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.     

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.     

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.     

Breeding season length predicts duet coordination and consistency in Neotropical wrens (Troglodytidae)

Many animals produce coordinated signals, but few are more striking than the elaborate male–female vocal duets produced by some tropical songbirds. Yet, little is known about the factors driving the extreme levels of vocal coordination between mated pairs in these taxa. We examined evolutionary patterns of duet coordination and their potential evolutionary drivers in Neotropical wrens (Troglodytidae), a songbird family well known for highly coordinated duets. Across 23 wren species, we show that the degree of coordination and precision with which pairs combine their songs into duets varies by species. This includes some species that alternate their song phrases with exceptional coordination to produce rapidly alternating duets that are highly consistent across renditions. These highly coordinated, consistent duets evolved independently in multiple wren species. Duet coordination and consistency are greatest in species with especially long breeding seasons, but neither duet coordination nor consistency are correlated with clutch size, conspecific abundance or vegetation density. These results suggest that tightly coordinated duets play an important role in mediating breeding behaviour, possibly by signalling commitment or coalition of the pair to mates and other conspecifics.     

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.     

Siberian crane duet as an individual signature of a pair: comparison of visual and statistical classification techniques

Vocal individuality varies between species and/or ontogenesis stages depending on needs in the vocal recognition, but also estimation of individual differences depends on the method of analysis. We studied pair-specific differences of duets elicited by mating pairs of Siberian crane Grus leucogeranus. We quantitatively described the duet structure and compared visual and statistical classification methods of pair identification by duet. Three methods were used: discriminant analysis, method of classification trees and visual classification of spectrogram. We found significant interpair differences. The pairs differ by duet structure that is by the ratio of male- and female-initiated duets and by the ratio of the number of male to female calls; temporal-frequency duet characteristics are pair-specific, too. All methods showed high interpair differences, which exceeded random values significantly. Discriminant analysis stepwise procedure based on 11 parameters resulted in 97.3% of correctly assigned duets. Human observers correctly assigned 80.7% of spectrograms. Our data provide a basis for remote monitoring of this endangered species with a wild population of only 3,000 birds.     

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.     

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.     

Duet function in the yellow-naped amazon, Amazona auropalliata: evidence from playbacks of duets and solos

The question of why animals participate in duets is an intriguing one, as many such displays appear to be more costly to produce than individual signals. Mated pairs of yellow-naped amazons, Amazona auropalliata, give duets on their nesting territories. We investigated the function of those duets with a playback experiment. We tested two hypotheses for the function of those duets: the joint territory defense hypothesis and the mate-guarding hypothesis, by presenting territorial pairs with three types of playback treatments: duets, male solos, and female solos. The joint territory defense hypothesis suggests that individuals engage in duets because they appear more threatening than solos and are thus more effective for the establishment, maintenance and/or defense of territories. It predicts that pairs will be coordinated in their response (pair members approach speakers and vocalize together) and will either respond more strongly (more calls and/or more movement) to duet treatments than to solo treatments, or respond equally to all treatments. Alternatively, the mate-guarding hypothesis suggests that individuals participate in duets because they allow them to acoustically guard their mate, and predicts uncoordinated responses by pairs, with weak responses to duet treatments and stronger responses by individuals to solos produced by the same sex. Yellow-naped amazon pairs responded to all treatments in an equivalently aggressive and coordinated manner by rapidly approaching speakers and vocalizing more. These responses generally support the joint territory defense hypothesis and further suggest that all intruders are viewed as a threat by resident pairs.     

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.     

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.     

A review of acoustic playback techniques for studying avian vocal duets

ABSTRACT Playback experiments involve the broadcast of natural or synthetic sound stimuli and provide a powerful tool for studying acoustic communication in birds. Playback is a valuable technique for exploring vocal duetting behavior because it allows investigators to test predictions of the various hypotheses for duet function. Here, we adopt a methodological perspective by considering various challenges specific to studying duetting behavior, and highlighting the utility of different playback designs for testing duet function. Single‐speaker playback experiments allow investigators to determine how duetting birds react to different stimuli, but do not simulate duets in a spatially realistic manner. Multi‐speaker playback experiments are superior to single‐speaker designs because duet stimuli are broadcast with spatial realism and unique and additional predictions can be generated for testing duet function. In particular, multi‐speaker playback allows investigators to evaluate how birds respond to male versus female duet contributions separately, based on reactions to the different loudspeakers. Interactive playback allows investigators to ask questions about the time‐ and pattern‐specific singing behavior of birds, and to understand how singing strategies correspond to physical behavior during vocal interactions. Although logistically challenging, interactive playback provides a powerful tool for examining specific elements of duets (such as the degree of coordination) and may permit greater insight into their functions from an operational perspective. Interactive playback designs where the investigator simulates half of a duet may be used to describe and investigate the function of pair‐specific and population‐wide duet codes. Regardless of experimental design, all playback experiments should be based on a sound understanding of the natural duetting behavior of the species of interest, and should aim to produce realistic and carefully controlled duet simulations. Future studies that couple playback techniques with other experimental procedures, such as Acoustic Location System recordings for monitoring the position of birds in dense vegetation or multimodal techniques that combine acoustic with visual stimuli, are expected to provide an even better understanding of these highly complex vocal displays.     

Behavioral Ecology of Elliot’s Laughingthrush (<Emphasis Type="Italic">Trochalopteron</Emphasis> (<Emphasis Type="Italic">Garrulax</Emphasis>) <Emphasis Type="Italic">elliotii</Emphasis>, Timaliidae,Aves): 2. Vocal Repertoire

Our work is the first study on the vocal repertoire of one of the babbler species, Elliot’s laughingthrush. Field studies were carried out in Hupingshan Nature Reserve, Hunan Province, China. There are three types of signals in the repertoire: songs, calls (chattering), and duets. Songs and calls are used by both males and females. The song consists of a quiet introduction (short note) and a louder main part (two or three tonal notes). There are up to four song variants in the repertoire of a given pair. All songs can be classified into three types. Types I and II represent songs with the main part consisting of three notes, while type III consists of two notes. Types I and II differ from each other in certain features of the frequency modulation shape of the first two notes. All song types can be used during the spontaneous vocalization of a single bird. However, their usage in other contexts differs. Songs of types I and II are more often uttered during vocal interactions of neighbor males. Songs of type III appear to be characteristic of male–female duets. Chattering (calls) is a continuous series of broadband notes. There are several (two to five) note variants in each series. Chattering can be used both during a male–female interaction and as an alarm call. We found an inverse correlation between (1) the duration of pauses between notes, and (2) the number of note variants (“repertoire size”) in a series. Both parameters probably reflect the internal state of an individual at a given moment.     

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.     

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.     

Migration and the evolution of duetting in songbirds

Many groups of animals defend shared resources with coordinated signals. The best-studied of these signals are the vocal duets produced by mated pairs of birds. Duets are believed to be more common among tropical-breeding species, but a comprehensive test of this hypothesis is lacking, and the mechanisms that generate latitudinal patterns in duetting are not known. We used a stratified sample of 372 songbird species to conduct the first broad-scale, phylogenetically explicit analysis of duet evolution. We found that duetting evolves in association with the absence of migration, but not with sexual monochromatism or tropical breeding. We conclude that the evolution of migration exerts a major influence on the evolution of duetting. The perceived association between tropical breeding and duetting may be a by-product of the migration–duetting relationship. Migration reduces the average duration of partnerships, potentially reducing the benefits of cooperative behaviour, including duetting. Ultimately, the evolution of coordinated resource-defence signals in songbirds may be driven by ecological conditions that favour sedentary lifestyles and social stability.     

Evidence of repertoire sharing and stability despite a high turnover rate in a duetting neotropical wren

In songbirds, the spatial pattern of song sharing among individuals is influenced by the song learning and dispersal strategies within each species. In birds where females and males sing and create joint acoustic displays (duets), the processes defining the patterns of song sharing become more complex as there might be different selection pressures shaping the behaviour of each sex. To provide further insight into the vocal development and the dispersal strategy of duetting tropical species, we investigated the patterns of individual and pair repertoire sharing, as well as the stability of these repertoires, in a colour-marked population of riverside wrens, Cantorchilus semibadius, located in the Osa Peninsula, Costa Rica. Using data collected over a five-year period, we found considerable variation in the sharing levels of phrase and duet type repertoires among neighbouring individuals coupled with a general decline of repertoire sharing as distance increased between birds’ territories. These results are consistent with a pattern predicted in age-restricted learners that establish preferentially near their tutors. Furthermore, we found no evidence of individuals changing their phrase type repertoires over time, including after remating events. Duet type repertoires were also stable when pairs remained together. However, we observed a surprisingly high turnover rate. When individuals remated, even though the majority of the previous duet type repertoire remained, several new duet types were included. Taken together, our findings suggest that riverside wrens might create their individual repertoires by copying their same-sex parent and neighbouring individuals before dispersal. Additionally, we speculate that even though birds were able to create new duet types after changing partners, a substantial portion of their duet type repertoire might also be copied from their parents and neighbouring pairs during the initial critical period of song learning.     

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.     

Vocal negotiation over parental care? Acoustic communication at the nest predicts partners' incubation share

In species with biparental care, individuals adjust their workload to that of their partner to either compensate or match its investment. Communication within a pair might be crucial for achieving this adjustment. Zebra finches, Taeniopygia guttata, form life‐long monogamous pair bonds, in which partners are highly coordinated and both incubate the eggs. When relieving each other during incubation, partners perform a structured call duet at the nest. If this duet functions to coordinate incubation workload, disrupting the pair's usual nest‐relief pattern by delaying the male's return to the nest should affect the structure of the duet. Using domesticated birds breeding in a large aviary, we found that delaying the male's return induced shorter duets with higher call rates. In addition, we tracked the location of individuals with a transponder at the nest and the feeder, and showed that these accelerated duets were associated with an increased haste of the partners to take turns incubating and foraging. Females also spent less time incubating during their subsequent shift, and females' time off‐nest was best predicted by their mate's calling behaviour in the previous duet. Taken together, these results suggest that duets may function as ‘vocal negotiation’ over parental care.