Chapter 3 A Review of Vocal Duetting in Birds

Abstract

Avian duetting is an unusual but taxonomically widespread phenomenon, occurring in over 400 species, representing 40% of bird families. Duets vary in form from loosely overlapping songs to highly coordinated duets where paired birds both adjust the timing and type of phrases they sing to fit those of their partner over the course of the duet. Duet coordination therefore signals how attentive an individual is to its partner, both to the partner and to other listeners. While some aspects of duetting are poorly understood, such as its ontogeny and causation (including hormonal and neural bases), it is clear that duetting serves multiple functions, including maintaining the year-round territories and partnerships that characterize many duetting species.

Introduction

There have been over one hundred years of research on duetting, most focusing on the form and function of duetting, and less is known about other aspects of this unusual communication strategy (Farabaugh, 1982, Hall, 2004, Langmore, 1998, Thorpe, 1972, von Helversen, 1980). The accurately timed duets of Australian magpie-larks (Grallina cyanoleuca) were first described in the Agricultural Gazette of New South Wales more than a century before experimental demonstration of the value of precise duet timing in creating threatening territorial displays by signaling the stability of partnerships to territorial rivals (Fig. 1, Cobb, 1897, Hall and Magrath, 2007). In that time, studies focusing on the form of duets have revealed considerable diversity among species, not only in the temporal precision with which partners coordinate their songs to form duets, but also in the consistency with which they associate particular song types in duets, and many other aspects of duet structure (Logue, 2006, Mann et al., 2009, Thorpe, 1963). An increasing focus on the individual behavioral patterns that underlie this variety in duet structure, and how these differ between species and between the sexes, is informing understanding of duet function (Grafe et al., 2004, Logue et al., 2008, Wright and Dahlin, 2007). Most detailed studies on duet function reveal multiple functions of duetting within a species in different contexts (Grafe and Bitz, 2004b, Logue, 2007b, Marshall-Ball et al., 2006, Mennill and Vehrencamp, 2008, Sonnenschein and Reyer, 1983). However, while much is known about duet structure and function, its ontogeny, hormonal basis, and underlying neural structures remain poorly understood.

Duets comprise a vocalization initiated by one individual and answered by its partner such that their vocalizations overlap or alternate (see Glossary in Box 1). Distinguishing between pair-level aspects of duetting and the underlying individual behaviors, as well as distinguishing between those individual-level behaviors that occur in response to a partner and those that occur independently, is essential to understanding how and why duetting occurs. Songs that initiate duets are independent of the partner, but song answering represents a response to the partner that is coordinated to varying degrees with respect to timing and song type (Todt and Naguib, 2000). The timing and types of phrases within duets may likewise be independent of the partner (following an autonomous tempo and pattern), or adjusted to suit the timing and sequence of phrase types used by the partner (von Helversen, 1980). Thus, though duet initiators begin singing independently of their partner, they may respond to their partner over the course of the duet by modifying the timing or types of phrases they use (Whitford, 1996). The ontogeny of those behaviors that are independent of the partner, such as calls, songs, phrases, syntax, singing styles, etc., is likely to be similar to their ontogeny in nonduetting species. However, behaviors that occur in response to a partner, such as song answering, consistent reaction times, and consistent answering rules, are likely to have ontogenetic pathways unique to duetting species, and perhaps differing between the sexes. Likewise, the neural and hormonal bases of duetting can be separated into those components that are shared with nonduetting species, and those that underlie responsive behaviors distinct to duetting species. Understanding how the two individuals contributing to the duet differ in the extent to which they modify their singing pattern in response to their partner, and how that is affected by context, is necessary to understand the function of duetting, and the evolution of the individual behaviors that underlie duets.

Vocal interactions between duetting partners have many parallels with vocal interactions between countersinging male songbirds (Todt and Naguib, 2000). Both are interactive processes that involve time- and pattern-specific relationships among the exchanged signals (Todt and Naguib, 2000). Since song is a broadcast signal, males overlap or match the song types of conspecifics to direct aggressive signals at particular rivals (Vehrencamp et al., 2007). Conspecifics in the wider communication network may pay attention to the temporal and song type aspects of these vocal interactions to obtain information about status relationships between countersinging rivals (Logue and Forstmeier, 2008, Mennill et al., 2002, Naguib and Todt, 1997, Naguib et al., 1999). In general, the vocal interactions between duetting partners occur on a much faster timescale, and responding to a partner in duet requires that an individual is attentive, not only to when its partner begins singing, but also, for certain styles of duetting, to the timing and phrase types of its partner throughout the duet. Duets that involve individuals reacting to their partner in just fractions of a second demonstrate how attentive they are to when and what their partner is singing (von Helversen, 1980), with this information available, not only to the partner, but also to other conspecifics in the communication network.

Avian duetting has some similarities with duetting in other taxa. Insect duets are also characterized by extremely short latencies between calls with, for example, latencies of just 25 ms between the end of the male call and the start of the female call in phaneropterid bushcrickets (Ancistrura nigrovittata) (Dobler et al., 1994). However, while avian duets may be initiated by males or females, insect duets are usually formed by females replying to male calls with species-specific latencies that allow males and females to find one another for mating (Bailey, 2003). In an unusual case of duetting among anurans, female South African clawed frogs (Xenopus laevis) advertise their fertility with a rapping call to which males respond with an answering call and approach for mating (Tobias et al., 1998). Duetting birds share more ecological characteristics with duetting primates, which form long-term socially monogamous partnerships and defend territories (Mitani, 1987). Gibbon duets comprise long bouts of sex-specific phrases, and their coordination into duets takes time to develop in newly formed pairs (Geissmann, 2002, Maples et al., 1989), sharing with some avian duets a greater complexity in structure.