Detection of frequency modulation (FM) and amplitude modulation (AM) by budgerigars ( Melopsittacus undulatus )

Many bird vocalizations contain elements consisting of both frequency-modulated (FM) and/or amplitude-modulated (AM) compo- nents. In humans, differences in perception of FM versus AM sounds are related to other psychoacoustic phenomena and have provided an interesting test of the excitation pattern model of hearing. Here, budgerigars were trained by operant conditioning and tested with the method of constant stimuli to detect FM and AM. The birds were tested at three carrier frequencies (1.0, 2.86, and 4.0 kHz) and the threshold for detecting modulation was measured at eight different modulation frequencies from 5 to 640 Hz. The threshold for detection of frequency modulation decreased with increasing modulation frequency, while the threshold for detecting amplitude modulation was fairly constant over the entire range of modulation frequencies. This pattern of results is similar to that found for humans. Critical modulation frequencies calculated from these functions will be compared to other psychoacoustic phenomena in budgerigars and the relevance of these findings for mechanisms of hearing in birds and the characteristics of vocal signals will be discussed. [Work supported by NIH DC-00198 and MH-00982 to RJD.]