The multifaceted vocal duets of white-browed sparrow weavers are based on complex duetting rules

In some bird species, mated pairs sing duet songs for pair bond maintenance and joint territory defense. Duet partners often follow specific duetting rules, such as answering rules and duet codes, to construct their duet songs. Previous studies mainly concentrated on species that repeat particular phrase pairings during duetting. Our study aims to add to the comparative aspect of duetting research by investigating duet structure in songs of white-browed sparrow weavers Plocepasser mahali, a songbird species that does not repeat the same type of vocalization but switches between different types of vocal elements within one duet song. We used lightweight radio-telemetric microphone transmitters to record the vocal activity in both individuals of duetting pairs simultaneously. This technique enabled us not only to investigate duetting behavior in naturally behaving animals, but also to assign each vocalization to the respective individual while maintaining the precise temporal relationship between the vocalizations of both partners. Our data reveal that although white-browed sparrow weavers share a large part of their sex-specific vocal repertoires with same-sex birds of other pairs, duetting partners adhere to pair-specific answering rules to generate their highly complex duet songs. Moreover, partners can combine answering rules into consistent duet trains. Duet trains generally represent only the core of the duet songs, while at the beginning and the end of duet songs, partners arrange their vocalization types more flexibly. In addition to completely pair-specific duet trains, duet trains of white-browed sparrow weaver pairs can include subsequences that are shared between different pairs. The sharing pattern we find in this species suggests that in combination to pair-specific duetting rules, mated pairs use duetting rules that have been acquired prior to pair formation.