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We studied the duet of the Caribbean-slope subspecies of the Plain Wren (Thryothorus modestus zeledoni) in Costa Rica. It is one of the most complex duets to have been described. The duet proper consists of rapid, highly coordinated alternation of “A-phrases” from the female and “B-phrases” from the male. While the female initiates this section with her A-phrase, this cyclical part of the duet is almost invariably preceded by an introductory “I-phrase” from the male, so that it is the male that initiates the performance. Each male has a repertoire of I- and B-phrases, and each female has a repertoire of A-phrases. These are specifically associated with each other to form a repertoire of duet types. We hypothesize that the pattern of song organization in this species facilitates more coordinated and precise duetting. The presence of the three components means that a full duet requires the cooperation of both members of the pair, strongly suggesting that it represents a mutually beneficial signal. El Canto en Dueto de Thryothorus modestus Resumen. En este trabajo estudiamos el canto en dueto de la subespecie caribeña Thryothorus modestus zeledoni en Costa Rica. El canto en dueto de esta especie es uno de los más complejos que se han descrito hasta la fecha. El dueto consiste en una serie de frases “A” por parte de la hembra y frases “B” por parte del macho alternadas de una manera altamente coordinada. A pesar de que la hembra inicia esta sesión de dueto con una frase “A”, esta parte del dueto es precedida invariablemente por una frase introductoria “I” del macho, de modo que el macho es el que inicia propiamente el canto. Cada macho tiene un repertorio de frases “I” y “B”, y cada hembra tiene un repertorio de frases “A.” Estas frases están estrechamente asociadas entre ellas para formar un repertorio de cantos en dueto. La hipótesis que proponemos es que el tipo de organización en el canto de esta especie facilita un canto en dueto más preciso y coordinado. La presencia de tres componentes hace que el canto en dueto requiera la cooperación absoluta de ambos miembros de la pareja. Esto sugiere que este tipo de señal beneficia a ambos sexos.

Song-type switching rate and song matching have been shown to function as territorial signals in male solo song but, to our knowledge, seem not to have been previously studied in a duetting species. We studied the plain wren (Thryothorus modestus zeledoni ), to test whether duets signalled threat through song-switching rates, or through phrase type or duet type matching. Increases in the rate of song switching appear to function as an anti-habituation device rather than as a specific signal of threat. Fitting with previous results that same-sex individuals share phrase types, but pairs do not share duet types, both males and females used duets to phrase type match to playback. Pairs, however, did not duet type match in response to playback, and this suggests that within the cooperative territory defence of the duet, each sex is targeting its aggression at same-sex competitors.

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.