Song structure variability in passerine birds: Random variation or direct informative changes

The variability of the fine song structure was studied in the wood warbler (Phylloscopus sibilatrix), willow warbler (Ph. trochilus), greenish warbler (Ph. throchiloides viridanus), chaffinch (Fringilla coelebs), and gray shrike thrush (Colluricincla harmonica). The increase in the number of similar elements per phrase is shown to be related to the conspecific and heterospecific male-male song interactions in these birds. The responses of wood warbler males to playback conspecific songs with standard and prolonged (in the number of similar elements) second phrases differ significantly. The initial phrases of willow warbler songs become significantly longer in the populations with a high density as compared to those in populations of low density. The increase in the variability of both phrase combinations and song duration is shown to be related also to the song interaction with conspecific males for the willow warbler, greenish warbler, chaffinch, and gray shrike thrush. The within-song type variations turned out to play an important role during male-male interactions of different species. The results obtained suggest that similar ways of song responses may be quite widespread among passerine birds and reflect the general principles of changes in the song structure during acoustic interactions.     

Phonological-dependent territorial responses in yellowhammers (Emberiza citrinella)

In songbirds, males sing to attract potential mate and to defend their territory. Information about the sender such as its sex, its motivational state, its strength or identity can be encoded through subtle modifications in songs. In this study, we investigated whether territorial responses of yellowhammer males may be affected by modifications of song syntax and phonology. Yellowhammer song is mainly composed of three elements among which the last one is a long and low frequency note that is supposed to be the main component used by males to assess their potential rival. We carried out field experiments and played back either fully natural songs or partly natural and partly artificial songs to test this hypothesis. We identified that phonology rather than syntax modifications altered territorial responses. Our results also suggest that the phonology of the first song element plays a critical role in driving territorial responses.     

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.     

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.     

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.     

Is the Soft Song of the Brownish‐Flanked Bush Warbler an Aggressive Signal?

Soft songs have been detected in many songbirds, but in most species, research on soft songs has lagged behind studies of broadcast songs. In this study, we describe the acoustic features of a soft song in the brownish‐flanked bush warbler Cettia fortipes. Compared with the broadcast song, the warbler's soft song was characterized by a lower minimum frequency and longer duration, and it had a higher proportion of rapid frequency modulation notes. Using playback experiments, some in combination with mounted specimens, we found different responses to soft and broadcast songs, and we found that soft song can predict aggressive escalation (attack). We conclude that the soft song is an aggressive signal in this species.     

Components of song used for species recognition in the common yellowthroat

Territorial male common yellowthroats (Geothlypis trichas) were presented modified songs to determine the species-identifying song components. Presentations were paired, with the response elicited by a synthetic normal song (control) compared to that elicited by modified songs. Modified songs in which the frequency range was held constant yet lowered by 1 kHz from the average elicited a response that was significantly lower than the control. Doubling internote time intervals resulted in lower responses. Removing downslurred and upslurred hooks on notes did not alter responses. A simple two-noted repeated phrase song, one note an upstroke, the other a downstroke, with a frequency envelope of approximately 2 to 6 kHz, may provide the minimum requirements for species recognition.     

Overlapping and matching in the song contests of black-capped chickadees

During vocal contests, animals alter both the timing and the patterning of responses to their opponents. Time-specific responses (such as overlapping an opponent's song) and pattern-specific responses (such as matching the type of an opponent's song) may reveal information about interacting animals. Here we explore the consequences of overlapping and frequency matching during song contests of male black-capped chickadees, Poecile atricapillus. Using interactive playback, we engaged birds of high and low dominance status in vocal interactions with a simulated territorial intruder. The playback intruder either overlapped or avoided overlapping the subjects' songs and either matched or avoided matching the frequency of the subjects' songs. Individuals who were overlapped by the playback intruder showed higher variability in their song length and song timing than individuals who were not overlapped. Individuals who were frequency-matched by the playback intruder responded with more agitated responses (more flights and passes over the speaker and closer distances of closest approach) and spent more time farther away from the loudspeaker. We argue that the timing of song delivery and the choice of song type are distinct functional components of vocal interactions, where overlapping and matching songs are threatening signals that have separate consequences for opponent behaviour and song performance. High-ranking males responded at greater distances from the loudspeaker in all treatments and responded with lower agitation levels than low-ranking males. We demonstrate that males of different quality show different behavioural responses to territorial intruders, where males of high status appear reluctant to engage an intruder as intensely as males of low status.     

Song Learning in Bengalese Finches: a Comparison with Zebra Finches

Male Bengalese finches, Lonchura striata var. domestica, learn their song from an adult male conspecific with whom they can interact at 35 to 70 days of age and normally-raised males fail to reproduce song which they have only heard before or after this time. Birds which have been raised by their mother alone and those which have been deprived of a song tutor during the learning phase produce abnormal songs with indistinct elements and little or no phrase structure; this is typical of males which fail to hear adult song during their development. These songs are unstable and are replaced by normal songs, if there is an opportunity to learn from an adult male conspecific. Presumably, this flexibility in the time when young males learn acts as a safeguard to ensure that normal conspecific song is produced. These results bear striking similarity to those on zebra finch song development. Differences between the two species, especially in the learning of call notes by female zebra finches, are discussed.     

Male Song Stimulation of Female Reproduction in Canaries: Features Relevant to Sexual Displays are not Relevant to Nest-building or Egg-laying

Recent studies conducted in our laboratory have demonstrated that a special type of song phrase (‘sexy’ phrases) containing bipartite syllables composed of abrupt frequency falls and short silences stimulate female canaries to solicit for copulation. The study was undertaken to determine whether sexy phrases also facilitated other aspects of the reproductive activity of the female canary, namely, nest-building and egg-laying. During the first experiment, we studied the effect of sexy and non-sexy songs on copulation solicitation displays in 1-year-old females without reproductive experience and in mature females with previous reproductive experience. We confirmed that sexy songs elicited more sexual responses than did non-sexy songs in yearlings and in mature females. During the second experiment, we studied the effect of male songs on nest-building activities and egg-laying in 1-year-old inexperienced females, and in mature, experienced females. The songs of conspecific males significantly triggered and increased nest-building behaviour in female canaries whatever their age or reproductive experience. In contrast, song effects on egg-laying were only found in young females. One-year-old inexperienced females exposed to sexy or non-sexy songs laid more eggs and laid earlier than did 1-year-old inexperienced controls; no such differences were observed in mature, experienced females. The efficiency of songs in promoting nest-building or egg-laying appeared to be unrelated to their efficiency in eliciting sexual responses. No difference was found between females exposed to sexy songs and females exposed to non-sexy songs; differences were only found between control and the two groups of song-exposed females. This result demands further experiments in order to determine whether other song phrase types may account for the stimulating effects of male song on female nest-building and egg-laying.     

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.     

Element repertoire: change and development with age in Whitethroat Sylvia communis song

Song repertoires are often important determining factors in sexual selection. In several species, older males have larger repertoires than 1-year-old males. The development of large song repertoires by an individual is, however, poorly understood. We studied song element repertoire changes in five individual male Whitethroats Sylvia communis sampled as 1- and 2-year olds. These males increased the size of their element repertoire between their first and second year, but song length and number of different elements per song did not change. On average, 44.3% of the song elements in the first-year repertoire were also found in the second-year repertoire. Elements shared between years were found earlier in the songs and tended to occur in sequences. Sequences of shared elements also seemed to be conserved between years. The study suggests that the song element repertoire of the second year is partly based on the first-year repertoire, which may explain why large song repertoires are mainly expressed by males at least 2 years of age. It would appear, therefore, that song element repertoire size could be a reliable signal of male age.     

Multiple signaling functions of song in a polymorphic species with alternative reproductive strategies

Vocal traits can be sexually selected to reflect male quality, but may also evolve to serve additional signaling functions. We used a long‐term dataset to examine the signaling potential of song in dimorphic white‐throated sparrows (Zonotrichia albicollis). We investigated whether song conveys multifaceted information about the vocalizing individual, including fitness, species identity, individual identity, and morph. We also evaluated whether song traits correlate differently with fitness in the two morphs, as the more promiscuous strategy of white, relative to tan, morph males might impose stronger sexual selection. Males with high song rates achieved higher lifetime reproductive success, and this pattern was driven by white morph males. In addition, males that sang songs with many notes survived longer, but this pattern was less robust. Thus, song traits reflect differences in fitness and may more strongly affect fitness in the white morph. Song frequency was unrelated to fitness, body size, or morph, but was individual specific and could signal individual identity. Songs of the two morphs displayed similar frequency ratios and bandwidths. However, tan morph males sang songs with longer first notes, fewer notes, and higher variability. Thus, song could be used in morph discrimination. Variation in frequency ratios between notes was low and could function in conspecific recognition, but pitch change dynamics did differ between four different song types observed. Our results support a multiple messages model for white‐throated sparrow song, in which different song traits communicate discrete information about the vocalizing individual.     

The Influence of Song Mode on Responses of Male American Redstarts

Male American redstarts (Setophaga ruticilla) exhibit two modes of singing: repeat and serial mode. Repeat mode consists of repetitions of a single song type, usually one possessing an accented terminal note. Serial mode is constituted by a series of different songs with unaccented terminal notes, sung in a versatile sequence. Different authors have observed that males discriminate between these two modes. In the experiments reported in this paper, we tested whether males use accented terminal notes, song versatility or both to discriminate between the two song modes. We observed that males responded more intensively to the accented song. The accented note seems a very important song characteristic for evoking male American redstart responses. In contrast, males did not show any difference in their response to single and varied serial songs.     

Red-winged blackbirds Ageliaus phoeniceus respond differently to song types with different performance levels

Physical constraints on animal signals can have important consequences for communication. For bird songs that include a trill, performance is physically constrained by a trade-off between the rate at which notes are repeated in the trill and the sound frequency range covered, such that a trill cannot exceed a certain frequency range for a given note repetition rate. Producing trills that are closer to this performance limit is presumably more challenging for birds than producing trills that are farther from it. Male red-winged blackbirds Ageliaus phoeniceus have repertoires of two to eight song types containing trills that span a range of performance levels. We determined the approximate trill performance maximum for a population of red-winged blackbirds, then conducted playback experiments to measure the responses of territorial males to song types with either high or low performance levels relative to this limit. Males responded significantly more strongly to songs containing low performance trills. Our results show that male red-winged blackbirds can discriminate between different song performance levels, suggesting that vocal proficiency plays a role in male-male interactions.     

Sound Spectrum Characteristics of Songs of Hainan Gibbon (Nomascus hainanus)

Gibbons are characterized by their species-specific calls, or songs. There are few studies of songs of Hainan gibbon (Nomascus hainanus). To study the sound spectrum characteristics and test for intergroup differences in Hainan gibbon song, we studied the singing behavior of Hainan gibbons in Bawangling National Nature Reserve, Hainan Province, China, intermittently from August 2002 to February 2013, collecting 184 recordings. Our results show that: 1) Hainan gibbon song bouts occur mainly 0–4 h after dawn. 2) The songs of adult males living in groups are composed mainly of one to three short notes and one to five long notes, while solitary adult male songs consist only of long frequency modulated notes and no short or single notes. 3) The song chorus is dominated by adult males, while females add a great call. Males do not have a great call, unlike those in other gibbon species. There are no female solos. 4) The sound spectrum frequency is similar in adult males living in two different groups, but the duration of the first long note differed significantly between the groups. The sonic frequencies of male and female songs are lower than those of other gibbons: no more than 2 kHz. Hainan gibbon sound structure is simple, although females participate in the chorus, reflecting their primitive status among gibbon species.     

Testosterone Induces Sexual Release Quality in the Song of Female Canaries

We tested the sexual responsiveness of female canaries, Serinus canaria, to two sets of different types of female conspecific songs versus an adult male conspecific song. Female songs were either spontaneously emitted (‘S-songs’) or were testosterone-induced (‘T-songs’). Copulation-solicitation displays (CSD) were used as an index of female sexual response. Playbacks were performed several days before and during egg laying, a period of natural sexual responsiveness of the females to song. We demonstrated that the weaker sexual displays of female canaries were recorded to S-songs, thus suggesting that these types of female songs do not contain fully functional sexual releasers. Three T-songs elicited high levels of sexual displays, thus demonstrating that testosterone treatment may induce sexual release quality in the female songs. Study of the phonology of these three T-songs strongly suggested that special song phrases may be good candidates as powerful sexual releasers. To test the sexual value of these female song phrases, we carried out a third experiment, using hybrid songs where each of these special T-song phrase types was included in a well-known heterospecific context. Two phrases elicited high levels of sexual responses in females. Essential features of the male full song, such as broadband rapid frequency modulations and high repetition rate, are retrieved in both female song phrases. Taken together, these data demonstrate that testosterone treatment not only induces a male-like structure in the songs of females, but also induces functionally ‘male-like’ songs. This result allows features of the vocal control network of testosterone-treated females to be compared with those of adult males singing full songs, to distinguish neural correlates of testosterone-dependent full songs. However, because testosterone does not induce functionally male-like songs in all the females, neuroanatomical structure-function correlations need detailed behavioural analysis.     

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.     

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.