Auditory intensity discrimination in blackbirds and pigeons.

Redwing blackbirds, brown-headed cowbirds, and pigeons were trained with operant conditioning techniques to respond to small increases in the intensity of pulsed tone trains at three frequencies: .5, 1.0, and 2.0 kHz. All three species produced similar intensity difference limens (DLs) at the frequencies tested. Intensity DLs decreased as sensation level (intensity level above absolute threshold) increased at all three frequencies, with the slopes of these sensation level functions being greatest at 2.0 kHz . The median intensity DLs at 50 dB sensation level were 3.3, 2.7, and 2.9 dB at .5, 1.0, and 2.0 kHz, respectively, averaged over the three species. Some subjects were also required to detect decreases in intensity. They produced intensity DLs two to three times larger than the DLs obtained when these same subjects were required to detect increases in intensity. Avian intensity DLs generally appear to be 1-2 dB higher than the DLs of those mammals that have been tested (rat, cat, monkey, humans).