Aspects of frequency discrimination in passerine birds and pigeons

With the use of a positive reinforcement operant technique, pulsed-tone, frequency difference limens (DLs) were measured in two Icterid passerine species, the redwing blackbird (Agelaius phoeniceus) and the brown-headed cowbird (Molothrus ater). Direct comparisons were made with two nonpasserine control (pigeon) subjects. Blackbirds were more sensitive than pigeons to frequency changes above 1.0 kHz but less sensitive below 1.0 kHz. Measured DLs in all these birds were within the range of nonhuman mammals but higher than those of humans. The lowest delta f/f ratio for all birds occurred at 4.0 kHz, a frequency for which they exhibit minimal absolute auditory thresholds and which is a dominant component in the song of the redwing. As in humans, blackbird sensitivity at 1.0 and 2.0 kHz declined as sensation level (SL) was decreased, but at 4.0 kHz, sensitivity showed no clear dependence on SL. At 8.0 kHz, near the upper limit of blackbird hearing, sensitivity increased with a decrease in SL. This latter frequency is a dominant component in the song of the cowbird. Differences in frequency sensitivty were also found as a function of the direction of frequency shift from the standard: Upward shifts produced lower DLs at 1.0 and 2.0 kHz, and downward shifts produced lower DLs at 8.0 kHz.