Versatility and Stereotypy of Free-Tailed Bat Songs

In mammals, complex songs are uncommon and few studies have examined song composition or the order of elements in songs, particularly with respect to regional and individual variation. In this study we examine how syllables and phrases are ordered and combined, ie “syntax”, of the song of Tadarida brasiliensis, the Brazilian free-tailed bat. Specifically, we test whether phrase and song composition differ among individuals and between two regions, we determine variability across renditions within individuals, and test whether phrases are randomly ordered and combined. We report three major findings. First, song phrases were highly stereotyped across two regions, so much so that some songs from the two colonies were almost indistinguishable. All males produced songs with the same four types of syllables and the same three types of phrases. Second, we found that although song construction was similar across regions, the number of syllables within phrases, and the number and order of phrases in songs varied greatly within and among individuals. Last, we determined that phrase order, although diverse, deviated from random models. We found broad scale phrase-order rules and certain higher order combinations that were highly preferred. We conclude that free-tailed bat songs are composed of highly stereotyped phrases hierarchically organized by a common set of syntactical rules. However, within global species-specific patterns, songs male free-tailed bats dynamically vary syllable number, phrase order, and phrase repetitions across song renditions.     

Complexity,Predictability and Time Homogeneity of Syntax in the Songs of Cassin’s Vireo (Vireo cassinii)

Many species of animals deliver vocalizations in sequences presumed to be governed by internal rules, though the nature and complexity of these syntactical rules have been investigated in relatively few species. Here I present an investigation into the song syntax of fourteen male Cassin’s Vireos (Vireo cassinii), a species whose song sequences are highly temporally structured. I compare their song sequences to three candidate models of varying levels of complexity–zero-order, first-order and second-order Markov models–and employ novel methods to interpolate between these three models. A variety of analyses, including sequence simulations, Fisher’s exact tests, and model likelihood analyses, showed that the songs of this species are too complex to be described by a zero-order or first-order Markov model. The model that best fit the data was intermediate in complexity between a first- and second-order model, though I also present evidence that some transition probabilities are conditioned on up to three preceding phrases. In addition, sequences were shown to be predictable with more than 54% accuracy overall, and predictability was positively correlated with the rate of song delivery. An assessment of the time homogeneity of syntax showed that transition probabilities between phrase types are largely stable over time, but that there was some evidence for modest changes in syntax within and between breeding seasons, a finding that I interpret to represent changes in breeding stage and social context rather than irreversible, secular shifts in syntax over time. These findings constitute a valuable addition to our understanding of bird song syntax in free-living birds, and will contribute to future attempts to understand the evolutionary importance of bird song syntax in avian communication.     

Sexually dimorphic phrase organization in the song of the indris (Indri indri)

Animal acoustic communication often takes the form of complex sequences, composed of multiple distinct acoustic units, which can vary in their degree of stereotypy. Studies of sequence variation may contribute to our understanding of the structural flexibility of primates' songs, which can provide essential ecological and behavioral information about variability at the individual, population, and specific level and provide insights into the mechanisms and drivers responsible for the evolutionary change of communicative traits. Several methods have been used for investigating different levels of structural information and sequence similarity in acoustic displays. We studied intra and interindividual variation in the song structuring of a singing primate, the indri (Indri indri), which inhabits the montane rain forests of Madagascar. Indri groups emit duets and choruses in which they combine long notes, short single units, and phrases consisting of a variable number of units (from two to six) with slightly descending frequency. Males' and females' contributions to the song differ in the temporal and frequency structure of song units and repertoire size. We calculated the similarity of phrase organization across different individual contributions using the Levenshtein distance, a logic distance that expressed the minimum cost to convert a sequence into another and can measure differences between two sequences of data. We then analyzed the degree of similarity within and between individuals and found that: (a) the phrase structure of songs varied between reproductive males and females: female structuring of the song showed a higher number of phrases if compared to males; (b) male contributions to the song were overall more similar to those of other males than were female contributions to the song of other females; (c) male contributions were more stereotyped than female contributions, which showed greater individual flexibility. The picture emerging from phrase combinatorics in the indris is in agreement with previous findings of rhythmic features and song repertoire size of the indris, which also suggested that female songs are potentially less stereotyped than those of males.     

Experimental evidence that distinct song phrases in the Grey‐cheeked Fulvetta Alcippe morrisonia permit species and local dialect recognition

Birds with small song repertoires have a limited number of song types which may serve multiple functions. The Grey‐cheeked Fulvetta Alcippe morrisonia is a non‐migratory species of lowland forests in Taiwan. Its song consists of two distinct phrases: a whistled phrase and a harmonic one. Each individual usually sings only one type of whistled phrase, and the geographical patterns of songs can be distinguished by the motif of this phrase. We proposed a dual adaptation hypothesis for the functions of these two phrases. Playback experiments including six sound stimuli (familiar whistled phrases, familiar harmonic phrases familiar complete songs, foreign whistled phrases, foreign harmonic phrases and foreign complete songs) were conducted at 12 sites in the Shoushan Nature Park of Kaohsiung, Taiwan. Grey‐cheeked Fulvettas came closer to the playback speaker and increased counter‐singing more in response to the familiar whistled phrase (local dialect) than to a foreign whistled phrase (distant dialect). However, birds responded equally to the playback of harmonic stimuli from familiar and foreign sources. We suggest that in this species, whistled phrases are used for local recognition and harmonic phrases are used for species recognition in short‐range communication.     

Structure,syntax and “small-world” organization in the complex songs of California Thrashers (Toxostoma redivivum)

We describe songs of the California Thrasher (Toxostoma redivivum), a territorial, monogamous species whose complex songs are composed of extended sequences of phonetically diverse phrases. We take a network approach, so that network nodes represent specific phrases, and links or transitions between nodes describe a subgroup structure that reveals the syntax of phrases within the songs. We found that individual birds have large and largely distinct repertoires, with limited phrase sharing between neighbours and repertoire similarity decaying between individuals with distance apart, decaying also over time within individuals. During song sequences, only a limited number of phrases (ca. 15–20) were found to be actually “in play” at any given time; these phrases can be grouped into themes within which transitions are much more common than among them, a feature contributing to a small-world structure. It appears that such “small-world themes” arise abruptly, while old themes are abandoned more gradually during extended song sequences; most individual thrashers switch among 3–4 themes over the course of several successive songs, and some small-world themes appear to have specific roles in starting or ending thrasher songs.     

The Mechanics of Duetting in a New Zealand Endemic, the Kokako (Callaeas cinerea wilsoni): Song at a Snail's Pace

New Zealand's endemic, duetting kokako (Callaeas cinerea wilsoni) produce one of the longest known bird songs (ca 30 s) and duets that differ strikingly from those of most duetters in their unusual length and non‐repetitive structure, long pauses between component phrases, and the great flexibility in sex roles. Here we present a structural analysis of the vocalizations of 17 kokako pairs collected during natural song bouts and in response to conspecific playback, to gain insight into the functional role of this extraordinary vocal behavior. Males tend to sing a greater proportion of the duet than females. Like many duetting species, kokako have a moderately sized repertoire of phrases (mean repertoire size =18) and pair members tend to sing antiphonally rather than in unison. Sharing of phrase types is high among neighboring kokako ( = 86%) and repertoires are not sex specific, as is typical of some but not all duetting species. Timing characteristics, broad sharing of phrase types, and countersinging behavior strongly suggest that kokako duets play an important role in territory defense. Additionally, differences in pairs’ sex role and phrase sequence flexibility suggest that these aspects of duet performance may reflect pair‐bond length or commitment, and require a time investment by pair members.     

What exactly is ‘local song’ in a population of ortolan buntings with a common dialect?

Previous studies have shown that ortolan buntings (Emberiza hortulana) exhibit apparent dialect variation. Neighbouring males typically share the same final phrase of a simple two-part song. Consequently, the final phrase was considered to be the dialect cue important for discriminating between males from a local population and strangers. Recently, it was shown that in an isolated and fragmented population of the ortolan buntings in Norway there was no local dialect in the above-mentioned sense. Norwegian males often had song types with different final phrases in their repertoire, and had larger repertoires and a lower level of song type sharing than in other populations. It was experimentally revealed that only local songs (L) evoked a strong response in Norwegian males, while these birds did not respond strongly to foreign (F) or hybrid songs composed of local and foreign initial and final phrases in any composition (i.e. both FL and LF songs). These results suggest that, in the Norwegian population, the final phrase of the song is not a sufficient cue for local song dialect recognition. This paper is a further study in which we tested the response of the ortolan bunting males to L, F, FL and LF songs in a typical continuous population (in Poland) in which males share the same single final phrase, i.e. have a common dialect. We found that ortolan bunting males in Poland responded with similar strength to L, FL and LF songs. The majority of males did not respond as strongly to playback of only F songs. Our results suggest that a common final phrase for a population should not be treated as the only signal of ‘locality’. Our results show clear asymmetry in response to hybrid songs with non-local phrases in different populations.     

Is female preference for male repertoires due to sensory bias?

There are several possible explanations for the female preference for male repertoires in birds. These males are older, and have better territories; thus there are functional reasons for females to prefer these males. However, there is an alternative explanation; females may habituate less quickly to song repertoires than single songs. I tested whether females have a non-functional, sensory bias for male song repertoires, by testing female preference for a repertoire in zebra finches (Taenopygia guttata), a species in which males possess a single stereotyped song. Females chose between a male repertoire of four different phrases created from the song phrase of one individual and that of one of those phrases repeated four times (natural zebra finch song). Females were also given a choice between the above repertoire and a song made from the phrases of four related males (''family'' stimulus). I tested female preference by training females to press a button for presentation of a song stimulus, and counting the number of button presses. Females preferred the song repertoire to a single phrase song, and did not differentiate between the repertoire and song phrases from four males. Evidence from the Estrildidae indicates that having a single song is the ancestral state for zebra finches, so the preference is not ancestral.     

Mechanisms of song differentiation in introduced populations of Chaffinches Fringilla coelebs in New Zealand

Chaffinches Fringilla coelebs were introduced to New Zealand from Great Britain over 100 years ago, and since then their songs have diverged from British ones in both syllable structure and basic organization of syllable sequences. The New Zealand populations seem to be unique in that their songs have undergone differentiation of trill segments by progressive elaboration of syllable morphology and hp ascending and descending changes of pitch. Published sonagrams of British song types have significantly more trill phrases and significantly fewer syllables in the end phrase than do Yew Zealand ones. Many New Zealand song types have much elaborated end phrases and concomitantly simpler trill segments, with songs quite commonly having only one trill phrase. This reversal of complexity between the trill and end phrase in New Zealand seems to have been derived by progressive reduction of the ultimate trill phrase to one syllable, and by recombination of end phrase syllables from different song types into one compound end phrase. The significance of the increased complexity in the end phrases of many song types may relate to the sound transmission properties of dense pine forests in New Zealand, in which Chaffinches are ubiquitous. Elaborate end phrases degrade much less from reverberation in pine forests because individual syllables have more dispersed temporal patterning.
Recombination of syllables to form new song types is a major mechanism of song differentiation in New Zealand. Although whole song copying is the predominant mode of replication, very few song types in a locality sample are composed of a unique set of syllables. Rather, song types in an area are interconnected by different combinations of shared syllables, suggesting that the incorporation of some local syllables in a bird's repertoire is sufficient to signal its status as a member of a neighbourhood and also allows the evolution of broadcast complexity.     

Song characterization in the spectacled warbler (Sylvia conspicillata): a circum-Mediterranean species with a complex song structure

The spectacled warbler (Sylvia conspicillata) is a small passerine with a patchy distribution throughout the circum-Mediterranean region, including the North Atlantic archipelagos of Madeira, Canary Islands and Cape Verde. Here we characterize the species song structure on the island of Fuerteventura, quantifying repertoire size, inter- and intra-individual spectrographic variation, to determine whether acoustic variation occurred within an island population. Male song display was organized in song bouts of a variable number of song phrases, which in turn were made up of 4–69 syllables. We classified syllable types to derive a measure of repertoire size (number of different syllables) per song bout, and then used rarefaction methods to calculate the estimated repertoire size for our population of males. Three categories of song bout length were considered in analyses: short song bouts of 10 phrases, average bouts of 19 phrases and long bouts of ≥ 29 phrases. The observed and estimated repertoire size per male (between 43 and 126 syllables per male) increased with song bout duration, although the relationship was not significant for the estimated values. To test whether songs could be individually specific, we measured 11 spectrotemporal parameters of the song. A discriminant analysis using these variables performed poorly in classifying songs to the individuals that uttered them, but we found less variation in the individual than in the population for three out of the 11 variables. These individually specific variables, involving the first or the most common syllable of the song, the trill, were the duration of the first syllable of the phrase, the duration and the dominant frequency of the trill syllable. Our study emphasizes the complexity of spectacled warbler songs, in which males continuously add novel syllables over the entire song bout. This complexity appears to be determined by individual innovation capabilities rather than by the behaviour of copying neighbour repertoires, since songs of close birds were not more similar than songs from far-away territories.     

The Role of Song Phonology and Syntax in Vocal Learning Preferences in the Song Sparrow,Melospiza melodia

Are young songbirds ready to learn virtually any song, or are they predisposed to learn songs of their own species? To explore this question tests were conducted on the equipotentiality of auditory song learning stimuli in the song sparrow. 23 males reared as nestlings were exposed to tape recordings of their own and other species songs in early life and subsequent song production was analyzed for imitations. Birds exposed to natural song sparrow songs, including their fathers', and equal numbers of swamp sparrow songs, strongly preferred conspecific songs. They neither favored nor eschewed paternal songs despite having had access to them for 6–10 days as nestlings. In three other experiments synthetic songs were used in which some properties were held constant and others were systematically varied. Birds were exposed to 1–4 segmented songs varying in phrase order, tempo and syllable number, each synthesized in two versions, one from conspecific and the other from heterospecific (swamp sparrow) song syllables. With one-segmented songs (alien syntax) subjects favored conspecific over heterospecific syllable songs. Heterospecific syllables were rendered more acceptable by incorporation into two-segmented trilled songs (more song sparrow-like syntax). Heterogeneous summation of phonological and syntactical cues appeared to occur. There was also evidence of interaction between phonology and syntax. When another phrase type, the note complex, was added, in three- and four-segmented songs, a preference for conspecific syllables reappeared. Heterospecific syllables may be more readily accepted as a trilled sequence than without repetition, as in a note-complex. When phrase structure within four-segmented songs was varied, birds favored patterns most like normal conspecific song. We conclude that there are innate learning preferences in the song sparrow, based on note and syllabic structure (phonology and syllabic syntax), and temporal organization of phrases (segmental syntax), differing from those of the closely related swamp sparrow, Melospiza georgiana, in which song syntax plays no role in learning preferences.     

Signalling through acoustic windows: nightingales avoid interspecific competition by short-term adjustment of song timing

The function of bird song is closely linked to sexual selection through female choice and male–male competition, and thus variation in communication success is likely to have major fitness consequences for a singing male. A crucial constraint on signal transmission is imposed by background noise, which may include songs from other species. I investigated whether singing nightingales (Luscinia megarhynchos) avoid temporal overlap with the songs of other bird species in a playback experiment. I analysed the temporal song patterns of six males, each of which were exposed to songs of other species. The nightingales significantly avoided overlapping their songs with the playback songs, and started singing preferentially during the silent intervals between the heterospecific songs. This timing of song onset produced a greater variability in pause duration compared to the nightingales’ undisturbed solo singing. These findings suggest that birds adjust their song timing to avoid acoustic interference on short temporal scales, and thus are able to improve the efficiency of acoustic communication in complex sonic environments. Moreover, the results indicate that temporal song patterns can be affected by the songs of other species, and thus such influences should be taken into account when studying bird song in the field.     

Songs of Holcostethus strictus (Fabricius): a different repertoire among landbugs (Heteroptera: Pentatomidae)

Songs emitted during mating by male and female Holcostethus strictus were recorded as substrate vibrations. Spectra of the vibrational signals have a dominant frequency peak between 100 and 260 Hz and in this respect reflect the general characteristic of the family Pentatomidae. Songs of H. strictus differ from the song repertoire of the southern green stink bug Nezara viridula (L.) (Pentatomidae) in many respects. The female calling and courtship songs differ in echeme and phrase duration. The male calling song is composed of spectrally different subunits. The male courtship song is characterised by three types of spectrally and temporally different echemes. The male copulatory song is composed of echemes of two types, which constitute a phrase of less regular temporal structure. In H. strictus, males start to sing first and female songs are less complex than in N. viridula. The female calling song is evoked by male calling and does not trigger male response. The female and male courtship song phrases are superimposed on one another and we have not observed any obvious regularity in their exchange. The possible role of different songs in H. strictus is discussed and compared with that in other pentatomide landbug species.     

Differences in Frequency of Shared Song Types Enables Neighbour‐Stranger Discrimination in a Songbird Species with Small Song Repertoire

Acoustic Neighbour‐Stranger (N‐S) discrimination is widespread in birds and has evolved to settle territorial disputes with low costs. N‐S discrimination was found both in song‐learning oscines and non‐song‐learning bird taxa, irrespective of the repertoire sizes they have. Therefore, it seems that more than just a single mechanism enable N‐S discrimination. Species with larger repertoires, where males have unique phrases or syllables may rely on such interindividual differences. The majority of birds have rather small repertoires, which often are shared among neighbours. In this case, males are facing the problem of individual recognition when rivals produce songs, at least superficially, identical. To better understand the acoustic basis of N‐S discrimination in species with small and shared repertoires, I studied the ortolan bunting (Emberiza hortulana). Males of this small oscine species are able to N‐S discrimination based on a single song rendition when presented in a playback experiment, regardless of song‐type diversity and song‐sharing level within a particular population. It was also found that songs of the same type sung by different males differ in the frequency of the initial song phrases and these differences persist over years. Here, I tested whether males are able to discriminate among the natural songs and the artificially modified songs of their neighbours in which the frequency was experimentally changed by relatively small value in comparison with the variation range found in this population. Subjects responded significantly more aggressively to the songs with an artificially modified frequency, suggesting that males treat such songs as having come from the repertoire of a non‐neighbour. These results confirm an earlier prediction that differences in the frequency of shared song types enable N‐S discrimination. The study presents one of the possible mechanisms enabling N‐S discrimination in songbirds with small repertoires and stress the role of within‐song‐type variation, which is still understudied song characteristic.     

Species Recognition by Song in the Veery (Catharus fuscescens: Aves)

Territorial male veeries (Catharus fuscescens) were presented with three series of reorganised songs to determine the functional properties of song structure. Songs consist of three parts: a low frequency initial phrase, a higher frequency middle phrase, and again a lower frequency end phrase. The latter two consist of vibrato syllables. Three series of experiments were performed, using various combinations of song syllables. In Series I, songs consisted of repetitions of a single syllable, one from each part of the song. Veeries responded to syllables from the middle and end phrases as to normal songs, both in localisation and in vocal responses. However, they showed little localisation and vocalisation responses to the introductory syllables. In Series II and III, veeries responded significantly more to songs in which syllables from the middle and end phrases were presented in proper sequence, rather than not. The minimum requirements for intraspecific recognition were determined in relation to the number and order of syllables in the song.     

Buzzwords in Females’ Ears? The Use of Buzz Songs in the Communication of Nightingales (Luscinia megarhynchos)

Differences in individual male birds’ singing may serve as honest indicators of male quality in male-male competition and female mate choice. This has been shown e.g. for overall song output and repertoire size in many bird species. More recently, differences in structural song characteristics such as the performance of physically challenging song components were analysed in this regard. Here we show that buzz elements in the song of nightingales (Luscinia megarhynchos) hold the potential to serve as indicators of male quality and may therefore serve a communicative function. Buzzes were produced with considerable differences between males. The body weight of the males was correlated with one measure of these buzzes, namely the repetition rate of the buzz subunits, and individuals with larger repertoires sang buzzes at higher subunit-rates. A model of buzz performance constraints suggested that buzzes were sung with different proficiencies. In playback experiments, female nightingales showed more active behaviour when hearing buzz songs. The results support the idea that performance differences in the acoustic fine structure of song components are used in the communication of a large repertoire species such as the nightingale.     

Repertoire sharing by the individual and the pair: Insights into duet function and development in the plain wren Thryothorus modestus

Although both repertoire sharing and individual variation have been studied widely in male solo song, few studies have investigated these aspects of song in a species that performs complex duets. This study examined song repertoires of both males and females of the plain wren (canebrake subspecies, Thryothorus modestus zeledoni ), which performs antiphonal duets, and tested for patterns of repertoire sharing at the level of the individual and of the pair. We suggest that both males and females of the plain wren develop their phrase repertoires by direct imitation from other individuals of their sex, but that males, who may exist for a period as 'floaters' after dispersal, learn their songs from a wider pool of tutors than do females. In contrast to considerable individual repertoire sharing, whole duet types were very rarely shared between pairs, despite the potential for this to occur. This suggests that duet types are developed within the pair without reference to other pairs and the implications of this for duet functions are discussed.     

Female recognition of trills in the male calling song of the field cricket,<Emphasis Type="Italic"> Teleogryllus taiwanemma</Emphasis>

The calling song of the field cricket, Teleogryllus taiwanemma, is usually considered to consist of sequences of separate chirps. However, sometimes it comprises a phrase of several chirps in a row, with one long chirp (chirp) and a few short chirps (trills). In this study, I compared the phrase containing only chirps with that containing both chirps and trills by analyzing male songs and conducting playback experiments of male songs to females. The song analyses showed significant differences between chirps and trills for all song parameters except bandwidth. To test whether female preference differed with respect to the two phrases, I performed two-speaker playback experiments. When the same numbers of phrases were presented per unit time, females preferred the song with trills to that without trills. This result may reflect female preference for songs with greater sound density. In subsequent playback experiments, I equalized the total sound duration per unit time (duty cycle) in songs with and without trills. The numbers of females that preferred songs with and without trills did not differ significantly. This suggests that trills can attract females like chirps do, even though the two sounds have different components.