In-air and underwater hearing of the cormorant

Numerous studies have mapped the hearing abilities of birds in air but currently there is little or no data on how diving birds hear or react to sound under water. Therefore, it is unknown whether the ears and auditory system of diving birds are adapted to underwater hearing. In the present study, we measured audiograms from wild-caught cormorant fledglings Phalacrocorax carbo sinensis) using auditory brainstem response (ABR). Each bird was anesthetized and the ABR was measured, first, in a sound attenuated and anechoic box in air and, secondly, with its head and neck submerged approximately 10 cm under water in a large water filled-tank while being artificially ventilated. ABR-responses to calibrated tone bursts
produced by a woofer and an underwater speaker, respectively, were measured at different intensities and frequencies to obtain hearing threshold values in air and under water. The shape of the audiogram was similar to that reported for birds of the same size in air. The bandwidth and slopes of the audiogram were similar in air and water. In air the highest sensitivity was found at 1-2 kHz, while the most sensitive response was displaced towards lower frequencies (lower than 1 kHz) under water. Generally, the cormorant ear was not very sensitive to sound, neither in air nor under water. The hearing abilities in water, however, were better than what would have been expected for a purely in-air adapted ear. (Supported by the Carlsberg Foundation
2009_01_0292 and the Danish Council for Independent Research 4002-00536)