Syntax of complex bird song in the large-billed reed warbler (Acrocephalus orinus)

Among songbirds there are species with rich and variable repertoires and very complex sequences of sounds that are hard to analyse. Here, we investigate repertoire and syntax organization in the large-billed reed warbler advertising vocalization. We provide the first detailed qualitative and quantitative description of a song of a recently rediscovered species with the analysis of song syntax by clustering. The song of A. orinus combines discrete and continuous types of singing. Although the vocalization can be produced continuously for minutes without any significant pauses, it is easy to split it into distinctive songs using the criterion of 500 ms pause. Songs themselves are grouped into song classes. Both songs and song classes are unique to each individual, though several simple notes could be shared between males. Linear syntax is found both in sequences of syllables and in sequences of songs. Each song starts with initiator syllable types. We also discovered the presence of the transitional songs that sometimes separate songs from different classes in the vocal stream. The repertoire size in our sample varies from 69 to 147 syllable types and from 10 to 24 song classes, and does not depend on the duration of recording. Only one male’s repertoire was estimated entirely. This male was singing alone while other males sang in the presence of other individuals. We discuss how the presence of listeners may affect the repertoire and temporal and sequential organization of the song.