Between order and chaos: contrasting syntax in the advertising song of Dusky (Phylloscopus fuscatus) and Radde’s (Ph. schwarzi) Warblers

Abstract

Dusky (Phylloscopus fuscatus) and Radde’s (Ph. schwarzi) Warblers are small insectivorous avian species occurring as regular inhabitants of the taiga in central and eastern Siberia. Their singing is characterized by a typically discontinuous performance, strict stereotypy of all song types and immediate variety in the course of a song. The songs of both species have a typical trill structure and the repeated trill syllables are composed of up to three notes. Individual repertoires of Dusky Warbler males include 60–70 song types, but each male has just 3 or 4 “favorite” song types which are produced more often than all the others and in total make up to a third of all vocal production. The rest of the vocalizations are represented by rare song types. The sequence order of different song types looks like a random process. Only a few transitions within the songs of some males appear more often than might be expected by chance. In contrast, vocalizations of the Radde’s Warbler represent a strictly determined periodic process. All song types are produced according to fixed sequences which include up to 22 different types. Because of prolonged pauses between the songs, the performance of such sequences takes up to 160–190 s. The rigid order of performance of the song types does not change even with the longest pauses (up to 20–30 s). After a pause, the male invariably executes that song type which is expected in its given place. The adaptive value of large repertoires with either fixed or random sequences of song types is discussed.