Acoustical features of song categories of the Adelaide's Warbler (Dendroica adelaidae)

Abstract

I identified acoustical features that distinguish the two song categories, A and B, of the Adelaide's Warbler (Dendroica adelaidae). Like many other wood-warbler species, male Adelaide's Warblers have functionally structured song repertoires such that A and B song categories are distinguished by common usage among males. The particular song types used in a given category, however, vary greatly among individual males. Therefore, I focused my analysis on acoustical features other than song types. Song categories were distinguished by three major features: (1) the sound energy within a given B song was concentrated into a narrower frequency band, which was shifted about 600 Hz lower than for A songs; (2) the structure of B songs tended to be more complex, as they contained more note types and greater contrast between successive types than did A songs (discriminant function analysis using structural and frequency data correctly classified 85% of songs to category); and (3) the singing behavior associated with B-song sequences was more complex, due to inclusion of a larger number of song types, more frequent switching or alternating between them, and a more rapid rate of song delivery. Overall, the B category appears to represent a more complex and lower frequency signal than the A category. Differences between the two song categories, which parallel data available for other paruline species, should reduce signal ambiguity and are consistent with several nonmutually exclusive functional interpretations: The B-song category, used in intrasexual interactions, might represent a more aggressive signal or might be specialized for carrying motivational information. Females might find the higher-frequency A-song category more appeasing or stimulating. Song categories A and B also might be specialized for traveling different distances, at different times of day, or through different microhabitats.