Although song development in songbirds has been much studied as an analogue of language development in humans, the development of vocal interaction rules has been relatively neglected in both groups. Duetting avian species provide an ideal model to address the acquisition of interaction rules as duet structure involves time and pattern-specific relationships among the vocalizations from different individuals. In this study, we address the development of the most striking properties of duets: the specific answering rules that individuals use to link their own phrase types to those of their partners (duet codes) and precise temporal coordination. By performing two removal experiments in canebrake wrens (Cantorchilus zeledoni), we show that individuals use a fixed phrase repertoire to create new phrase pairings when they acquire a new partner. Furthermore, immediately after pairing, individuals perform duets with poor coordination and poor duet code adherence, but both aspects improve with time. These results indicate that individuals need a learning period to be able to perform well-coordinated duets that follow a consistent duet code. We conclude that both duet coordination and duet code adherence are honest indicators of pair-bond duration.

Exchange of vocal signals is an important aspect of animal communication. Although birdsong is the premier model for understanding vocal development, the development of vocal interaction rules in birds and possible parallels to humans have been little studied. Many tropical songbirds engage in complex vocal interactions in the form of duets between mated pairs. In some species, duets show precise temporal coordination and follow rules (duet codes) governing which song type one bird uses to reply to each of the song types of its mate. We determined whether these duetting rules are acquired during early development in canebrake wrens. Results show that juveniles acquire a duet code by singing with a mated pair of adults and that juveniles gradually increase their fidelity to the code over time. Additionally, we found that juveniles exhibit poorer temporal coordination than adults and improve their coordination as time progresses. Human turn-taking, an analogous rule to temporal coordination, is learned during early development. We report that the ontogeny of vocal interaction rules in songbirds is analogous to that of human conversation rules.

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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 fi xed 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 fi eld. 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 modifi cations 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 diff erences 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. Th at precise coordination is achieved by a complex and dynamic process supports the idea that this behavior could signal pair bond strength.

Avian song learning has a rich history of study and has become the preeminent system for understanding the ontogeny of vocal communication in animals. Song learning in birds has many parallels with human language learning, ranging from the neural mechanisms involved to the importance of social factors in shaping signal acquisition. While much has been learned about the process of song learning, virtually all of the research done to date has focused on temperate species, where often only one sex (the male) sings. Duetting species, in which both males and females learn to sing and learn to combine their songs into temporally coordinated joint displays, could provide many insights into the processes by which vocal learning takes place. Here we highlight three key features of song learning-neuroendocrine control mechanisms, timing and life history stages of song acquisition, and the role of social factors in song selection and use-that have been elucidated from species where only males sing, and compare these with duetting species. We summarize what is known about song learning in duetting species and then provide several suggestions for fruitful directions for future research. We suggest that focusing research efforts on duetting species could significantly advance our understanding of vocal learning in birds and further cement the importance of avian species as models for understanding human conversations and the processes of vocal learning more broadly.