Characteristics, Use, and Significance of Two Singing Behaviors in Grace's Warbler (Dendroica graciae)

Each male Grace's Warbler (Dendroica graciae) possessed two distinct subsets (A and B) of preferred songs and sang each subset with a different singing behavior. In singing behavior A, males sang subset A songs in a more repetitive manner (e.g. XXXX...); in singing behavior B, males sang subset B songs in a more variable, often alternating, manner (e.g. YZYZ...). Furthermore, singing behavior B was significantly higher in song diversity, rate, and complexity than singing behavior A; and the structure of B songs was more variable than the structure of A songs. The particular song types used in the two behaviors varied among males and, in several cases, the same song type was used in the opposite singing behavior by different males. I suggest that singing behaviors rather than song types are key features in song communication in this species. Pairing status, time of season and day, location in the territory, and type of social interaction significantly affected the choice of singing behavior. Prior to pair formation and whenever males interacted with females, singing behavior A predominated. After pairing, singing behavior B predominated, particularly in countersinging between neighboring males, at songposts, and at dawn. Thus, in a relative comparison of functions, singing behavior A appears to have a stronger intersexual component, and singing behavior B a stronger intrasexual component. Presumably, the two singing behaviors differ in function and have been shaped by different selection pressures. They are used in ways parallel to, and thus functionally similar to, the two kinds of song identified in several other species of wood-warbler.