An analysis of avian vocal performance at the note and song levels
Sexual displays that require extreme feats of physiological performance have the potential to reliably indicate the signallers skill. The hypothesis that the structure of bird song is physiologically constrained remains controversial. We tested for evidence of performance constraints in Adelaides warblers (Setophaga adelaidae) songs. At the note level, we identified three trade-offs with well-defined limits. At the song level, we identified two trade-offs, but their limits were less well-defined than the note-level limits. Trade-offs at both levels suggest that song structure is constrained by limits to the speed of both frequency modulation (while vocalizing and between notes) and respiration. Individual males experience the same trade-offs that characterize the population, but the intensity of those trade-offs varies among individuals. Performance metrics derived from the observed limits to performance varied moderately among individuals and strongly among song types. Note-level performance metrics were positively skewed, as predicted by the hypothesis that this population has experienced positive selection for constrained performance. We conclude that physiological limits on frequency modulation and respiration constrain song structure in male Adelaides warblers. Further work is needed to determine whether receivers respond to natural levels of variation in performance, and whether performance correlates with singer quality.