WHITE-THROATED SPARROWS (ZONOTRICHIA-ALBICOLLIS) CAN PERCEIVE PITCH CHANGE IN CONSPECIFIC SONG BY USING THE FREQUENCY RATIO INDEPENDENT OF THE FREQUENCY DIFFERENCE

In the ascending songs of white-throated sparrows (Zonotrichia albicollis), the first note (Phrase 1) is sung lower than the remaining notes (Phrase 2). The production of this frequency change is more reliably predicted by the frequency ratio (Phrase 2 divided-by Phrase 1) than by the frequency difference (Phrase 2 - Phrase 1). To determine whether sparrows use the ratio rather than the difference to identify ascending songs, we transposed a low-frequency song upward in frequency, maintaining a normal frequency difference but altering the frequency ratio, and we transposed a high-frequency song downward in the same way. Territorial males tested in the wild sang more and approached more closely to songs with normal ratios than to songs with altered ratios, even when the frequency difference was identical in both. This is the first evidence that the perception of pitch change by songbirds is related to the frequency ratio independent of the difference.