A Comparative Study of Hearing in Owls of the Family Strigidae

Abstract

In view of throwing new light on some zoogeographical and systematical problems in owls, we started a comparative study on hearing in these birds. On the basis of conditioning experiments audibility curves were obtained from 6 Tawny Owls (Strix aluco L.) and 6 Long-eared Owls (Asio otus (L.)). In addition cochlear potentials, evoked by tone-bursts, were derived from 5 Tawny Owls and 4 Long-eared Owls. The size of the potentials was correlated to the applied sound levels. Curves of varying frequency and sound pressure at constant size of the cochlear potential were obtained, showing to some extent the owls' frequency response. The audible frequency ranges found appeared to rank among the best high-frequency hearing known in birds at present, the high-frequency cut-off (defined as frequency with hearing threshold as loud as 0 db = 1μbar/cm 2 ) being about 13 kHz. With regard to sensitivity of hearing, the owls proved to be extremely good hearers with lowest thresholds of - 91 db for the Tawny Owl and - 95 db for the Long-eared Owl. The frequency range of very good sensitivity extends from about 0.4-7 kHz in Tawny Owls and from about 0.5-8 kHz in Long-eared Owls (thresholds for these frequencies do not exceed - 80 db). This stands for 4 octaves of very sensitive hearing. These hearing characteristics have been shown in the Figs 3 and 4. The results obtained from the cochlear potentials corresponded rather well with the behavioral findings (Figs 7 and 8). Some systematical lack of correspondence could be explained to a certain extent, as being caused by differences in applied methods. In additional experiments behavioral audibility curves were also collected from 9 owls belonging to 8 other species of which the external ears are less specialized than those of Tawny and Long-eared Owls (Fig. 9). In these 8 species we found a wide range of hearing sensitivity. The best hearing individuals showed a sensitivity of the same order as that found in Tawny and Long-eared Owls. However, their frequency range of very high sensitivity never exceeded 6 kHz in contrast to the more extended ranges of Tawny and Long-eared Owls (Fig. 10). The relevancy of these data for owl systematics was discussed.