COMMON sense suggests that birds can hear tones of the same range as those which they produce. However, precise investigations have revealed exceptions and limitations to this rule. All birds, for instance, can hear frequencies down to a minimum of about 50 cycles (table 1); but even the voice of the Great Horned Owl (Bubo virginianus) is considerably higher than this. The upper limit of hearing is usually reached at 20,000 cycles (reached after conditioning), but cochlear potentials have been detected at 25,000 to 30,000 cycles by using greater intensities of sound than those normally encountered by the ear. The notes of song birds, however, frequently contain still higher harmonics (Brand, 1938). The highest sensitivity of hearing in a given species--which in song birds reaches the human threshold (Schwartzkopff, 1949) and in owls probably exceeds it--is mostly found at the modal center of the voice of that species. Among related species, the larger ones have lower voices, and hearing sensitivity shows a corresponding trend (compare Corvus and Pica with Pyrrhula and Fringilla in table 1). Such a shift is undoubtedly owing to differences in the dimensions of the vibrating parts (specific resonance). Exceptions to this size-voice relation are of ecological and physiological interest. For example, owls are very sensitive to tones above the midpoint of the voice of song birds (6000 cycles), but which correspond to the squeak of some mice. Baby chicks hear almost nothing but the low clucking of their mother (400 cycles) while the hen reacts with preference to the very high cheeping (above 3000 cycles) of her offspring (Collias and Joos, 1953). Though one cannot always with certainty apply to physiology conclusions drawn from ethology, it seems that in the present case something having to do with the developmental stage of the middle ear of chicks impedes the reception of higher notes (table 2). There is probably a selective value in the chicks being able to hear only the mother and not their fellow chicks.