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 signaller’s skill or
motivation. We tested for evidence of performance constraints in
Adelaide’s 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. 
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 performance is constrained. We conclude that physiological limits on
frequency modulation and respiration constrain song structure in male
Adelaide’s warblers. Further work is needed to determine whether receivers
respond to natural levels of variation in performance, and whether
performance correlates with singer quality or motivation.