Prosody in birdsong: A review and perspective

Sensitivity to the sonority sequencing principle in rats (Rattus norvegicus)

Albeit diverse, human languages exhibit universal structures. A salient example is the syllable, an important structure of language acquisition. The structure of syllables is determined by the Sonority Sequencing Principle (SSP), a linguistic constraint according to which phoneme intensity must increase at onset, reaching a peak at nucleus (vowel), and decline at offset. Such structure generates an intensity pattern with an arch shape. In humans, sensitivity to restrictions imposed by the SSP on syllables appears at birth, raising questions about its emergence. We investigated the biological mechanisms at the foundations of the SSP, testing a nonhuman, non-vocal-learner species with the same language materials used with humans. Rats discriminated well-structured syllables (e.g., pras) from ill-structured ones (e.g., lbug) after being familiarized with syllabic structures conforming to the SSP. In contrast, we did not observe evidence that rats familiarized with syllables that violate such constraint discriminated at test. This research provides the first evidence of sensitivity to the SSP in a nonhuman species, which likely stems from evolutionary-ancient cross-species biological predispositions for natural acoustic patterns. Humans' early sensitivity to the SSP possibly emerges from general auditory processing that favors sounds depicting an arch-shaped envelope, common amongst animal vocalizations. Ancient sensory mechanisms, responsible for processing vocalizations in the wild, would constitute an entry-gate for human language acquisition.

Lessons learned in animal acoustic cognition through comparisons with humans

Humans are an interesting subject of study in comparative cognition. While humans have a lot of anecdotal and subjective knowledge about their own minds and behaviors, researchers tend not to study humans the way they study other species. Instead, comparisons between humans and other animals tend to be based on either assumptions about human behavior and cognition, or very different testing methods. Here we emphasize the importance of using insider knowledge about humans to form interesting research questions about animal cognition while simultaneously stepping back and treating humans like just another species as if one were an alien researcher. This perspective is extremely helpful to identify what aspects of cognitive processes may be interesting and relevant across the animal kingdom. Here we outline some examples of how this objective human-centric approach has helped us to move forward knowledge in several areas of animal acoustic cognition (rhythm, harmonicity, and vocal units). We describe how this approach works, what kind of benefits we obtain, and how it can be applied to other areas of animal cognition. While an objective human-centric approach is not useful when studying traits that do not occur in humans (e.g., magnetic spatial navigation), it can be extremely helpful when studying traits that are relevant to humans (e.g., communication). Overall, we hope to entice more people working in animal cognition to use a similar approach to maximize the benefits of being part of the animal kingdom while maintaining a detached and scientific perspective on the human species.

On the representation of hierarchical structure: Revisiting Darwin's musical protolanguage

In this article, we address the tenability of Darwin's musical protolanguage, arguing that a more compelling evolutionary scenario is one where a prosodic protolanguage is taken to be the preliminary step to represent the hierarchy involved in linguistic structures within a linear auditory signal. We hypothesize that the establishment of a prosodic protolanguage results from an enhancement of a rhythmic system that transformed linear signals into speech prosody, which in turn can mark syntactic hierarchical relations. To develop this claim, we explore the role of prosodic cues on the parsing of syntactic structures, as well as neuroscientific evidence connecting the evolutionary development of music and linguistic capacities. Finally, we entertain the assumption that the capacity to generate hierarchical structure might have developed as part of tool-making in human prehistory, and hence was established prior to the enhancement of a prosodic protolinguistic system.

Syntactic modulation of rhythm in Australian pied butcherbird song

The acoustic structure of birdsong is spectrally and temporally complex. Temporal complexity is often investigated in a syntactic framework focusing on the statistical features of symbolic song sequences. Alternatively, temporal patterns can be investigated in a rhythmic framework that focuses on the relative timing between song elements. Here, we investigate the merits of combining both frameworks by integrating syntactic and rhythmic analyses of Australian pied butcherbird (Cracticus nigrogularis) songs, which exhibit organized syntax and diverse rhythms. We show that rhythms of the pied butcherbird song bouts in our sample are categorically organized and predictable by the song's first-order sequential syntax. These song rhythms remain categorically distributed and strongly associated with the first-order sequential syntax even after controlling for variance in note length, suggesting that the silent intervals between notes induce a rhythmic structure on note sequences. We discuss the implication of syntactic-rhythmic relations as a relevant feature of song complexity with respect to signals such as human speech and music, and advocate for a broader conception of song complexity that takes into account syntax, rhythm, and their interaction with other acoustic and perceptual features.

Syntactic modulation of rhythm in Australian pied butcherbird song

The acoustic structure of birdsong is spectrally and temporally complex. Temporal complexity is often investigated in a syntactic framework focusing on the statistical features of symbolic song sequences. Alternatively, temporal patterns can be investigated in a rhythmic framework that focuses on the relative timing between song elements. Here, we investigate the merits of combining both frameworks by integrating syntactic and rhythmic analyses of Australian pied butcherbird (Cracticus nigrogularis) songs, which exhibit organized syntax and diverse rhythms. We show that rhythms of the pied butcherbird song bouts in our sample are categorically organized and predictable by the song's first-order sequential syntax. These song rhythms remain categorically distributed and strongly associated with the first-order sequential syntax even after controlling for variance in note length, suggesting that the silent intervals between notes induce a rhythmic structure on note sequences. We discuss the implication of syntactic-rhythmic relations as a relevant feature of song complexity with respect to signals such as human speech and music, and advocate for a broader conception of song complexity that takes into account syntax, rhythm, and their interaction with other acoustic and perceptual features.

Auditory Pattern Discrimination in Budgerigars (Melopsittacus undulatus)

Auditory patterns carry information in human speech at multiple levels, including the surface relationships between sounds within words in phonology and the abstract structures of syntax. The sequences of other animal vocalizations, such as birdsong, can also be described as auditory patterns, but few studies have probed how the sequences are perceived at multiple levels. Past work shows that a small parrot species, the budgerigar (Melopsittacus undulatus), exceeds other birds in sequence perception and is even sensitive to abstract structure. But it is not known what level of auditory analysis is dominant in perception or what limits might exist in sensitivity to abstract structure. Here, budgerigars were tested on their ability to discriminate changes in an auditory pattern, AAB, i.e. sound-same different, to ask how they attended to surface relationships among the sounds and the abstract relationships of same/different among the elements. The results show that the budgerigars primarily used surface transitions between the sounds when discriminating the sequences, but were able to use the abstract relationships to a limited extent, largely restricted to two elements. This study provides insight into how budgerigars extract information from conspecific vocalizations and how their capacities compare to human speech perception.

Birdsong and the Neural Regulation of Positive Emotion

Birds are not commonly admired for emotional expression, and when they are, the focus is typically on negative states; yet vocal behavior is considered a direct reflection of an individual's emotional state. Given that over 4000 species of songbird produce learned, complex, context-specific vocalizations, we make the case that songbirds are conspicuously broadcasting distinct positive emotional states and that hearing songs can also induce positive states in other birds. Studies are reviewed that demonstrate that that the production of sexually motivated song reflects an emotional state of anticipatory reward-seeking (i.e., mate-seeking), while outside the mating context song in gregarious flocks reflects a state of intrinsic reward. Studies are also reviewed that demonstrate that hearing song induces states of positive anticipation and reward. This review brings together numerous studies that highlight a potentially important role for the songbird nucleus accumbens, a region nearly synonymous with reward in mammals, in positive emotional states that underlie singing behavior and responses to song. It is proposed that the nucleus accumbens is part of an evolutionarily conserved circuitry that contributes context-dependently to positive emotional states that motivate and reward singing behavior and responses to song. Neural mechanisms that underlie basic emotions appear to be conserved and similar across vertebrates. Thus, these findings in songbirds have the potential to provide insights into interventions that can restore positive social interactions disrupted by mental health disorders in humans.

ALS/FTD: Evolution, Aging, and Cellular Metabolic Exhaustion

Amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) are neurodegenerations with evolutionary underpinnings, expansive clinical presentations, and multiple genetic risk factors involving a complex network of pathways. This perspective considers the complex cellular pathology of aging motoneuronal and frontal/prefrontal cortical networks in the context of evolutionary, clinical, and biochemical features of the disease. We emphasize the importance of evolution in the development of the higher cortical function, within the influence of increasing lifespan. Particularly, the role of aging on the metabolic competence of delicately optimized neurons, age-related increased proteostatic costs, and specific genetic risk factors that gradually reduce the energy available for neuronal function leading to neuronal failure and disease.

Vocal Learning and Behaviors in Birds and Human Bilinguals: Parallels, Divergences and Directions for Research

Comparisons between the communication systems of humans and animals are instrumental in contextualizing speech and language into an evolutionary and biological framework and for illuminating mechanisms of human communication. As a complement to previous work that compares developmental vocal learning and use among humans and songbirds, in this article we highlight phenomena associated with vocal learning subsequent to the development of primary vocalizations (i.e., the primary language (L1) in humans and the primary song (S1) in songbirds). By framing avian “second-song” (S2) learning and use within the human second-language (L2) context, we lay the groundwork for a scientifically-rich dialogue between disciplines. We begin by summarizing basic birdsong research, focusing on how songs are learned and on constraints on learning. We then consider commonalities in vocal learning across humans and birds, in particular the timing and neural mechanisms of learning, variability of input, and variability of outcomes. For S2 and L2 learning outcomes, we address the respective roles of age, entrenchment, and social interactions. We proceed to orient current and future birdsong inquiry around foundational features of human bilingualism: L1 effects on the L2, L1 attrition, and L1<–>L2 switching. Throughout, we highlight characteristics that are shared across species as well as the need for caution in interpreting birdsong research. Thus, from multiple instructive perspectives, our interdisciplinary dialogue sheds light on biological and experiential principles of L2 acquisition that are informed by birdsong research, and leverages well-studied characteristics of bilingualism in order to clarify, contextualize, and further explore S2 learning and use in songbirds.

Vocal learning in songbirds: the role of syllable order in song recognition

Songbird vocal learning has interesting behavioural and neural parallels with speech acquisition in human infants. Zebra finch males sing one unique song that they imitate from conspecific males, and both sexes learn to recognize their father's song. Although males copy the stereotyped syllable sequence of their father's song, the role of sequential information in recognition remains unclear. Here, we investigated father's song recognition after changing the serial order of syllables (switching the middle syllables, first and last syllables, or playing all syllables in inverse order). Behavioural approach and call responses of adult male and female zebra finches to their father's versus unfamiliar songs in playback tests demonstrated significant recognition of father's song with all syllable-order manipulations. We then measured behavioural responses to normal versus inversed-order father's song. In line with our first results, the subjects did not differentiate between the two. Interestingly, when males' strength of song learning was taken into account, we found a significant correlation between song imitation scores and the approach responses to the father's song. These findings suggest that syllable sequence is not essential for recognition of father's song in zebra finches, but that it does affect responsiveness of males in proportion to the strength of vocal learning. This article is part of the theme issue 'Vocal learning in animals and humans'.

Phylogeny and mechanisms of shared hierarchical patterns in birdsong

Organizational patterns can be shared across biological systems, and revealing the factors shaping common patterns can provide insight into fundamental biological mechanisms. The behavioral pattern that elements with more constituents tend to consist of shorter constituents (Menzerath's law [ML]) was described first in speech and language (e.g., words with more syllables consist of shorter syllables) and subsequently in music and animal communication. Menzerath's law is hypothesized to reflect efficiency in information transfer, but biases and constraints in motor production can also lead to this pattern. We investigated the evolutionary breadth of ML and the contribution of production mechanisms to ML in the songs of 15 songbird species. Negative relationships between the number and duration of constituents (e.g., syllables in phrases) were observed in all 15 species. However, negative relationships were also observed in null models in which constituents were randomly allocated into observed element durations, and the observed negative relationship for numerous species did not differ from the null model; consequently, ML in these species could simply reflect production constraints and not communicative efficiency. By contrast, ML was significantly different from the null model for more than half the cases, suggesting additional organizational rules are imposed onto birdsongs. Production mechanisms are also underscored by the finding that canaries and zebra finches reared without auditory experiences that guide vocal development produced songs with nearly identical ML patterning as typically reared birds. These analyses highlight the breadth with which production mechanisms contribute to this prevalent organizational pattern in behavior.

Higher-Order Musical Temporal Structure in Bird Song

Bird songs often display musical acoustic features such as tonal pitch selection, rhythmicity, and melodic contouring. We investigated higher-order musical temporal structure in bird song using an experimental method called “music scrambling” with human subjects. Recorded songs from a phylogenetically diverse group of 20 avian taxa were split into constituent elements (“notes” or “syllables”) and recombined in original and random order. Human subjects were asked to evaluate which version sounded more “musical” on a per-species basis. Species identity and stimulus treatment were concealed from subjects, and stimulus presentation order was randomized within and between taxa. Two recordings of human music were included as a control for attentiveness. Participants varied in their assessments of individual species musicality, but overall they were significantly more likely to rate bird songs with original temporal sequence as more musical than those with randomized temporal sequence. We discuss alternative hypotheses for the origins of avian musicality, including honest signaling, perceptual bias, and arbitrary aesthetic coevolution.

Behavioral discrimination and time-series phenotyping of birdsong performance

Variation in the acoustic structure of vocal signals is important to communicate social information. However, relatively little is known about the features that receivers extract to decipher relevant social information. Here, we took an expansive, bottom-up approach to delineate the feature space that could be important for processing social information in zebra finch song. Using operant techniques, we discovered that female zebra finches can consistently discriminate brief song phrases ("motifs") from different social contexts. We then applied machine learning algorithms to classify motifs based on thousands of time-series features and to uncover acoustic features for motif discrimination. In addition to highlighting classic acoustic features, the resulting algorithm revealed novel features for song discrimination, for example, measures of time irreversibility (i.e., the degree to which the statistical properties of the actual and time-reversed signal differ). Moreover, the algorithm accurately predicted female performance on individual motif exemplars. These data underscore and expand the promise of broad time-series phenotyping to acoustic analyses and social decision-making.

Birdsong fails to support object categorization in human infants

Recent evidence reveals a precocious link between language and cognition in human infants: listening to their native language supports infants' core cognitive processes, including object categorization, and does so in a way that other acoustic signals (e.g., time-reversed speech; sine-wave tone sequences) do not. Moreover, language is not the only signal that confers this cognitive advantage: listening to vocalizations of non-human primates also supports object categorization in 3- and 4-month-olds. Here, we move beyond primate vocalizations to clarify the breadth of acoustic signals that promote infant cognition. We ask whether listening to birdsong, another naturally produced animal vocalization, also supports object categorization in 3- and 4-month-old infants. We report that listening to zebra finch song failed to confer a cognitive advantage. This outcome brings us closer to identifying a boundary condition on the range of non-linguistic acoustic signals that initially support infant cognition.

Arc-shaped pitch contours facilitate item recognition in non-human animals

Acoustic changes linked to natural prosody are a key source of information about the organization of language. Both human infants and adults readily take advantage of such changes to discover and memorize linguistic patterns. Do they so because our brain is efficiently wired to specifically process linguistic stimuli? Or are we co-opting for language acquisition purposes more general principles that might be inherited from our animal ancestors? Here, we address this question by exploring if other species profit from prosody to better process acoustic sequences. More specifically, we test whether arc-shaped pitch contours defining natural prosody might facilitate item recognition and memorization in rats. In two experiments, we presented to the rats nonsense words with flat, natural, inverted and random prosodic contours. We observed that the animals correctly recognized the familiarization words only when arc-shaped pitch contours were implemented over them. Our results suggest that other species might also benefit from prosody for the memorization of items in a sequence. Such capacity seems to be rooted in general principles of how biological sounds are produced and processed.

Discrimination of natural acoustic variation in vocal signals

Studies of acoustic communication often focus on the categories and units of vocalizations, but subtle variation also occurs in how these signals are uttered. In human speech, it is not only phonemes and words that carry information but also the timbre, intonation, and stress of how speech sounds are delivered (often referred to as "paralinguistic content"). In non-human animals, variation across utterances of vocal signals also carries behaviorally relevant information across taxa. However, the discriminability of these cues has been rarely tested in a psychophysical paradigm. Here, we focus on acoustic communication in the zebra finch (Taeniopygia guttata), a songbird species in which the male produces a single stereotyped motif repeatedly in song bouts. These motif renditions, like the song repetitions of many birds, sound very similar to the casual human listener. In this study, we show that zebra finches can easily discriminate between the renditions, even at the level of single song syllables, much as humans can discriminate renditions of speech sounds. These results support the notion that sensitivity to fine acoustic details may be a primary channel of information in zebra finch song, as well as a shared, foundational property of vocal communication systems across species.

Scaling of the avian middle ear

This article presents a comparative study of morphology of the avian middle ear. The general morphology of the columella shows considerable variation across species, yet few studies have attempted to provide quantitative comparisons, and basic anatomical data has not been thoroughly reported. In this study, we examined the middle ear in 49 taxonomically diverse species of bird. We found significant correlations between measurements of several features (columellar length, mass, tympanic membrane area, footplate area) and interaural diameter. While scaling of columellar length with interaural diameter is consistent with isometry, masses and areas showed negative allometry, or a non-proportional scaling with interaural diameter. These observations remained true even for species with unusual middle ear morphology, such as Alcedinidae (Kingfishers) in which the basal struts of the columella form a structure almost resembling a mammalian stapes, or Tytonidae (Barn Owls) which have a highly bulbous footplate. It therefore appears that allometry cannot help explain the morphological variation in the columella.

Manipulations of sensory experiences during development reveal mechanisms underlying vocal learning biases in zebra finches

Biological predispositions in learning can bias and constrain the cultural evolution of social and communicative behaviors (e.g., speech and birdsong), and lead to the emergence of behavioral and cultural "universals." For example, surveys of laboratory and wild populations of zebra finches (Taeniopygia guttata) document consistent patterning of vocal elements ("syllables") with respect to their acoustic properties (e.g., duration, mean frequency). Furthermore, such universal patterns are also produced by birds that are experimentally tutored with songs containing randomly sequenced syllables ("tutored birds"). Despite extensive demonstrations of learning biases, much remains to be uncovered about the nature of biological predispositions that bias song learning and production in songbirds. Here, we examined the degree to which "innate" auditory templates and/or biases in vocal motor production contribute to vocal learning biases and production in zebra finches. Such contributions can be revealed by examining acoustic patterns in the songs of birds raised without sensory exposure to song ("untutored birds") or of birds that are unable to hear from early in development ("early-deafened birds"). We observed that untutored zebra finches and early-deafened zebra finches produce songs with positional variation in some acoustic features (e.g., mean frequency) that resemble universal patterns observed in tutored birds. Similar to tutored birds, early-deafened birds also produced song motifs with alternation in acoustic features across adjacent syllables. That universal acoustic patterns are observed in the songs of both untutored and early-deafened birds highlights the contribution motor production biases to the emergence of universals in culturally transmitted behaviors.

The Drosophila Gene Sulfateless Modulates Autism-Like Behaviors

Major challenges to identifying genes that contribute to autism spectrum disorder (ASD) risk include the availability of large ASD cohorts, the contribution of many genes overall, and small effect sizes attributable to common gene variants. An alternative approach is to use a model organism to detect alleles that impact ASD-relevant behaviors and ask whether homologous human genes infer ASD risk. Here we utilized the Drosophila genetic reference panel (DGRP) as a tool to probe for perturbation in naturally occurring behaviors in Drosophila melanogaster that are analogous to three behavior domains: impaired social communication, social reciprocity and repetitive behaviors or restricted interests. Using 40 of the available DGRP lines, we identified single nucleotide polymorphisms (SNPs) in or near genes controlling these behavior domains, including ASD gene orthologs (neurexin 4 and neuroligin 2), an intellectual disability (ID) gene homolog (kirre), and a gene encoding a heparan sulfate (HS) modifying enzyme called sulfateless (sfl). SNPs in sfl were associated with all three ASD-like behaviors. Using RNAi knock-down of neuronal sfl expression, we observed significant changes in expressive and receptive communication during mating, decreased grooming behavior, and increased social spacing. These results suggest a role for HS proteoglycan synthesis and/or modification in normal social communication, repetitive behavior, and social interaction in flies. Finally, using the DGRP to directly identify genetic effects relevant to a neuropsychiatric disorder further demonstrates the utility of the Drosophila system in the discovery of genes relevant to human disease.

The role of prosody in infants' preference for speech: A comparison between speech and birdsong

Speech preferences emerge very early in infancy, pointing to a special status for speech in auditory processing and a crucial role of prosody in driving infant preferences. Recent theoretical models suggest that infant auditory perception may initially encompass a broad range of human and nonhuman vocalizations, then tune in to relevant sounds for the acquisition of species-specific communication sounds. However, little is known about sound properties eliciting infants' tuning-in to speech. To address this issue, we presented a group of 4-month-olds with segments of non-native speech (Mandarin Chinese) and birdsong, a nonhuman vocalization that shares some prosodic components with speech. A second group of infants was presented with the same segment of birdsong paired with Mandarin played in reverse. Infants showed an overall preference for birdsong over non-native speech. Moreover, infants in the Backward condition preferred birdsong over backward speech whereas infants in the Forward condition did not show clear preference. These results confirm the prominent role of prosody in early auditory processing and suggest that infants' preferences may privilege communicative vocalizations featured by certain prosodic dimensions regardless of the biological source of the sound, human or nonhuman.

The Development of Structured Vocalizations in Songbirds and Humans: A Comparative Analysis

Humans and songbirds face a common challenge: acquiring the complex vocal repertoire of their social group. Although humans are thought to be unique in their ability to convey symbolic meaning through speech, speech and birdsong are comparable in their acoustic complexity and the mastery with which the vocalizations of adults are acquired by young individuals. In this review, we focus on recent advances in the study of vocal development in humans and songbirds that shed new light on the emergence of distinct structural levels of vocal behavior and point to new possible parallels between both groups.

A Comparative Perspective on the Role of Acoustic Cues in Detecting Language Structure

Most human language learners acquire language primarily via the auditory modality. This is one reason why auditory artificial grammars play a prominent role in the investigation of the development and evolutionary roots of human syntax. The present position paper brings together findings from human and non-human research on the impact of auditory cues on learning about linguistic structures with a special focus on how different types of cues and biases in auditory cognition may contribute to success and failure in artificial grammar learning (AGL). The basis of our argument is the link between auditory cues and syntactic structure across languages and development. Cross-species comparison suggests that many aspects of auditory cognition that are relevant for language are not human specific and are present even in rather distantly related species. Furthermore, auditory cues and biases impact on learning, which we will discuss in the example of auditory perception and AGL studies. This observation, together with the significant role of auditory cues in language processing, supports the idea that auditory cues served as a bootstrap to syntax during language evolution. Yet this also means that potentially human-specific syntactic abilities are not due to basic auditory differences between humans and non-human animals but are based upon more advanced cognitive processes.

Multifractal analysis reveals music-like dynamic structure in songbird rhythms

Music is thought to engage its listeners by driving feelings of surprise, tension, and relief through a dynamic mixture of predictable and unpredictable patterns, a property summarized here as “expressiveness”. Birdsong shares with music the goal to attract its listeners’ attention and might use similar strategies to achieve this. We here tested a thrush nightingale’s (Luscinia luscinia) rhythm, as represented by song amplitude envelope (containing information on note timing, duration, and intensity), for evidence of expressiveness. We used multifractal analysis, which is designed to detect in a signal dynamic fluctuations between predictable and unpredictable states on multiple timescales (e.g. notes, subphrases, songs). Results show that rhythm is strongly multifractal, indicating fluctuations between predictable and unpredictable patterns. Moreover, comparing original songs with re-synthesized songs that lack all subtle deviations from the “standard” note envelopes, we find that deviations in note intensity and duration significantly contributed to multifractality. This suggests that birdsong is more dynamic due to subtle note timing patterns, often similar to musical operations like accelerando or crescendo. While different sources of these dynamics are conceivable, this study shows that multi-timescale rhythm fluctuations can be detected in birdsong, paving the path to studying mechanisms and function behind such patterns.

Learning Biases Underlie "Universals" in Avian Vocal Sequencing

Biological predispositions in vocal learning have been proposed to underlie commonalities in vocal sequences, including for speech and birdsong, but cultural propagation could also account for such commonalities [1-4]. Songbirds such as the zebra finch learn the sequencing of their acoustic elements ("syllables") during development [5-8]. Zebra finches are not constrained to learn a specific sequence of syllables, but significant consistencies in the positioning and sequencing of syllables have been observed between individuals within populations and between populations [8-10]. To reveal biological predispositions in vocal sequence learning, we individually tutored juvenile zebra finches with randomized and unbiased sequences of syllables and analyzed the extent to which birds produced common sequences. In support of biological predispositions, birds tutored with randomized sequences produced songs with striking similarities. Birds preferentially started and ended their song sequence with particular syllables, consistently positioned shorter and higher frequency syllables in the middle of their song, and sequenced their syllables such that pitch alternated across adjacent syllables. These patterns are reminiscent of those observed in normally tutored birds, suggesting that birds "creolize" aberrant sequence inputs to produce normal sequence outputs. Similar patterns were also observed for syllables that were not used for tutoring (i.e., unlearned syllables), suggesting that motor biases could contribute to sequence learning biases. Furthermore, zebra finches spontaneously produced acoustic patterns that are commonly observed in speech and music, suggesting that sensorimotor processes that are shared across a wide range of vertebrates could underlie these patterns in humans.

Regularities in zebra finch song beyond the repeated motif

The proliferation of birdsong research into the neural mechanisms of vocal learning is indebted to the remarkable stereotypy of the zebra finch's song motif. Motifs are composed of several syllables, which birds learn to produce in a fixed order. But at a higher level of organization-the bout-zebra finch song is no longer stereotyped. Song bouts include several repetitions of the motif, which are often linked by a variable number of short "connector" vocalizations. In this conceptual methods paper, we show that combinatorial analysis alone yields an incomplete description of this bout-level structure. In contrast, studying birdsong as a time-varying analog signal can reveal patterns of flexibility in the rhythmic organization of song bouts. Visualizing large song-samples in sorted raster plots shows that motifs are strung together via two distinct categories of connections: tight or loose. Loose connections allow considerable timing variation across renditions. Even among co-tutored birds that acquired similar motifs, we observe strong individual variability in rhythms and temporal plasticity of song bouts. These findings suggest that vocal flexibility could potentially allow individuals to express a variety of behavioral states through their songs, even in species that sing only a single stereotyped motif.