The Responses of Duetting Antbirds to Stereo Duet Playback Provide Support for the Joint Territory Defence Hypothesis

In multiple animal taxa, including many birds and primates, members of mated pairs produce coordinated acoustic displays known as duets. By observing the behaviour of territorial animals as they respond to playback‐simulated duets of rivals, we can gain insight into the behavioural significance of vocal duets. Playback experiments, however, have been conducted across a very narrow range of duetting animals. Furthermore, many studies have been conducted with single‐speaker playback, whereas stereo‐speaker playback offers more spatially realistic simulation of duets. Moreover, by evaluating the reactions of animals to separate loudspeakers broadcasting male and female duet contributions, we can study the interactions of both males and females with same‐sex vs. opposite‐sex rivals. We used a paired experimental design to broadcast duet stimuli through a single‐speaker and a stereo‐speaker apparatus to 30 pairs of duetting barred antshrikes Thamnophilus doliatus in Costa Rica. Our goals were (1) to evaluate whether territorial antbirds respond more aggressively to male vs. female duet components and (2) to assess aggressive responses of antbirds towards single‐speaker vs. stereo‐speaker playback. Neither males nor females differentiated between the loudspeaker simulating the male vs. female duet contribution during stereo‐speaker playback trials. Barred antshrikes displayed significantly stronger responses to stereo‐speaker playback compared with single‐speaker playback. Males displayed stronger playback responses than females with closer, quicker and more vocal responses. These results provide evidence for a joint resource defence function of antbird duets given that pairs responded together with equivalent intensity to male and female simulated intruders. This is the first study to show that although duetting is an aggressive territorial signal, birds do not necessarily respond to sex‐specific components of duets. Our results support the idea that spatially realistic stereo presentation of duet stimuli is critical for experimental duet research.     

Function of pair duets in the eastern whipbird: cooperative defense or sexual conflict?

Paired male and female eastern whipbirds, Psophodes olivaceus,sing precisely coordinated, male-led duets. Four broad explanationshave been proposed for the function of duets: 1) cooperativeresource defense, 2) prevention of partner usurpation, 3) defenseof an individual's own position within the partnership, or 4)mate identification and localization. These 4 hypotheses makedifferent predictions about how male and female residents shouldrespond to simulated intrusion by other pairs or individuals.We compared the behavioral and vocal responses of 20 pairs ofeastern whipbirds to simulated territorial intrusions by: 1)a solitary singing male, 2) a solitary singing female, and 3)a duetting pair. Males and females did not coordinate theirapproach to the playback speaker and showed sex-specific responsesto playback. Males did not respond differently to duetting versussolo singing intruders. By contrast, females approached moreclosely during solo female song than during solo male song orduet playback. Females also produced specific vocalizationsonly in response to duet and solo female playback. Both sexesapproached the speaker more closely and quickly during playbackof same-sex solo songs than opposite-sex solo songs. Finally,females answered more of their mate's songs during simulatedintrusion by a lone female than during simulated intrusion bya lone male. Our results suggest that duets in this speciesprimarily function to allow females to defend their exclusiveposition in a partnership. Mate defense by females is unusualin birds but may be promoted in eastern whipbirds by a female-biasedsex ratio and the need for exclusive access to male care. Thus,duets result from independent and conflicting strategies ofmate and territory defense in males and females.     

Functions of duetting in the tropical boubou, Laniarius aethiopicus: territorial defence and mutual mate guarding

Duetting, found in many animal taxa, is still a poorly understood form of signalling behaviour despite numerous hypotheses to explain its function. One contentious issue is whether duetting is a cooperative endeavour or signals conflict between the sexes. We studied the function of duetting in the tropical boubou, using interactive playback experiments. We staged encounters by presenting paired birds with four variants of each of four duet types (neighbour and stranger solos and neighbour and stranger duets). Multiple lines of evidence suggest that both joint territorial defence and mutual mate guarding are important functions of duetting, and that both cooperation and conflict between males and females have shaped duetting behaviour. Of a repertoire of 12 duet types, eight were used for joint territorial defence. Two of these eight duet types were probably also used for mutual mate guarding, suggesting that individual duets can have multiple functions. We found an unusual response for duetting birds in that females often synchronized notes with male solo playback to form precise duets. In turn, males attempted to jam the song of rivals when females joined the song of male solo playback, providing further evidence for the importance of acoustic mate guarding. Song jamming by males has not previously been described for duetting species. Finally, we suggest that the repertoire of duet types is used to fine-tune territorial encounters in a network environment of multiple interactants.     

Song repertoire and duetting behaviour of the tropical boubou, Laniarius aethiopicus

Female song and the significance of duetting in birds remain puzzling, in part because of the limited number of species studied. We investigated duetting behaviour in the tropical boubou in West Africa. Birds produced a diverse song repertoire consisting of solos, duets and trios, with duets being the most conspicuous vocalization. We identified 12 distinct duet types in which tonal and broadband notes were combined by males and females in highly synchronized and temporally precise patterns. Molecular sexing and observations of colour-banded birds revealed that duets were initiated by both sexes, with strict sex-specific roles maintained within the duet. An equal number of duet types was initiated by males and females. Seven of 12 duets were terminated by the male. Male-initiated duets were also more common (89% of total) and were repeated more often, i.e. sung more frequently. Solo singing occurred when partners did not respond and was also used by unpaired birds. Trios were produced by subadult birds joining the duet of a resident pair. The wide variety of contexts in which duets were sung suggests that they serve several functions, including territorial defence and mutual mate guarding.     

Context-dependent functions of avian duets revealed by microphone-array recordings and multispeaker playback

In many tropical animals, male and female breeding partners combine their songs to produce vocal duets [1-5]. Duets are often so highly coordinated that human listeners mistake them for the songs of a single animal [6]. Behavioral ecologists rank duets among the most complex vocal performances in the animal kingdom [7, 8]. Despite much research, the evolutionary significance of duets remains elusive [9], in part because many duetting animals live in tropical habitats where dense vegetation makes behavioral observation difficult or impossible. Here, we evaluate the duetting behavior of rufous-and-white wrens (Thryothorus rufalbus) in the humid forests of Costa Rica. We employ two innovative technical approaches to study duetting behavior: an eight-microphone acoustic location system capable of triangulating animals' positions on the basis of recordings of their vocalizations [10] and dual-speaker playback capable of simulating duets in a spatially realistic manner [11]. Our analyses provide the first detailed spatial information on duetting in both a natural context and during confrontations with rivals. We demonstrate that birds perform duets across highly variable distances, that birds approach their partner after performing duets, and that duets of rivals induce aggressive, sex-specific responses. We conclude that duets serve distinct functions in aggressive and nonaggressive contexts.     

Description of the acoustical interaction and synchronization between duetters of the Large-footed Finch (Pezopetes capitalis)

Duets are highly coordinated acoustical displays produced by two individuals. Studying the structure of duet songs and its causes is essential to understand the ecological role and evolution of this form of communication. Our goals in this study were to describe the structure of the duet of the Large-footed Finch, the temporal synchronization of each individual, and to test the effect of provoked (playback) duets. We recorded birds at Cerro de la Muerte, Costa Rica and simulated territory intrusions using playback trials. The duet of this species consisted of overlapping notes between both members of a mated pair, which were emitted either randomly or periodically. Temporal synchronization was similar between both individuals in a mated pair. Both partners adjust the silent intervals between notes in response to the duration of its partner’s note. The high frequency of the second part of the duet of this species decreased with time and the second individual, in duetting this part, not only synchronized its notes temporally with the first individual, but also synchronized the high frequency. The high frequency of the first note the second individual sung was a bit lower than the previous note sung by the first individual. Finally, birds responding to provoked duets produced duets with higher frequency and higher rate of notes compared to unprovoked duets. Our results provide information on the temporal and frequency synchronization in duets of an endemic emberizid and provide new information about a less reported vocal behaviour in bird duets, the acoustic frequency coordination.     

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.     

Context, Structural Variability and Distinctiveness of California Towhee (Pipilo crissalis) Vocal Duets

Avian vocal duets occur when paired birds produce temporally and structurally coordinated vocalizations. Duets are given by members of many species from taxonomically distinct lineages and show great variety in form that often reflects function. By describing the structure of vocal duets we can learn about the diversity of communication signals present in nature and also gain insight into the evolution and operation of those signals. This study quantified the usage patterns and acoustic structure of California towhee duet vocalizations, and tested the distinctiveness of duets among different pairs. California towhee duets consist of a 'squeal' vocalization that is highly unlike the species-typical call note or male advertizement song. California towhees duet an average of three times per hour during the breeding season, and all duets are accompanied by an approach response that brings duetting partners into close spatial proximity. Males and females produce duet contributions with the same syllabic structure. Individual birds produce highly variable squeal vocalizations that are distinctive enough to signal identity. California towhee duet characteristics indicate that the squeal vocalization has evolved separately from other vocal traits in this species and was promoted by natural selection as a duet-specific vocalization. Duet usage patterns and structure suggest that these communication signals function in a cooperative context.     

Duetting behavior in a Neotropical ovenbird: sexual and seasonal variation and adaptive signaling functions

Duetting is a collective behavior and might have multiple functions, including joint territory defense and mate guarding. An important step toward understanding the adaptive function of bird song is to determine if and how singing behavior varies seasonally. However, seasonal patterns for duetting species are different from the pattern described for species in which only the male sings, because song function may vary according to sex, singing role (initiator vs responder) and level of duet organization (individual vs pair). We investigated whether patterns of seasonal variation in duetting depends on these factors, which would suggest different interpretations of song function. We studied social pairs of a Neotropical bird species (rufous hornero Furnarius rufus) for seven consecutive months, recording vocal and territorial behaviors. Overall, partners coordinated 61% of their songs into duets and many song traits (song initiation rate, song output and duet rate) peaked in territorial contexts. Males engaged in territorial interactions with strangers more often, initiated more songs, and answered proportionately more of their partners’ songs than females. Male song initiation rate peaked during the pre‐ and post‐breeding stages, whereas females initiated more songs during the non‐breeding season. Both sexes answered partner songs faster and at higher rates during the pre‐breeding and female fertile stages. Partners duetted at a higher rate during the pre‐ and post‐breeding stages. Finally, song initiation rates and duet rate, but not song answering rates, correlated with frequency of territorial interactions with strangers. Although our findings indicate that song function may vary with sex, singing role and level of duet organization, our results suggest that in general duet functions to defend common territories and as a mutual mate guarding strategy in the rufous hornero.     

Antiphonal duetting in Steere’s liocichla (Liocichla steerii): male song individuality and correlation between habitat and duetting behavior

We studied duet song and vocal duetting behavior in an endemic Taiwanese passerine, Steeres liocichla (Liocichla steerii). We found that the leading male song in duets was highly individualistic. Also, we found duetting behavior varied significantly across different habitat types. Females were more likely to answer male songs in densely vegetated, steep forest habitat compared to open agricultural habitat. These findings provide quantitative evidence for vocal individuality for a duet song and provide tentative support for the idea that females are duetting to reveal their location to their mates.     

Duet-splitting and the evolution of gibbon songs

Unlike the great apes and most other primates, all species of gibbons are known to produce elaborate, species-specific and sex-specific patterns of vocalisation usually referred to as "songs". In most, but not all, species, mated pairs may characteristically combine their songs in a relatively rigid pattern to produce coordinated duet songs. Previous studies disagree on whether duetting or the absence of duetting represented the primitive condition in gibbons. The present study compares singing behaviour in all gibbon species. Various vocal characteristics were subjected to a phylogenetic analysis using previously published phylogenetic trees of the gibbon radiation as a framework. Variables included the degree of sex-specificity of the vocal repertoire, the occurrence of solo songs, and the preference for a specific time of day for song-production. The results suggest the following scenario for the evolution of gibbon songs: (1) The last common ancestor of recent gibbons produced duet songs. (2) Gibbon duets probably evolved from a song which was common to both sexes and which only later became separated into male-specific and female-specific parts (song-splitting theory). (3) A process tentatively called "duet-splitting" is suggested to have led secondarily from a duetting species to a non-duetting species, in that the contributions of the pair-partners split into temporally segregated solo songs. This appears to be the first time that a non-duetting animal can be shown to be derived from a duetting form. (4) The return to exclusive solo singing may be related to the isolated island distribution of the non-duetting species.     

Duets defend mates in a suboscine passerine, the warbling antbird (Hypocnemis cantator)

Despite the widespread occurrence of avian duets, their adaptivesignificance is poorly understood. It is generally assumed thatthey function in the joint defense of territories, but no studyhas successfully distinguished between this hypothesis, whichinvokes cooperation between the sexes, and mate defense, whichinvokes conflict. Further, most duetting studies have focusedon oscine passerines, the songs of which are learnt and relativelycomplex. We therefore tested the mate defense hypothesis inthe warbling antbird (Hypocnemis cantator), an Amazonian suboscinethat produces simple sex-specific songs and duets. Acousticanalysis of songs showed (1) that solos were often producedby males, but rarely by females; (2) that duets consisted ofa male song and a female reply; and (3) that, although femalesong was invariable, a swift reply resulted in males producingshorter songs with fewer notes. These results suggest that duetting,and the structure of duets, is chiefly a product of female behavior,a scenario more suggestive of conflict than cooperation. Toinvestigate this idea we carried out playback experiments, whichshowed that (4) the response to solo songs was sex specific(i.e., male solos elicited a strong response from paired males,and female solos elicited a strong response from paired females);(5) males and females responded to same-sex solos more stronglythan to duets; and that (6) females answered their partner'ssongs more often, and more rapidly, in response to female solosthan male solos or duets. Although it can be argued that sex-specificresponses to solo song result from intrasexual territorial defense,we cannot use the same reasoning to explain (5) or (6). Instead,these observations imply that solitary intruders were more threateningthan paired intruders, and thus that the perceived threat wasto the partnership rather than the territory. Taken together,findings (1) to (6) suggest that females adjust their vocalbehavior in relation to the level of perceived threat to thepartnership, and duet with males in order to repel same-sexrivals. This study therefore strengthens support for the matedefense hypothesis, and suggests that conflict—ratherthan cooperation—may have played a major role in the evolutionand maintenance of avian duets.     

Angry Birds Need Not Apply: A Perspective on the Flexible form and Multifunctionality of Avian Vocal Duets

Research into avian vocal duets has revealed much about the social interactions of mated pairs, including how communication signals mediate important components of fitness such as reproductive success and resource‐holding ability. Excellent reviews have been written on avian duets, but none have quantified the prevalence of different duet features and functions, or tested for potential links between structure and function. Here we have compiled a thorough list of studies at the species level and investigated whether there are trends in duet form, trends in duet function and matches between form and function. We have also provided recommendations for reporting of duet structure to help standardize future data collection and facilitate comparative analyses. We found that duet vocalizations are typically tightly coordinated (69% of 33 species), initiated by both partners (80% of 35 species), and have sex‐specific contributions (73% of 48 species). Duets vary substantially in structure both across and within species, and this flexibility in duet form may contribute to their use in multiple contexts. Among the well‐studied species (functions well supported by experimental evidence and studied post‐1983), duets are usually multifunctional, with functions including joint resource defence, signalling commitment, maintaining contact, ensuring reproductive synchrony and mate‐guarding. Levels of support for different hypotheses have changed over time as theory within the field developed, but overall, the best‐supported function of duets across avian species (81% of 26 species) has been joint resource defence. In all but one of these 26 species, duets are sometimes cooperative (meaning that duet participation benefits both duetting partners), and they often produce multiple benefits. We found associations between form and function, in that duets directed to non‐pair individuals have higher amplitude and are more likely to have sex‐specific notes than duets directed within the pair. We see the field moving away from labelling duets as representing ‘cooperation’ or ‘conflict’ and moving towards a general multifunctionality framework within which researchers test multiple hypotheses for duet function under the assumption that both duet form and function can vary moment to moment and that such flexibility can provide multiple benefits.     

VOCAL AND OTHER BEHAVIOUR OF KENYAN BLACK-COLLARED BARBETS LYBIUS TORQUATUS

We describe 12 visual and six acoustical displays of Black-collared Barbets, all associated with interactions of these sexually monomorphic barbets in greeting ceremonies and in antiphonal duets. Certain of the displays, e. g., bill directing, smooth posture and chatter calls largely are associated with multiple-barbet interactions and greeting ceremonies involving more than two barbets, and seem to represent purely agonistic displays. Other displays, most notably the bobbing and floating flight displays, and the tyaw call, are associated with preduet greeting ceremonies involving the presumed pair. In every case duets were performed by a presumed pair, following a greeting ceremony and visual displays accompany the duetting, which usually becomes precisely synchronized after the initial few notes. Four to 17 notes of each of two types, a usually lower pitched, short and mainly double type A note that introduces most duets, and a higher pitched, usually single, clear type B note that often terminates the duet. Variation in the notes and in their tempos within duets suggests no clearly defined temporal indicators for achieving, maintaining, and concluding the duets. The A and B notes cannot yet be assigned to one or other sex and similarity in form and development of the notes, from notes of greeting ceremonies, suggests the possibility that under some circumstances a bird of one sex may be able to deliver notes of either type. The duets are delivered from singing posts within the group's (or pair's) territory and usually only one duet is uttered from a site before the birds shift to another singing post. The duetting is compared with that of other barbets and appears to be among the best synchronized and most precise in the family. Comparisons with non-barbet duetting species are difficult because of the special habits and ecology of the barbets and uncertainty as to the sexual rôle of the duetters.     

Memory-dependent adjustment of vocal response latencies in a territorial songbird

Vocal interactions in songbirds can be used as a model system to investigate the interplay of intrinsic singing programmes (e.g. influences from vocal memories) and external variables (e.g. social factors). When characterizing vocal interactions between territorial rivals two aspects are important: (1) the timing of songs in relation to the conspecific’s singing and (2) the use of a song pattern that matches the rival’s song. Responses in both domains can be used to address a territorial rival. This study is the first to investigate the relation of the timing of vocal responses to (1) the vocal memory of a responding subject and (2) the selection of the song pattern that the subject uses as a response. To this end, we conducted interactive playback experiments with adult nightingales (Luscinia megarhynchos) that had been hand-reared and tutored in the laboratory. We analysed the subjects’ vocal response latencies towards broadcast playback stimuli that they either had in their own vocal repertoire (songs shared with playback) or that they had not heard before (unknown songs). Likewise, we compared vocal response latencies between responses that matched the stimulus song and those that did not. Our findings showed that the latency of singing in response to the playback was shorter for shared versus unknown song stimuli when subjects overlapped the playback stimuli with their own song. Moreover birds tended to overlap faster when vocally matching the stimulus song rather than when replying with a non-matching song type. We conclude that memory of song patterns influenced response latencies and discuss possible mechanisms.     

Differentiating the evolution of female song and male–female duets in the New World blackbirds: Can tropical natural history traits explain duet evolution?

Female bird song and combined vocal duets of mated pairs are both frequently associated with tropical, monogamous, sedentary natural histories. Little is known, however, about what selects for duetting behavior versus female song. Female song likely preceded duet evolution and could drive apparent relationships between duets and these natural histories. We compared the evolution of female song and male–female duets in the New World blackbirds (Icteridae) by investigating patterns of gains and losses of both traits and their relationships with breeding latitude, mating system, nesting pattern, and migratory behavior. We found that duets evolved only in lineages in which female song was likely ancestral. Both female song and duets were correlated with tropical breeding, social monogamy, territorial nesting, and sedentary behavior when all taxa were included; however, correlations between duets and these natural history traits disappeared when comparisons were limited to taxa with female song. Also, likelihood values supported stronger relationships between the natural history traits and female song than between these traits and duets. Our results suggest that the natural histories thought to favor the evolution of duetting may in fact be associated with female song and that additional selection pressures are responsible for the evolution of duets.     

Plain wrens Cantorchilus modestus zeledoni adjust their singing tempo based on self and partner's cues to perform precisely coordinated duets

Precise coordination appears to be an important signal in several duetting species. However, little attention has been directed to the proximate mechanisms that might drive this behavior. To perform highly coordinated duets, individuals can either have an intrinsic fixed singing tempo or modify their singing tempo based on cues in their own and their partner's songs. In this study I determined whether autogenous and/or heterogenous factors are associated with duet coordination in plain wrens Cantorchilus modestus zeledoni by analyzing recorded duets from 8 territorial pairs in the field. Previous research has determined that plain wrens perform highly coordinated antiphonal duets with almost no overlap. I found that to achieve such precise coordination individuals perform phrase‐by‐phrase modifications to the duration between two consecutive phrases (inter‐phrase interval) based on a) whether their song is answered, b) the phrase type used in the duet and c) the position of the inter‐phrase interval within the duet. Moreover, there are several sex differences in how individuals use these cues to modify their inter‐phrase intervals. Females produce longer inter‐phrase intervals when their mates do not answer a phrase, whereas males produce shorter inter‐phrase intervals when their mates do not answer. Females modify their inter‐phrase intervals based only on the phrase type their mates sing, whereas males modify their inter‐phrase intervals based on both the phrase that they sing and the phrase the females use to answer. Both males and females produce longer inter‐phrase intervals for longer phrase types sung by their partners, but males do so with more precision than do females. Finally both sexes increase their inter‐phrase intervals as the duet progresses. That precise coordination is achieved by a complex and dynamic process supports the idea that this behavior could signal pair bond strength.     

Acoustic Monitoring Reveals Congruent Patterns of Territorial Singing Behaviour in Male and Female Tropical Wrens

Many animals defend territories against conspecific individuals using acoustic signals. In birds, male vocalizations are known to play a critical role in territory defence. Territorial acoustic signals in females have been poorly studied, perhaps because female song is uncommon in north‐temperate ecosystems. In this study, we compare male vs. female territorial singing behaviour in Neotropical rufous‐and‐white wrens Thryothorus rufalbus, a species where both sexes produce solo songs and often coordinate their songs in vocal duets. We recorded free‐living birds in Costa Rica using an eight‐microphone Acoustic Location System capable of passively triangulating the position of animals based on their vocalizations. We recorded 17 pairs of birds for 2–4 consecutive mornings and calculated the territory of each individual as a 95% fixed kernel estimate around their song posts. We compared territories calculated around male vs. female song posts, including separate analyses of solo vs. duet song posts. These spatial analyses of singing behaviour reveal that males and females use similarly sized territories with more than 60% overlap between breeding partners. Territories calculated based on solo vs. duet song posts were of similar size and similar degrees of overlap. Solos and duets were performed at similar distances from the nest for both sexes. Overall, male and female rufous‐and‐white wrens exhibit very similar spatial territorial singing behaviour, demonstrating congruent patterns of male and female territoriality.     

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.     

Duetting in space: a radio-telemetry study of the black-bellied wren

In many birds, individuals 'answer' the songs of their pair-mates to produce vocal 'duets'. One hypothesized function of song answering is that it prevents extra-pair birds from intruding into the duetting pair's territory to obtain copulations or usurp one of the pair-mates. In this capacity, answering may signal that the pair-mates are close together, and so are prepared to defend against such an intrusion. Another functional hypothesis states that answering helps pair-mates maintain contact, and so predicts that a bird is more likely to approach its mate after a duet than after a solo song. I used radio-telemetry to monitor the distance between mated black-bellied wrens (Pheugopedius fasciatoventris). I found that birds of both sexes were more likely to answer their mate's song when the mate was close, and that maximum duet length was negatively related to the distance between pair-mates. Furthermore, song answering positively affected the likelihood of one pair-mate approaching the other after a song. In a significant majority of the approaches after duet songs, the answering bird approached the initiator. I conclude that in the black-bellied wren, (i) the occurrence and duration of vocal duets covary with physical closeness and (ii) contact maintenance is a secondary function of duet participation.