The ability of birds to distinguish acoustic intensities has so far received little attention. Here it was measured for the pigeon, dependent on frequency and sound intensity. Heart-rate conditioning was used as an indication of the distinction. With the help of this classical conditioning method, an associative connection between an unconditioned (acoustic) stimulus and a conditioned one (a weak electric shock) is developed and the reaction of the animal's heart-rate is evaluated [1, 2].
Eight homing pigeons of either sex were tested in a sound-proof box with sinus tones of 3.5 s duration. The intensity difference limens were received at five frequencies between 400 Hz and 6 kHz and five different sound intensities between 30 and 70 dB (SPL) each.
It became obvious that the success of the examination depended on the proceeding and presentation of the task of discrimination. Learning a task could be attained best by presenting the conditioned stimulus (= test tone) louder than the other sinus tone.
Prior to the test tone an accidental
number of lower unconditioned reference tones were played to the
birds. The sound intensity of the reference tones was approximated
to the test tone intensity until the discrimination of the acoustic
signals could no longer be secured statistically. The smallest intensity
difference percepted with certainty over several sessions (Student's
t-test, one-tailed error probability 2.5%) was termed the difference