William R. Rice’s research while affiliated with Oregon State University and other places

The ability of Marsh Hawks (Circus cyaneus) to locate prey acoustically was measured in both the laboratory and the field. Laboratory experiments indicated that the directional hearing of the Marsh Hawk was substantially better than that of a sample of typical diurnal raptors and similar to that of owls capable of capturing prey in total darkness. Angular resolution along the horizontal axis was 2° for the Marsh Hawks, 1-2° for the owls, and 8-12° for the sample of typical diurnal raptors. For the Marsh Hawks, angular resolution along the vertical axis was at least 2°. The maximum range at which prey could be detected by sound was estimated to be 3-4 m for the Marsh Hawk and 7 m for the Barn Owl (Tyto alba). Field experiments indicated that free-ranging Marsh Hawks could locate vole vocalizations (squeaks) accurately and attack prey successfully without the aid of visual or olfactory cues. Additional field experiments were conducted to determine how the Marsh Hawk integrates auditory and visual cues while capturing concealed prey. These experiments show that the Marsh Hawk does not require motion cues or auditory depth perception to determine the elevation of a sound source.