Avian interaural canal enhances interaural delay

Using a simple model of the birds' binaural pressure difference acoustic receivers it was predicted that the interaural delay achieved by birds at low frequencies is far greater than that of mammals with a similar head-size. This was upheld when interaural delay was recorded, between the cochlear microphonics, for six species. Stimulus positions were varied over the azimuthal range from the frontal midline to the interaural axis. Predicted delays were frequency dependent (higher frequency, smaller delay), as were the actual delays, and the magnitude of the measured delays were comparable with predictions. Delays measured at high frequencies were close to those expected from pathlength around the head, but delays measured at low frequencies could be more than three times this expectation. This finding raises the possibility that interaural delay may be a useful localization cue in birds, even for those species with very small heads, since the large delays at low frequencies are sufficient to provide a physiological cue to azimuth.