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A field test was conducted on the accuracy of an eight-microphone acoustic location system designed to triangulate the position of duetting rufous-and-white wrens (Thryothorus rufalbus) in Costa Rica's humid evergreen forest. Eight microphones were set up in the breeding territories of 20 pairs of wrens, with an average intermicrophone distance of 75.2+/-2.6 m. The array of microphones was used to record antiphonal duets broadcast through stereo loudspeakers. The positions of the loudspeakers were then estimated by evaluating the delay with which the eight microphones recorded the broadcast sounds. Position estimates were compared to coordinates surveyed with a global-positioning system (GPS). The acoustic location system estimated the position of loudspeakers with an error of 2.82+/-0.26 m and calculated the distance between the "male" and "female" loudspeakers with an error of 2.12+/-0.42 m. Given the large range of distances between duetting birds, this relatively low level of error demonstrates that the acoustic location system is a useful tool for studying avian duets. Location error was influenced partly by the difficulties inherent in collecting high accuracy GPS coordinates of microphone positions underneath a lush tropical canopy and partly by the complicating influence of irregular topography and thick vegetation on sound transmission.

Summary In many tropical animals, male and female breeding partners combine their songs to produce vocal duets [1–5]. The temporal precision of these coordinated vocalizations is often so astonishing that human listeners mistake duets for the songs of a single animal [6]. Behavioural ecologists rank duets among the most complex and sophisticated vocal performances in the animal kingdom [7,8]. Despite decades of study, the evolutionary significance of vocal duets remains elusive [9], in large part because many duetting animals live in tropical habitats where dense vegetation makes direct behavioural observation difficult or impossible. Here we evaluate the vocal duetting behaviour of neotropical rufous- and-white wrens (Thryothorus rufalbus) in the humid forests of Costa Rica. We employ two innovative technical approaches to study vocal behaviour: an eight-microphone Acoustic Location System capable of passively triangulating the position of animals based on recordings of their vocalizations [10], and dual-speaker playback capable of simulating vocal duets in a spatially realistic manner [11]. Our analyses provide the first detailed spatial information on duetting animals in both a natural context and during confrontations with territorial rivals. We demonstrate that birds perform duets across distances that are more variable than previously imagined, that birds approach their partner after performing duets, and that birds respond to the duets of rivals with aggressive, sex-specific strategies. We conclude that the vocal duets serve distinct functions in aggressive and non-aggressive contexts.

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Song-type matching is a singing strategy found in some oscine songbirds with repertoires of song types and at least partial sharing of song types between males. Males reply to the song of a rival male by subsequently singing the same song type. For type matching to serve as an e¡ective long-distance threat signal, it must be backed up by some probability of aggressive approach and impose some type of cost on senders that minimizes the temptation to blu¡. Western subspecies of the song sparrow exhibit moderate levels of song-type sharing between adjacent males and sometimes type match in response to playback of song types they possess in their repertoires. Interactive playback experiments were used in order to examine the subsequent behaviour of type-matching birds and to quantify the responses of focal birds to type-matching versus non-matching stimuli. Birds that chose to type match the playback of a shared song type subsequently approached the speaker much more aggressively than birds that did not type match. Moreover, birds approached a type-matching stimulus much more aggressively than a non-matching stimulus. These results and consideration of alternatives suggest that type matching in song sparrows is a conventional signal in which honesty is maintained by a receiver retaliation cost against blu¡ers.