation is that young swamp sparrows almost invariably crystallize their songs with a simple trilled syntax, irrespective of the syntax of vocal models from which they learn. A striking exception to this pattern was recently identified by Podos (1996, Animal Behaviour, 51, 1061–1070), who found that large-scale organizational changes in vocal syntax, including the production of an intermit- tent or ‘broken’ syntax, were produced when birds faced limits on vocal performance capacities during motor ontogeny. Our goal in the present study was to determine whether song models with broken syntax could serve as suitable training models for young swamp sparrows, and, if so, if broken syntax could be faithfully reproduced. We hand-reared 10 male swamp sparrows and exposed them to control, rapid and broken song models. Control song models were copied with a high degree of accuracy, as in previous studies. Rapid song models were copied with deficiencies that suggested performance limits on vocal production; such deficiencies included the production of songs with broken syntax and the production of songs in which notes were dropped out as songs progressed. Broken songs proved suitable as training models. Furthermore, copies of broken song models were crystallized either with normal or with broken syntax. These data identify an unexpected direction of permissiveness in the types of songs swamp sparrows will memorize and accurately reproduce, and also point to a possible proximate basis for syntactical changes in the evolution of sparrow songs. 1999 The Association for the Study of Animal Behaviour Most songbirds learn their songs by imitation, through a song learning, during which time exposure to song pat- process that includes an early ‘sensory phase’ when song terns of other birds can help to determine the subset of models are committed to memory, and a subsequent previously memorized song models that an individual ‘sensorimotor phase’ when stored representations of will eventually express (e.g. Nelson 1992; Nelson & these models are accessed and reproduced (Slater 1989). Marler 1994). Learning by imitation is inherently open-ended or ‘per- Song learning is not completely permissive, however, missive’, insofar as sensory information directs the devel- despite the open-endedness associated with imitation. opment of motor patterns (Marler 1976, 1984). In Instead, the range of songs a young bird may memorize songbird vocal learning, permissiveness is illustrated by and subsequently reproduce is restricted in some dimen- the fact that young birds are able to reproduce precisely sions by both sensory and motor factors. For example, the acoustic features of a broad range of models to which young birds typically display a strong predisposition to they are exposed. The range of acoustic stimuli that may memorize conspecific songs in preference to hetero- be imitated includes not only widely divergent variants of specific songs, as revealed by their greater attentiveness to conspecific song (e.g. Marler 1970) but even heterospe- conspecific models (e.g. Dooling & Searcy 1980; Nelson & cific songs under some rearing conditions (e.g. Baptista & Marler 1993) and by their eventual reproduction of con- Petrinovitch 1984; Marler & Peters 1989). Permissiveness specific song models (e.g. Marler & Peters 1977, 1989). also plays an important role in the sensorimotor phase of Evidence for sensorimotor predispositions in song devel- opment comes from the observation that some species- Correspondence: J. Podos, Department of Ecology and Evolutionary Biology, Room 310 Biological Sciences West, University of Arizona, typical motor patterns are produced even when young Tucson, AZ 85721, U.S.A. (email: [email protected]). S. Nowicki birds are deprived of all external song models (Marler & and S. Peters are at the Department of Zoology, Duke University, Sherman 1985). In addition, motor limitations on the Durham, NC 27708-0325, U.S.A. performance capacities of the vocal apparatus may restrict 0003–3472/99/070093+11 $30.00/0 93 1999 The Association for the Study of Animal Behaviour 94 ANIMAL BEHAVIOUR, 58, 1 Figure 1. Expression of broken syntax as an outcome of learning from rapid models (examples from Podos 1996). (a) Typical wild swamp sparrow song. (b) Digitally manipulated version of that wild song, used as a training model. The trill rate of this model was increased by 92%, compared with the trill rate of the original wild song. (c) Copy of model, produced with broken syntax. *Syllable period within multisyllable segments; **syllable period across gaps. Sonagrams were made using a Kay Elemetric Digital Sonagraph (Model 5500) at 300-Hz filter bandwidth and a frequency range of 0–8 kHz. a bird’s ability to reproduce accurately songs that had swamp sparrow songs are produced as continuous trills of been successfully memorized during the sensory phase identical syllables repeated at a relatively constant rates. (Podos 1996, 1997). In almost every case, these trills include only one type of The swamp sparrow, Melospiza georgiana, is a species in syllable; much more rarely, songs begin with one syllable which sensory and motor restrictions appear to play an type and then switch to a second syllable type (i.e. to a especially strong role in the song-learning process. This different set of repeated notes) about halfway through the species’ song lasts about 2 s and is composed of a repeat- song, but in such cases the trill remains continuous with ing sequence of one to five individual notes; the repeat- no obvious break between the repetitions of the first and ing sequence of notes is referred to as a ‘syllable’, and the second syllable type. In the laboratory, swamp sparrows repeated syllables form a ‘trill’ (Fig. 1a; Marler & Pickert show a strong preference for imitating conspecific songs 1984; Podos 1996). Although the species is widely distrib- and are especially resistant to learning phonology that uted across eastern and central North America (Mowbrey deviates from the set of six species-typical note types, 1997), all swamp sparrow songs show a relatively narrow even when given no other choice of models (Marler & range of structural characteristics. On the phonological Peters 1977, 1980, 1989). When young birds learn from level, all notes produced by individuals across the entire swamp sparrow phonology presented in a more complex species range fall into only six categories of perceptually syntactical structure, they typically reproduce this pho- distinct note types (Marler & Pickert 1984; Clark et al. nology as simple continuous trills (Marler & Peters 1980). 1987; Nelson & Marler 1989). On the syntactical level, all Even birds raised in total isolation with no exposure at all PODOS ET AL.: PERMISSIVENESS IN SONG LEARNING 95 to song models produce songs as continuous trills (Marler broken syntax when faced with a performance limitation & Sherman 1985). The fact that swamp sparrows rarely, if is mirrored by an unexpected permissiveness to learn and ever, deviate from production of continuous trills, irre- reproduce this novel vocal pattern. spective of early experience, led Marler (1984) to suggest that an innate central motor programme underlies this METHODS species-typical feature of swamp sparrow song. A striking exception to the tendency for swamp spar- We collected 10 male swamp sparrows from eight nests, rows to develop songs with continuous trilled syntax was 4–7 days after hatching, on 25–26 May 1995, from a revealed in a laboratory study of song learning, in which population in Crawford County, Pennsylvania. Birds young birds were tutored with song models that were were hand-reared (as in Marler & Peters 1988) and housed digitally manipulated to have trill rates higher than in individual cages inside two walk-in acoustic isolation species norms (Podos 1996; see e.g. Fig. 1a versus b). Birds chambers (5 birds/chamber). Cages were positioned were able to memorize these fast-trill models, but proved inside these chambers so that birds could not see each unable to reproduce them accurately. Instead, birds’ other, although they were able to hear each other. reproductions of these models included several kinds of Using ‘Signal’ software (Engineering Design 1996), we modifications in timing and syllable structure, all of constructed 10 songs to be used as learning models: two which appeared to result from an inability of the vocal ‘control’ models, four ‘rapid’ models and four ‘broken’ apparatus to produce the faster trill rates of the models. models. Control and rapid models were constructed as in The most unusual of these modifications was the appear- Podos (1996). Briefly, we digitized songs recorded in the ance of a ‘broken’ syntax (Fig. 1c), in which short bursts wild at a sample rate of 25 kpt/s (Data Translation 2128G of syllables (reproduced accurately and at the fast rate of A/D board), after high-pass filtering to reduce low- the model) were separated by silent gaps, as if the motor frequency environmental noise (Krohn-Hite 3500, 1-kHz system could not sustain the high trill rate without breaks corner frequency) and low-pass filtering to prevent alias- for the entire duration of the song. The appearance of ing (Stanford Research Systems SR640, 10-kHz corner broken syntax songs is of interest not only because it is frequency; see Stoddard 1998 for explanation of sampling consistent with the hypothesis that motor limitations and aliasing).