Song fine structure as a cue to male quality in black-capped chickadees (Poecile atricapilla)

Among explanations for the evolution of birdsong, the hypothesis that song fine structure can provide cues to relative male quality is not well tested. A less recognized requirement of this honest advertisement model, however, is that these signals must be reliably discriminable by females who often listen to singing males from considerable distances. The two-note fee-bee of male black-capped chickadees, Poecile atricapilla , is considered a single-type song that singers transpose up and down a continuous frequency range. The song functions as a sexual signal, but how it encodes information about male quality remains unclear. We studied the acoustic structure of songs of male chickadees whose flock dominance status we determined the previous winter. We used analysis of variance and discriminant function analysis to demonstrate that five structural features of song can predict individual identity but not the male quality categories of dominant birds versus their most subordinate flockmates. Further, after transmitting chickadee songs through the forest and re-recording them at multiple broadcast distances, we found that despite significant acoustic degradation, structure continued to predict singer identity as long as songs remained audible (up to 80 m). We then used correlation analysis to show changes in pitch are accompanied by significant corresponding changes in song structure. While the ratio measure of the internote frequency interval remained constant for songs of high-rank birds despite changes in absolute pitch, low-rank males sang a significantly smaller ratio as they shifted to higher absolute pitches. We conclude that song structure may function to signal the identity of singers of known quality or it may provide a general indicator of male quality if songs are used in a series accompanied by shifts in pitch.