Use of song types by the Chestnut-sided Warbler: evidence for both intra- and inter-sexual functions

Abstract

Males of many wood warbler (Parulinae) species use different song types in different contexts, yet the exact functions of the two main song type categories remain unclear. We studied the use of songs by both experimental (males whose mate had been removed from the territory) and control male Chestnut-sided Warblers (Dendroica pensylvanica) during the dawn hour and midmorning throughout the breeding season. Unpaired males sang more accented-ending songs and fewer unaccented-ending songs than paired males during all observation periods. Accented-ending songs appeared to be used primarily in the absence of intrasexual stimuli, and the percentage of unaccented-ending songs that was used during the nesting cycle appeared to fluctuate directly with the intensity of defense by the male of both his female and his territory. During courtship the male sang accented-ending songs on those infrequent occasions when he did sing in the immediate presence of his female, regardless of her location and the presence or absence of other males. These critical observations seem most consistent with the conclusion that the accented-ending songs are primarily intersexual. The unaccented- and accented-ending categories of song types appear to be used mainly as intra- and inter-sexual messages, respectively.