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.     

Loss of complex female song but not duetting in the ancestors of Carolina wrens

Female singing and coordinated male–female duetting are often but not always found in the same species. Both behaviors are more common in tropical than temperate songbirds, but few studies have differentiated between the factors selecting for each. Here we investigate the evolution of female vocal complexity and male–female vocal coordination in Carolina wrens (Thryothorus ludovicianus), one of the few non-tropical members of a songbird family (Troglodytidae) that is well known for producing coordinated male–female duets. Female Carolina wrens are not known to sing; rather, they produce relatively simple, sex-specific chatters, often during territorial encounters. We analyzed field recordings to show that females coordinate these chatters with male songs at rates similar to those observed in some tropical duetting wren species. We then used phylogenetic comparative methods to show that the evolutionary ancestors of Carolina wrens had female songs that were more acoustically complex than the vocalizations of current females, suggesting past selection against female vocal complexity. Levels of vocal coordination with males, in contrast, have changed relatively little from those of tropical ancestors. Our results suggest that these two aspects of female behavior, acoustic complexity and vocal coordination with males, have evolved independently and have different functions in communication.     

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.     

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.     

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.     

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 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.     

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.     

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.     

Partners coordinate territorial defense against simulated intruders in a duetting ovenbird

Duets in breeding pairs may reflect a situation of conflict, whereby an individual answers its partner's song as a form of unilateral acoustic mate guarding or, alternatively, it may reflect cooperation, when individuals share in territory defense or safeguard the partnership. The degree of coordination between the sexes when responding to solo versus paired intruders may elucidate the function of songs in duets. We examined this issue in a study with rufous horneros (Furnarius rufus), a duetting, socially monogamous Neotropical species with low levels of extrapair paternity. We exposed social pairs during the nonbreeding season to playbacks of duets, male solos, female solos, and control heterospecific songs. Partners approached all conspecific stimuli together and responded by singing quickly, at higher rates and by coordinating ~80% of their songs into duets. For both sexes, most response variables (seven of nine) did not vary across conspecific treatments. These results suggest that partners duet and coordinate behaviors to cooperatively defend common territories. However, females spent more time in territorial vigilance, and partners were highly coordinated (correlated responses) in response to duets and female solos in comparison with male solos. This indicates that female intrusions (paired or solo) might be more threatening than male intrusions in the nonbreeding season, especially for territorial females, and that females are less cooperative with their partners in territory defense against male intruders.     

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.     

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.     

Duet codes do not enhance neighbour recognition in two closely related species of duetting neotropical wrens

Numerous studies have shown that territorial animals exhibit less aggression in response to neighbours than to strangers, a phenomenon known as dear enemy effect. The influence of acoustic features, such as song type sharing and repertoire sizes, in neighbour recognition has been widely documented in male songbirds. However, few studies have focused on duetting species, and particularly on those where pairs have pair‐specific duet codes (consistent associations of their individual phrase types). Given that each pair in the population can have a unique repertoire of duet types, duet codes have been hypothesized to enhance discrimination. In this context, we tested for evidence of neighbour recognition and duet code discrimination in two closely related species of neotropical wrens, the riverside wren, Cantorchilus semibadius, and the canebrake wren, C. modestus zeledoni. Although both species have moderately large repertoires, riverside wrens have higher levels of phrase type and duet type sharing across the population. We compared the approach and vocal responses of focal pairs to three playback treatments: neighbours' correct duet type, neighbours' incorrect duet type and a strangers' duet type. We found that riverside wrens displayed a stronger response to the strangers' playback than to both neighbours' playbacks, whereas no differences among treatments were found in canebrake wrens. Given that both species exhibited similar levels of aggression during neighbour playbacks, regardless of whether the correct duet code was used, our findings suggest duet codes do not facilitate neighbour recognition. We conclude that the function of duet codes in these species might be more closely related to intra‐pair communication. Finally, we suggest that the level of dear enemy effect a species exhibits depends on ecological factors that influence the perceived level of threat of territory intruders.     

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.     

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.     

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.     

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.     

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.     

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.     

Duets in Yellow-Naped Amazons: Variation in Syntax, Note Composition and Phonology at Different Levels of Social Organization

Complex vocal signals composed of multiple notes are used by many species. Such signals may vary in a number of features such as the rules that govern note order and timing (syntax), the relative number and types of different notes (note composition), and the acoustic structure of notes (phonology). Previous research examining male song in songbirds typically has shown greater conservation of syntax than phonology. Here we investigated whether these patterns of variation are also found in the duets of parrots. We examined geographic variation in the pair duets of yellow-naped amazons, Amazona auropalliata , at five sites within one vocal dialect. We also examined variation at several levels of social organization, including within pairs, among pairs and among sites, to assess where variability was greatest. Variation was highest at the within-pair level, although variation was also present at all other levels and for all of the duet factors. We hypothesize that variation at the among-site level allows duets to indicate site membership, while variation at the within-pair level allows pairs to change their duet to match their current social or physical environment. Syntax and phonology are also more conserved than note composition for all levels of social organization. This pattern may result because note composition is less important in duet function than syntax or phonology, or because development of syntax and phonology is more heavily influenced by genetic templates.