Dynamics of frequency and amplitude modulations in vocalizations produced by eastern towhees, Pipilo erythrophthalmus.

Eastern towhees, Pipilo erythrophthalmus (Emberizidae, Passeriformes), appear to estimate source-SPL using spectral or temporal variables. Nevertheless, vocalizations are dynamic by nature and it remains unclear whether subjects pay attention to correlations between discrete variables or pay attention to the dynamics that these variables summarize. Sine functions are used to describe coarse (slow, < 10 Hz) frequency and amplitude modulations in towhee calls and correlations between variables are identified. Towhee calls are also finely (rapidly, > 400 Hz) modulated in both amplitude and frequency. Fine amplitude and frequency modulations correlate well (occur in phase) over relatively low fundamental frequencies (< approximately 3.5 kHz) and tend to have greater amplitudes and frequencies over these same frequencies. Modulations and correlations between modulations might exist due to stable dynamic interactions that occur within and between the physical forces that function to produce modulations in vocalizations. Results support the hypothesis that towhees communicate within separate sound frequency channels defined to each side of approximately 3.5 kHz.