Song organisation and singing patterns of the willow warbler, Phylloscopus trochilus

In this paper we describe the organisation of the song of the willow warbler and its pattern of singing, with the objective of providing robust measurements of song complexity that can be related to the functional and evolutionary aspects of song. Songs are composed of distinct individual elements, which can be reliably identified. These elements are not randomly assembled within each song: most songs start always with the same element, and thereafter elements are produced in a highly predictable way. The organisation of elements corresponds to a hierarchically branching pattern, where elements either connect predictably to a second one, or constitute a decision point where two or more transitions are possible. This branching pattern pervades the whole length of the sequence of elements, and suggests that the concept of song type is not appropriate in this species. The singing pattern could be characterised by a random delivery: consecutive songs are not more similar from each other than if they were delivered at random. Individuals differ greatly in several song characteristics. A principal component analysis performed in a comprehensive set of song complexity measurements produced 5 meaningful components: Song repertoire size (PC1); Song length (PC2); Song versatility (PC3); Element rate (PC4); and Repertoire fixation (PC5). The first component - song repertoire size - showed the highest variation between individuals and a high same-year repeatability. This makes of song repertoire size a likely target for sexual selection in this species.