Sound sequences in birdsong: how much do birds really care?

Auditory discrimination learning and acoustic cue weighing in female zebra finches with localized FoxP1 knockdowns

Rare disruptions of the transcription factor FOXP1 are implicated in a human neurodevelopmental disorder characterized by autism and/or intellectual disability with prominent problems in speech and language abilities. Avian orthologues of this transcription factor are evolutionarily conserved and highly expressed in specific regions of songbird brains, including areas associated with vocal production learning and auditory perception. Here, we investigated possible contributions of FoxP1 to song discrimination and auditory perception in juvenile and adult female zebra finches. They received lentiviral knockdowns of FoxP1 in one of two brain areas involved in auditory stimulus processing, HVC (proper name) or CMM (caudomedial mesopallium). Ninety-six females, distributed over different experimental and control groups were trained to discriminate between two stimulus songs in an operant Go/Nogo paradigm and subsequently tested with an array of stimuli. This made it possible to assess how well they recognized and categorized altered versions of training stimuli and whether localized FoxP1 knockdowns affected the role of different features during discrimination and categorization of song. Although FoxP1 expression was significantly reduced by the knockdowns, neither discrimination of the stimulus songs nor categorization of songs modified in pitch, sequential order of syllables or by reversed playback were affected. Subsequently, we analyzed the full dataset to assess the impact of the different stimulus manipulations for cue weighing in song discrimination. Our findings show that zebra finches rely on multiple parameters for song discrimination, but with relatively more prominent roles for spectral parameters and syllable sequencing as cues for song discrimination.NEW & NOTEWORTHY In humans, mutations of the transcription factor FoxP1 are implicated in speech and language problems. In songbirds, FoxP1 has been linked to male song learning and female preference strength. We found that FoxP1 knockdowns in female HVC and caudomedial mesopallium (CMM) did not alter song discrimination or categorization based on spectral and temporal information. However, this large dataset allowed to validate different cue weights for spectral over temporal information for song recognition.

Network analysis reveals context-dependent structural complexity of social calls in serrate-legged small treefrogs

Vocal communication plays an important role in survival, reproduction, and animal social association. Birds and mammals produce complex vocal sequence to convey context-dependent information. Vocalizations are conspicuous features of the behavior of most anuran species (frogs and toads), and males usually alter their calling strategies according to ecological context to improve the attractiveness/competitiveness. However, very few studies have focused on the variation of vocal sequence in anurans. In the present study, we used both conventional method and network analysis to investigate the context-dependent vocal repertoire, vocal sequence, and call network structure in serrate-legged small treefrogs Kurixalus odontotarsus. We found that male K. odontotarsus modified their vocal sequence by switching to different call types and increasing repertoire size in the presence of a competitive rival. Specifically, compared with before and after the playback of advertisement calls, males emitted fewer advertisement calls, but more aggressive calls, encounter calls, and compound calls during the playback period. Network analysis revealed that the mean degree, mean closeness, and mean betweenness of the call networks significantly decreased during the playback period, which resulted in lower connectivity. In addition, the increased proportion of one-way motifs and average path length also indicated that the connectivity of the call network decreased in competitive context. However, the vocal sequence of K. odontotarsus did not display a clear small-world network structure, regardless of context. Our study presents a paradigm to apply network analysis to vocal sequence in anurans and has important implications for understanding the evolution and function of sequence patterns.

Novel sound exposure drives dynamic changes in auditory lateralization that are associated with perceptual learning in zebra finches

Songbirds provide a model for adult plasticity in the auditory cortex as a function of recent experience due to parallels with human auditory processing. As for speech processing in humans, activity in songbirds' higher auditory cortex (caudomedial nidopallium, NCM) is lateralized for complex vocalization sounds. However, in Zebra finches exposed to a novel heterospecific (canary) acoustic environment for 4-9 days, the typical pattern of right-lateralization is reversed. We now report that, in birds passively exposed to a novel heterospecific environment for extended periods (up to 21 days), the right-lateralized pattern of epidural auditory potentials first reverses transiently then returns to the typical pattern. Using acute, bilateral multi-unit electrophysiology, we confirm that this dynamic pattern occurs in NCM. Furthermore, extended exposure enhances discrimination for heterospecific stimuli. We conclude that lateralization is functionally labile and, when engaged by novel sensory experience, contributes to discrimination of novel stimuli that may be ethologically relevant. Future studies seek to determine whether, (1) the dynamicity of lateralized processes engaged by novel sensory experiences recurs with every novel challenge in the same organism; (2) the dynamic pattern extends to other cortical, thalamic or midbrain structures; and (3) the phenomenon generalizes across sensory modalities.

Higher-order dialectic variation and syntactic convergence in the complex warble song of budgerigars

Dialectic signatures in animal acoustic signals are key in the identification of and association with group members. Complex vocal sequences may also convey information about behavioral state, and may thus vary according to social environment. Some bird species, such as psittaciforms, learn and modify their complex acoustic signals throughout their lives. However, the structure and function of vocal sequences in open-ended vocal learners remains understudied. Here, we examined vocal sequence variation in the warble song of budgerigars, and how these change upon contact between social groups. Budgerigars are open-ended vocal learners which exhibit fission-fusion flock dynamics in the wild. We found that two captive colonies of budgerigars exhibited colony-specific differences in the syntactic structure of their vocal sequences. Individuals from the two colonies differed in the propensity to repeat certain note types, forming repetitive motifs which served as higher-order signatures of colony identity. When the two groups were brought into contact, their vocal sequences converged, and these colony-specific repetitive patterns disappeared, with males from both erstwhile colonies now producing similar sequences with similar syntactic structure. We present data suggesting that the higher-order temporal arrangement of notes/vocal units is modified throughout life by social learning as groups of birds continually associate and dissociate. Our study sheds light on the importance of examining signal structure at multiple levels of organization, and the potential for psittaciform birds as model systems to examine the influence of learning and social environment on acoustic signals.

Notch1 is involved in cell proliferation and neuronal differentiation in the HVC of zebra finch (Taeniopygia guttata)

Significant sex differences are found in songbirds' song control nuclei and their controlled song behaviors. To elucidate the underlying mechanisms, we explored the role of Notch1 during the development of the high vocal centre (HVC) and song learning in zebra finch. Our study first found that Notch1 positive cells were distributed in HVC with female-biased densities at posthatching day (PHD) 15, but male-biased at PHD 45 and adult. There were about 60 putative oestrogen-responsive elements within 2.5 kb upstream of Notch1, and Notch1 mRNA in the explants that contained the developing male HVC was significantly increased after estrogen addition into the cultured medium for 48 h. After injecting Notch1-interfering lentivirus into the male or female HVC at PHD 15, cell proliferation was significantly promoted in the ventricle zone overlying the HVC at PHD 23. In addition, neuronal differentiation towards Hu+ /BrdU+ at PHD 31, mature neurons (NeuN+/BrdU+) including those projecting to RA in HVC and the sizes of HVC and RA at adult increased significantly after Notch1-interfering lentiviruses were injected into the male HVC at PHD 15. However, the above measurements decreased, following the injection of the lentiviruses expressing Notch intracellular domain (NICD). Finally, the repeat numbers of syllables 'b' or 'c' of learned songs changed after the injection of Notch1-interfering or NICD-expressing lentiviruses into the HVC at PHD15. Our study suggests that Notch1 is related to the development of HVC and song learning in the zebra finch.

Sound properties affect measurement of vocal consistency in birdsong: Validation of the spectrogram cross correlation method (SPCC)

In songbirds, singing with precision (vocal consistency) has been proposed to reflect whole-organism performance. Vocal consistency is measured using spectrogram cross correlation (SPCC) to assess the acoustic similarity between subsequent renditions of the same note. To quantify how SPCC is sensitive to the acoustic discrepancies found in birdsong, we created a set of 40 000 synthetic sounds that were designed based on the songs of 345 species. This set included 10 000 reference sounds and 30 000 inexact variants with quantified differences in frequency, bandwidth, or duration with respect to the reference sounds. We found that SPCC is sensitive to acoustic discrepancies within the natural range of vocal consistency, supporting the use of this method as a tool to assess vocal consistency in songbirds. Importantly, the sensitivity of SPCC was significantly affected by the bandwidth of sounds. The predictions derived from the analysis of synthetic sounds were then validated using 954 song recordings from 345 species (20 families). Based on psychoacoustic studies from birds and humans, we propose that the sensitivity of SPCC to acoustic discrepancies mirrors a perceptual bias in sound discrimination. Nevertheless, we suggest the tool be used with care, since sound bandwidth varies considerably between singing styles and therefore, SPCC scores may not be comparable.

Male song of the Aquatic Warbler, a promiscuous bird without paternal care, is more complex than previously thought

The Aquatic Warbler Acrocephalus paludicola is one of the rarest European passerines and is characterised by promiscuity, lack of pair-bonds and female-only parental care. This makes the species an important model for studying the function of avian courtship song. The song of the Aquatic Warbler consists of whistle and rattle phrases produced as discontinuous A-, B- and C-songs, which are built by a single rattle, a rattle and a whistle, and more than two phrases of both kinds, respectively. The A- and B-songs are thought to be aggressive signals in male-male interactions, while C-songs are thought to be important for female choice. Here, we analysed recordings of 40 individually marked males, and determined the phrase repertoire. The enumerated repertoire (males recorded for ≥ 10 min) ranged from 16 to 158 (mean 99), however, it did not capture the complete phrase repertoires. We then used models from species diversity ecology to estimate the actual phrase repertoire size, which ranged between 18 and 300 phrases (mean 155). The estimated repertoire was predicted by the number of C-songs. The rattle repertoire was larger than the whistle repertoire, and both positively correlated with the number of C-songs. Our study indicates that male Aquatic Warblers have highly complex phrase repertoires that vary widely in size. Their courtship song is flexible and efficient, enabling relative song complexity to be demonstrated in a short sample, thus facilitating both female attraction through the quick presentation of large phrase repertoires and rival deterrence through the production of many short and simple A- and B-songs.

Zebra finches (Taeniopygia guttata) demonstrate cognitive flexibility in using phonology and sequence of syllables in auditory discrimination

Zebra finches rely mainly on syllable phonology rather than on syllable sequence when they discriminate between two songs. However, they can also learn to discriminate two strings containing the same set of syllables by their sequence. How learning about the phonological characteristics of syllables and their sequence relate to each other and to the composition of the stimuli is still an open question. We compared whether and how the zebra finches' relative sensitivity for syllable phonology and syllable sequence depends on the differences between syllable strings. Two groups of zebra finches were trained in a Go-Left/Go-Right task to discriminate either between two strings in which each string contained a unique set of song syllables ('Different-syllables group') or two strings in which both strings contained the same set of syllables, but in a different sequential order ('Same-syllables group'). We assessed to what extent the birds in the two experimental groups attend to the spectral characteristics and the sequence of the syllables by measuring the responses to test strings consisting of spectral modifications or sequence changes. Our results showed no difference in the number of trials needed to discriminate strings consisting of either different or identical sets of syllables. Both experimental groups attended to changes in spectral features in a similar way, but the group for which both training strings consisted of the same set of syllables responded more strongly to changes in sequence than the group for which the training strings consisted of different sets of syllables. This outcome suggests the presence of an additional learning process to learn about syllable sequence when learning about syllable phonology is not sufficient to discriminate two strings. Our study thus demonstrates that the relative importance of syllable phonology and sequence depends on how these features vary among stimuli. This indicates cognitive flexibility in the acoustic features that songbirds might use in their song recognition.

Multi-level combinatoriality in magpie non-song vocalizations

Comparative studies conducted over the past few decades have provided important insights into the capacity for animals to combine vocal segments at either one of two levels: within- or between-calls. There remains, however, a distinct gap in knowledge as to whether animal combinatoriality can extend beyond one level. Investigating this requires a comprehensive analysis of the combinatorial features characterizing a species' vocal system. Here, we used a nonlinear dimensionality reduction analysis and sequential transition analysis to quantitatively describe the non-song combinatorial repertoire of the Western Australian magpie (Gymnorhina tibicen dorsalis). We found that (i) magpies recombine four distinct acoustic segments to create a larger number of calls, and (ii) the resultant calls are further combined into larger call combinations. Our work demonstrates two levels in the combining of magpie vocal units. These results are incongruous with the notion that a capacity for multi-level combinatoriality is unique to human language, wherein the combining of meaningless sounds and meaningful words interactively occurs across different combinatorial levels. Our study thus provides novel insights into the combinatorial capacities of a non-human species, adding to the growing evidence of analogues of language-specific traits present in the animal kingdom.

The social role of song in wild zebra finches

Male songbirds sing to establish territories and to attract mates.^1,2 However, increasing reports of singing in non-reproductive contexts³ and by females^4,5 show that song use is more diverse than previously considered. Therefore, alternative functions of song, such as social cohesion³ and synchronization of breeding, by and large, were overlooked even in such well-studied species such as the zebra finch (Taeniopygia guttata). In these social songbirds, only the males sing, and pairs breed synchronously in loose colonies,^6,7 following aseasonal rain events in their arid habitat.^8,9 As males are not territorial, and pairs form long-term monogamous bonds early in life, conventional theory predicts that zebra finches should not sing much at all; however, they do and their song is the focus of hundreds of lab-based studies.^{10,11,12,13,14,15,16,17,18,19,20,21,22} We hypothesize that zebra finch song functions to maintain social cohesion and to synchronize breeding. Here, we test this idea using data from 5 years of field studies, including observational transects, focal and year-round audio recordings, and a large-scale playback experiment. We show that zebra finches frequently sing while in groups, that breeding status influences song output at the nest and at aggregations, that they sing year round, and that they predominantly sing when with their partner, suggesting that the song remains important after pair formation. Our playback reveals that song actively features in social aggregations as it attracts conspecifics. Together, these results demonstrate that birdsong has important functions beyond territoriality and mate choice, illustrating its importance in coordination and cohesion of social units within larger societies.

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.

Syntax errors do not disrupt acoustic communication in the common cuckoo

When acoustic communication signals are distorted, receivers may misunderstand the signal, rendering it ineffective. Common cuckoos (Cuculus canorus) are popularly known for the males' simple, two-note advertisement calls, the "cu-coo" used for declaring the male's breeding territories. Cuckoos do not learn their calls (vocal non-learners), so they are expected to have a limited ability to produce different acoustic signals. Nevertheless, male cuckoos appear to make syntax errors (e.g., repeated, reversed, or fragmented elements) even in their simple advertisement calls. We conducted a playback experiment with male cuckoos, broadcasting ten call types, including seven modified calls with errors (e.g. "cu-cu", and "coo-cu") and three natural calls used for comparisons ("cu-coo", "cu-cu-coo", and interspecific control). Male cuckoos responded in a manner suggesting that the presence of the first ("cu") note of the natural 2-note "cu-coo" call in any form or combination yield effective signals. However, through the elevated frequency (by about 200 Hz) and greater speed of the "cu" note, the natural 3-note version "cu-cu-coo" call appears to have gained a novel communicative function in signalling with female cuckoos. Thus, syntax errors in calls with the "cu" element are not responsible for changing the function of the male cuckoos' "cu-coo" call.

Long-distance dependencies in birdsong syntax

Songbird syntax is generally thought to be simple, in particular lacking long-distance dependencies in which one element affects choice of another occurring considerably later in the sequence. Here, we test for long-distance dependencies in the sequences of songs produced by song sparrows (Melospiza melodia). Song sparrows sing with eventual variety, repeating each song type in a consecutive series termed a 'bout'. We show that in switching between song types, song sparrows follow a 'cycling rule', cycling through their repertoires in close to the minimum possible number of bouts. Song sparrows do not cycle in a set order but rather vary the order of song types from cycle to cycle. Cycling in a variable order strongly implies long-distance dependencies, in which choice of the next type depends on the song types sung over the past cycle, in the range of 9-10 bouts. Song sparrows also follow a 'bout length rule', whereby the number of repetitions of a song type in a bout is positively associated with the length of the interval until that type recurs. This rule requires even longer distance dependencies that cross one another; such dependencies are characteristic of more complex levels of syntax than previously attributed to non-human animals.

Beyond the Chicken: Alternative Avian Models for Developmental Physiological Research

Biomedical research focusing on physiological, morphological, behavioral, and other aspects of development has long depended upon the chicken (Gallus gallus domesticus) as a key animal model that is presumed to be typical of birds and generally applicable to mammals. Yet, the modern chicken in its many forms is the result of artificial selection more intense than almost any other domesticated animal. A consequence of great variation in genotype and phenotype is that some breeds have inherent aberrant physiological and morphological traits that may show up relatively early in development (e.g., hypertension, hyperglycemia, and limb defects in the broiler chickens). While such traits can be useful as models of specific diseases, this high degree of specialization can color general experimental results and affect their translational value. Against this background, in this review we first consider the characteristics that make an animal model attractive for developmental research (e.g., accessibility, ease of rearing, size, fecundity, development rates, genetic variation, etc.). We then explore opportunities presented by the embryo to adult continuum of alternative bird models, including quail, ratites, songbirds, birds of prey, and corvids. We conclude by indicating that expanding developmental studies beyond the chicken model to include additional avian groups will both validate the chicken model as well as potentially identify even more suitable avian models for answering questions applicable to both basic biology and the human condition.

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'.

Balanced imitation sustains song culture in zebra finches

Songbirds acquire songs by imitation, as humans do speech. Although imitation should drive convergence within a group and divergence through drift between groups, zebra finch songs sustain high diversity within a colony, but mild variation across colonies. We investigated this phenomenon by analyzing vocal learning statistics in 160 tutor-pupil pairs from a large breeding colony. Song imitation is persistently accurate in some families, but poor in others. This is not attributed to genetic differences, as fostered pupils copied their tutors’ songs as accurately or poorly as biological pupils. Rather, pupils of tutors with low song diversity make more improvisations compared to pupils of tutors with high song diversity. We suggest that a frequency dependent balanced imitation prevents extinction of rare song elements and overabundance of common ones, promoting repertoire diversity within groups, while constraining drift across groups, which together prevents the collapse of vocal culture into either complete uniformity or chaos.

Studying how songbirds learn songs can shed light on the development of human speech. An analysis of 160 tutor-pupil zebra finch pairs suggests that frequency dependent balanced imitation prevents the extinction of rare song elements and the overabundance of common ones, promoting song diversity within groups and species recognition across groups.

Evidence for Teaching in an Australian Songbird

Song in oscine birds (as in human speech and song) relies upon the rare capacity of vocal learning. Transmission can be vertical, horizontal, or oblique. As a rule, memorization and production by a naïve bird are not simultaneous: the long-term storage of song phrases precedes their first vocal rehearsal by months. While a wealth of detail regarding songbird enculturation has been uncovered by focusing on the apprentice, whether observational learning can fully account for the ontogeny of birdsong, or whether there could also be an element of active teaching involved, has remained an open question. Given the paucity of knowledge on animal cultures, I argue for the utility of an inclusive definition of teaching that encourages data be collected across a wide range of taxa. Borrowing insights from musicology, I introduce the Australian pied butcherbird (Cracticus nigrogularis) into the debate surrounding mechanisms of cultural transmission. I probe the relevance and utility of mentalistic, culture-based, and functionalist approaches to teaching in this species. Sonographic analysis of birdsong recordings and observational data (including photographs) of pied butcherbird behavior at one field site provide evidence that I assess based on criteria laid down by Caro and Hauser, along with later refinements to their functionalist definition. The candidate case of teaching reviewed here adds to a limited but growing body of reports supporting the notion that teaching may be more widespread than is currently realized. Nonetheless, I describe the challenges of confirming that learning has occurred in songbird pupils, given the delay between vocal instruction and production, as well as the low status accorded to anecdote and other observational evidence commonly mustered in instances of purported teaching. As a corrective, I press for an emphasis on biodiversity that will guide the study of teaching beyond human accounts and intractable discipline-specific burdens of proof.

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.

Individual vocal recognition in zebra finches relies on song syllable structure rather than song syllable order

Many species are able to vocally recognize individual conspecifics and this capacity seems widespread in oscine songbirds. The exact acoustic features used for such recognition are often not clear. In the zebra finch (Taeniopygia guttata), the song motif is composed of a few syllables repeated in a fixed sequential order and song bouts include several repetitions of the motif. Here, we used an operant discrimination task, the GO/NOGO procedure, to show that zebra finches are capable of individual vocal recognition even if the bird has to distinguish males that all produce an imitation of the same song model. Furthermore, we studied whether such individual vocal recognition was based on spectro-temporal details of song syllables, i.e. the local fine structure of the song, or on the sequential order in which song syllables are arranged in the song bout. To this end, we trained male and female zebra finches to discriminate songs of one male conspecific from those of four others. After learning this baseline discrimination, subjects were exposed to a novel set of stimuli originating from the same individuals, in order to test for their capability to generalise. Subjects correctly classified those novel stimuli, illustrating their ability for individual vocal recognition. Then they were exposed to hybrid stimuli combining the syllable sequences of one individual with the spectro-temporal features of another. Behavioural responses of subjects to hybrid stimuli suggest that they rely on spectro-temporal details of syllables and might pay less attention to syllable sequences for individual vocal recognition.

What can animal communication teach us about human language?

Language has been considered by many to be uniquely human. Numerous theories for how it evolved have been proposed but rarely tested. The articles in this theme issue consider the extent to which aspects of language, such as vocal learning, phonology, syntax, semantics, intentionality, cognition and neurobiological adaptations, are shared with other animals. By adopting a comparative approach, insights into the mechanisms and origins of human language can be gained. While points of agreement exist among the authors, conflicting viewpoints are expressed on several issues, such as the presence of proto-syntax in animal communication, the neural basis of the Merge operation, and the neurogenetic changes necessary for vocal learning. Future comparative research in animal communication has the potential to teach us even more about the evolution, neurobiology and cognitive basis of human language. This article is part of the theme issue 'What can animal communication teach us about human language?'