Chickadees fail standardized operant tests for octave equivalence

Octave equivalence occurs when an observer judges notes separated by a doubling in frequency perceptually similar. The octave appears to form the basis of pitch change in all human cultures and thus may be of biological origin. Previously, we developed a nonverbal operant conditioning test of octave generalization and transfer in humans. The results of this testing showed that humans with and without musical training perceive the octave relationship between pitches. Our goal in the current study was to determine whether black-capped chickadees, a North American songbird, perceive octave equivalence. We chose these chickadees because of their reliance on pitch in assessing conspecific vocalizations, our strong background knowledge on their pitch height perception (log-linear perception of frequency), and the phylogenetic disparity between them and humans. Compared to humans, songbirds are highly skilled at using pitch height perception to classify pitches into ranges, independent of the octave. Our results suggest that chickadees used that skill, rather than octave equivalence, to transfer the note-range discrimination from one octave to the next. In contrast, there is evidence that at least some mammals, including humans, do perceive octave equivalence.