Song Types, Repertoires and Song Variability in a Population of Chestnut-Sided Warblers

To help assess the function of multiple song types in the singing of the Chestnut-sided Warbler (Dendroica pensylvanica), a species with two song categories, I analyzed song diversity and variability in a Massachusetts population. Individual song repertoires contained up to 12 different song types (median 6 or 7), and included both accented-ending (AE) and unaccented-ending (UE) songs. AE song types were few, and were highly stereotyped both within and between individuals. In contrast, UE songs were variable, with many different types present in the population, and with different renditions of a type showing a larger range of variation. The distinction between constrained, stereotyped AE singing and flexible, variable UE singing suggests that song forms in the two song categories are specialized for different uses. AE songs are suited to comparative assessment of singers, long-range communication, or messages in which reduced ambiguity is crucial. UE songs may be specialized for communication over short distances or during interactions in which motivations and messages change rapidly. Chestnut-sided Warblers use most of the songs in their repertoires only infrequently, and no set of song types is shared by all members of a population. Consequently, the multiple song types within each song category are probably not connected to functions in which individuals benefit by displaying repertoire size, or in which particular song types encode special messages. Repertoire function instead probably involves a communication function for switches between commonly- and uncommonly-used song types.