Biology

The relationship between musical structures and birdsong

Abstract

Birdsong has inspired musicians from around the world for many years. These vocalisations are produced by the avian voice-box: the syrinx. Research has discovered many links between music and bird vocalisation through the comparison of musical elements to bird songs and calls.

The objective of this investigative study is to see whether musical structures (including contrast, continuity, and repetition) can be applied to birdsong. Further research is done to investigate whether the complexity of the structure is influenced by the musculature of the syrinx and the habitat of the bird. Syringeal complexity is determined by examining the taxonomy of the birds being studied; sub-orders of passeriformes (songbirds): oscines & sub-oscines are studied. Habitat is considered by categorising the location of birds into open country or forest.

The results show that musical structures can be applied to bird songs. Oscines, proportionally, have significantly more complex songs than sub-oscines. Although there is no statistical significance between open country and woodland birds’ structural complexity, there seems to be a trend that open country birds have more complex structures than woodland birds.

Introduction

Birdsong has inspired many composers from different musical backgrounds over the years. From classic-romantic Beethoven[1] to 20th century Messiaen[2] to modern jazz musician Rothenberg[3], either direct or indirect references to the melodious models of birdsong are featured in many genres of music. Birdsongs are among the most beautiful, complex sounds produced in the natural world [4].

The purpose of birdsong is for communication; Slater defines communication as, ‘the signal that causes modified behaviour of the receiving animal’[4], for example to attract females and serve as a territorial signal to keep out rivals. This signal in birds is in the form of vocalisations that are divided into songs and calls. As a generalisation, songs are long, complex vocalisations produced by males in the breeding season, whereas calls are short , simple and less spontaneous vocalisations [4]. Birdsong is transmitted horizontally (across society) and vertically (from parent to offspring) through a community, leading to cultural evolution: a cultural change described by the Darwinian evolutionary process [5]. The distinct sections of a song are called motifs or phrases. The phrases are built using units called syllables, which are further broken down into blocks called elements. Catchpole defines an element (in this context) as a continuous line on a sonogram [4].

Structure and complexity

Generally, there are five main components of music:

  • melody (frequency changes);
  • harmony (combination of pitches);
  • rhythm (temporal arrangement of elements);
  • articulation (performance techniques); and
  • structure (order of musical concepts or motifs in a song/composition, made up of three components: contrast, repetition, and continuity).

Motifs can contrast with each other by differing in the other four building blocks of music. There may also be contrast within a motif (e.g. changes in frequency through the use of a trill).

Comparisons between music and birdsong have been made previously through the relation of each musical component to birdsong. Melodic structure (the shape of the melody) has been studied using computer software to track the fundamental frequency contours of all notes on sound spectrograms. The frequency contours were converted into pitch contours and melodic shapes were identified [6]. This showed how birdsong follows clear melodic structures.

However, the primary objective of this study is to research a different, motivic aspect of structure in relation to birdsong. This study also provides a formal definition of structural complexity to allow for comparison between habitats and taxonomical sub-order. Structural complexity is determined by the song density: the number of motifs (and variations of motifs) sung in ten seconds; the inter-syllable frequency shift (the difference in frequency between motifs); and inter-syllable gap length (the rest period between motifs where the syringeal muscles realign).

Research shows that the minimum gap duration between syllables is longer when the frequency ratio between the end of one syllable and the start of the next syllable (inter-syllable frequency shift) is large in birds with complex structures (e.g Skylarks (Alauda arvensis)) [7]. This means that birds whose songs have motifs with complex/wide alterations of pitches (known as vocal deviation) in a trill or a series of repeated syllables often have longer gaps between each motif [8]. For motifs where the repeated syllables have unidirectional frequency sweeps, execution may be harder as the vocal tract has to reconfigure in order to repeat the correct starting frequency like the Banded Wren (Thryothorus pleurostictus) [8] [9]. Therefore, songs with a higher vocal deviation may have longer inter-syllable gaps. In order to increase syllable production, and therefore song density, some birds have consecutive up and down frequency sweeping syllables in their trills. As the vocal tract does not need to substantially reconfigure, they are able to shorten gaps between syllables.

Studies involving the other four musical elements

Many studies have been done in comparison and application of melody, harmony, rhythm and articulation to birdsong. The study would enable the judgment of motif formation based on the musical characterization of each motif/theme. It is clear that melodic contours can individualise motifs through the changes in frequencies and the use of intervals (leaps between notes). For example, the Black-capped Chickadee (Poecile atricapillus) sings in two pitches usually separated by a minor third interval. The Eastern Peewee sings up an augmented minor third [10]. Although most birdsong does not involve direct harmonic relationships [11], counter-singing (antiphonal dialoguing between individuals) and doubling (when individuals sing the same part simultaneously) can provide the harmonic basis of identification of each motif. The song of the Brown Thrasher provides a good example of this ability as the bird switches sound production back and forth from side to side, even within a single syllable, or uses both sides of the syrinx to produce non-harmonically related sounds [12]. Some birds even have the ability to sing rising and falling notes simultaneously, like the Wood Thrush (Hylocichla mustelina) in its final trill.

Research shows that some birds do not discern rhythm well. Zebra Finches (Taeniopygia guttata), for example, seem to pay attention to pauses between notes on short time scales but have trouble recognizing overarching rhythmic patterns [13] [14]. This leads to the belief that birds interpret sound in terms of pitch rather than rhythm fragments. Considering that both building blocks are essential for music, the lack of the latter suggests that birdsong’s structure cannot be defined by rhythm. The articulatory component of motifs can be used to recognise their individuality. Qualitative observations by Charles Hartshorne show that birds adopt various performance techniques for each motif. Here are some of his documented examples: accelerando (gradually getting faster) in the Field Sparrow (Spizella pusilla) and diminuendo (gradually getting quieter) in the Misto Yellowfinch (Sicalis luteola) of South America [15] [16].

The syrinx, passeriform subdivision and birdsong

Birds produce sound using a uniquely avian vocal organ, the syrinx, which is located at the “bifurcation of the trachea into the two primary bronchi of the lungs” [17]. In general, passerines (a taxonomical order of birds also known as the songbirds or perching birds) have more complex syrinxes than non-passerines. Within the passerines, there is a subdivision between the oscines (with complex syrinxes) and the tyranni or sub-oscines (with simple syrinxes). The syrinx of sub-oscines is restricted to the trachea and therefore these birds are often known as tracheophonae [18]. Both oscines and suboscines have a prominent sternotracheal muscle (see Figure 1). The oscines or ‘true song-birds’ have highly developed songs due to their syringeal muscles, which translate the motor commands into syringeal behaviour, which, in turn, produces the song [17].

In addition to this, respiratory muscles affect the acoustic properties of vocalisation by adjusting the pattern, frequency and amplitude of ventilation [19]. Sound can also be modified by the vocal tract in oscines. This was demonstrated by observing the effect of the inhalation of a light gas mixture of helium and oxygen, in birds, on the spectral properties of the song. The light gas mixture increases the speed of sound production and the experimental results indicated that songbirds are able to vary the tuning or resonant frequency of their vocal tract as they sing [12][19]. In oscines, each side of the syrinx is independently controlled, allowing birds to produce two harmonically unrelated pitches at once; this is known as lateralization [20]. The learnt songs are another major distinction between the two suborders within the passerines. In sub-oscine passerines, vocalizations are almost entirely innate and in the oscine passerines songs are largely learned through a process known as vocal learning [21]. This is because of sub-oscines’ lack forebrain neural specializations, known as the song system. The song system is involved in vocal learning and is prominent in the oscine suborder [22]. The second objective of this study is to compare the differences in structural complexities of the songs of the passeriform subdivisions – the oscines and the sub-oscines.

This image has an empty alt attribute; its file name is bird.png

Figure 1: Diagram and position of the syrinx. The sternotracheal muscle is circled in green. Modified from Bird Academy.

Habitat and birdsong

Birds living in dense vegetation or needing to communicate over long distances often use low frequency sounds [23]. This is because low-frequency sounds travel longer distances and are less likely to be affected by interference than higher frequencies. Simple calls (e.g whistles) tend to be less affected by dense vegetation than complex songs (e.g buzzes). Hence, many forest birds have songs that are simple whistles. In contrast, complex buzz-like songs have advantages in open habitats because they are not distorted by, for example, windy conditions. Hence many grassland and open country birds (including various sparrows) have complex buzzy songs. The ‘Acoustic Adaptation Hypothesis (AAH)’ [23] suggests that selection dependent on habitat structure has shaped the evolution of acoustic properties of birdsongs. The AAH predicts that songs with low frequencies and modulations (whistles), long elements, and inter‐element intervals should be prevalent in forested habitats, while high frequencies and modulations (trills), short elements and inter-element are expected in open country habitats [23]. Though most of this was confirmed in an experiment, the results also showed that habitat has no effect on the rhythm (i.e. temporal arrangements of the elements of the songs) [24]. Closed, wooded habitats may cause greater attenuation (reduction) in songs, and reverberation is thought to influence the length of notes and the amount of time between notes [25]. Therefore, birds that live in habitats with heavy foliage (i.e, woodland) may adopt the following strategies: avoid using rapidly modulated (varying amplitude or frequency) signals; use shorter notes; and put more space between notes to mitigate reverberations [25]. The third objective of this study is to compare the differences in structural complexities of the bird songs in the same genus but at different habitats (i.e. Open Country and Woodland).

Methods

In order to identify and organise the birdsongs into structures, a list of bird species was created (Appendix 1). These passerine birds were organised into two habitat types: Open Country and Woodland. This allowed for comparison of the complexity of structure based on habitat location. In addition to this, the species were further divided into two categories: oscines and sub-oscines. This allowed for comparison between birds with developed and birds with simple syrinxes.

Forty open country passerines were matched with forty forest passerines of the same genus. Of these 40 pairs, 30 pairs were oscines and 10 pairs were sub-oscines. The recordings of the sub-oscines used were South American birds from the Furnariidae (ovenbirds) and Tyrannidae (tyrant flycatchers) families. These were also organised into Open and Woodland categories.

All the bird song recordings were obtained from the approved birding website – www.xeno-canto.com. Each recording and sonogram had a ten-second time-span to ensure consistency across all species. Using the sonogram of each recording (three recordings per species to reduce error), as well as the actual audio, a structure was identified. Structural identification was achieved by giving each motif an alphabetical letter (e.g: A, B or C). If there was a variation of one motif (i.e. a slightly more florid/ ornamented melody featuring the same overall contour), a subscript number to denote the number of the variation with reference to the original motif was given. (see Figure 2). If other features could be identified, a qualitative musical description was given. The presence of a unidirectional frequency sweep (indicating a large inter-syllable frequency shift) and the gap length between motifs were also recorded. Each sonogram lasted for ten seconds, therefore, by calculating the number of motifs in ten seconds, the song density was recorded. The differences in structural complexities of the

This image has an empty alt attribute; its file name is bird2.png

Figure 2: Analysis of the Eurasion Wren song (Troglodytes troglodytes). Motifs divided with lines. Modified from xeno-canto.

bird songs between the different habitats and between the passeriform subdivisions were analysed. Fisher’s exact test was performed on the results to test for significance using a 2×2 contingency table with 1 degree of freedom on QuickCalcs Instat-2018 GraphPad Software [27]. As only 10 pairs of sub-oscines were studied, in comparison to the 30 pairs of oscines, all sub-oscine data was multiplied by a factor of three. The data was iterated proportionally to match the other dataset.

Results

Structure and birdsong

Structures determined by changing, repeating, and connected motifs could be identified in all songs. Some birdsongs consisted of similar phrases differing in only one musical element, however, this still impacted the overarching structure. Some birds, such as the Yellowhammer (Emberiza citrinella), use a continuous stream of clicks for their song. By considering each phrase to be a ‘section’, the Yellowhammer’s song is comprised of a single A section (distinguishable phrase)[4] with a three to four second gap between each repeated A section. Although the song had two of the three components of musical structure – repetition and continuity – it lacked structural contrast as there was only one phrase. However, contrast may not necessarily be between motifs; it can also apply to contrast within a motif. By altering the pitch of the 2-note clicked trill, the Yellowhammer achieved motivic contrast within its song.

The song of the Bolivian Earthcreeper (Tarphonomus harterti) consisted of a single note repeated 6 times a second. The contrasting feature was the increase in tempo and the increase in dynamics. By manipulating the performance techniques (articulatory component), this sub-oscine was able to provide contrast.

The most frequent form of contrast observed in this study was through variation in frequency. Every song tested involved repetition of at least one motif. Interestingly, almost all oscines and sub-oscines used silence to connect sections together. Some oscines, like that of the Eurasian Wren (Troglodytes troglodytes), seamlessly connected all the phrases, as these motifs are a stream of rapid clicks differing only in pitch and contour.

Taxonomic sub-order and birdsong

Observations of the sub-oscines showed that songs often only contained one or two motifs with longer gap lengths. Large inter-syllable frequency shifts were more common in oscines than in sub-oscines. The number of motifs per ten-second period (i.e. song density) for oscines and sub-oscines is shown Table 3a. The iterated data (not shown) was tested for statistical significance. The Fisher’s exact test showed that the association between taxonomic sub-order and motif number was statistically significant. The two-tailed P value=0.0035.

 

Number of Motifs per ten seconds- Song Density

(including variation)

1-2

3+

Oscines

41

19

Sub-oscines

17

3

Table 3a: Table comparing the number of motifs per ten seconds (i.e. song density) based on taxonomical sub-division

Habitat and birdsong

Open country birds tended to have shorter gap lengths and often connected motifs with a continuous stream of clicks. Larger inter-syllable frequency shifts were also more common in open country birds in comparison to their woodland cousins. However, there were some anomalies. The European pied flycatcher (Ficedula hypoleuca) had a more complex structure involving a larger number of motifs (including variations) than its open country counterpart, the European robin (Erithacus rubecula). Another anomalous result was the Fieldfare (Turdus pilaris), whose song not only had a higher song density but also had shorter gap lengths in comparison to its pair, the Redwing (Turdus iliacus). The number of motifs per ten-second period for open country and woodland birds is shown in Table 3b. The Fisher’s exact test showed no statistical significance between habitat and motif number. However, there seems to be a trend towards birds living in open country habitats having more complex structures based on song density.

 

Number of Motifs per ten seconds- Song Density

(including variation)

1-2

3+

Open

27

13

Woodland

31

9

Table 3b: Table comparing the number of motifs per ten seconds (i.e. song density) based on habitat in passerines

The structural complexities of the birdsongs, based on song density, inter-syllable frequency shifts and inter-syllable gap lengths, from lowest to highest is as follows: sub-oscine woodland, sub-oscine open country, oscine woodland and oscine open country.

Discussion

From this study it can be concluded that bird songs have a musical structure. By altering the pitch/melodic contour of each motif, birds are able to provide the contrast needed for their song. Repetition of motifs was observed in all analysed songs. The use of gaps between motifs is most likely due to respiratory requirements and recalibration of syrinx muscles. These gaps act as bridging points between motifs and thus ensure continuity of the song. These qualitative observations allow a reasonable conclusion to be drawn about the application of musical structures to birdsong, as stated in the objectives of this experiment. Although some songs had very little contrast and appeared to be anomalous, slight deviations in frequency permitted the identification of variated motifs.

The statistical analysis showed that oscines have structurally more complex songs in comparison to sub-oscines thus, supporting the original hypothesis. One explanation for this trend is that oscines have more developed syringeal muscles and a lack of syrinx restriction. The shorter gap lengths in oscines may be due to their refined vocal and respiratory apparatus allowing for well-controlled vocalisations. Another possible explanation may be that the oscines’ ability to learn songs during the critical period when nestlings listen to the adults singing around them[28]. This ability to learn songs through recognition and imitation may allow oscines to amalgamate a larger number of motifs which they can present. In doing so, they obtain a higher song density and, therefore, a more complex structure. The results from this experiment are in agreement with current avian anatomical knowledge and provide a practical usage to the theories about differing musculature distinguished by taxonomical sub-divisions. If this study was to be redone, a larger number of sub-oscines would be studied to avoid the need to iterate the data for comparison. Additionally, a larger number of species would be studied to prevent outliers from skewing the data. Musculoskeletal examinations could be done to see the exact movements of the sternotracheal muscle of the syrinx for each motif and study how this differs between oscines and sub-oscines, as it is the only prominent syringeal muscle common to both.

Although there is no statistical significance, the data collected suggests a trend in the relationship between habitat and structural complexity as predicted by the AAH. This may be because of cultural evolution. Birdsongs are transmitted vertically from parent to offspring as well as horizontally between individuals of a population. This means regional dialects and stylistic traits can form in the song, leading to cultural evolution [29][30]. This could explain why birds living in different habitats have adapted their musical structures, by means of cultural evolution, to communicate effectively depending on their environmental circumstances. These results can be used to monitor the effect of deforestation and habitat loss. The musical structures of woodland birds may increase in complexity (through cultural evolution) to adapt to the lack of vegetative interference. Anomalous results between pairs may be due to the age of observed bird and coordination of their syrinx. Contextual information (i.e. if the song is performed for courtship, territorial, communicatory purposes, etc.) may also impact the structural complexity of the song.

Limitations of the study

It was difficult to pair oscines of the same genus living in different habitats. This was because the two species of the same genus often had similar characteristics like for example an affiliation for water. This meant that their habitats were likely to be similar if not, the same. However, the final compiled list of genus-matched birds was verified by a professional ornithologist. Another limitation was the quality of audio recording. Some audio entries in xeno-canto.com have been edited; therefore, resulting in potential inaccuracies in the structural analysis. In addition, background noise and disturbance could have affected the visual sonogram and audio quality. However, the use of three recordings minimised the effects of faulty recordings. Apart from this, some birdsongs consisted of only one note/motif; thus, making it almost impossible to present a structure. However, careful analysis of the sonogram allowed for motivic identification.

Conclusion

The observations of this study suggest that birdsongs do follow musical structures where structure can be assigned to songs with contrast, continuity and repetition. Sub-oscines have less complex structures than oscines due to their morphologically primitive syrinx. The complex musculature, robust respiratory apparatus and prominent vocal tract of oscines enables them to produce a higher number of motifs per ten second/ song density. Passerines living in open country usually have more complex songs than birds living in forest habitats regardless of whether they are an oscine or sub-oscine. The results of the study concur with the AAH even though there was no statistical significance observed, possibly due to small numbers studied.

In future investigations, it may be beneficial to study the song and call structures of other orders of birds (i.e. non-passerine orders like Columbiformes) to see if musical structures could be applied to those as well. Additionally, further investigations could be done into the varying habitats (from small-scale ecological niches to global biomes) and their effect on the song’s structure. Future studies may choose to use computer software (such as Sounanalysispro) to discern the motifs of bird vocalisations, rather than manual identification, as the software could be more efficient. For the purposes of this investigation, manual identification was done due to financial constraints. It would be beneficial to record all birdsongs with the same recording device and to ensure that all the individual birds are roughly of the same age as research has shown that age has a positive correlation with individual quality amongst birds [25]. Thus, a future study could analyse the correlation between the structural complexity of a bird song and the age of a bird.

Acknowledgments

My heartfelt thanks go to Dr Michael Brooke (Strickland Curator of the Natural History Museum, University of Cambridge) for his guidance and insightful suggestions through this project.

I would also like to express my sincere gratitude to the staff at Bablake School, Coventry, for their continued support in my academic career.

A special thank you to my mother and grandmother for all their encouragement, help and chauffeuring!

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About the Author

This image has an empty alt attribute; its file name is Sathvika-Krishnans-Profile-Picture-1-730x1024.jpg

Sathvika Krishnan is a Year 13 student at Bablake School, Coventry, where she is also the School Captain. She is passionate about the welfare of the environment and is an aspiring zoologist. Aside from her academic interests, Sathvika is an avid South-Indian classical vocalist and Western diploma-level pianist. Additionally, she enjoys acting and public speaking and is Biology Ambassador for YSJ.

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