Minimum Intensities of Illumination Under Which Owls Can Find Dead Prey by Sight

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Abstract
Owls were placed in a reaction room in which the amt. of light could be controlled. The floor of the room was covered with either a light or gray sand, and either live or freshly killed deer mice were left in the room for various lengths of time as prey for the owls. Whether the owls pounced directly on their food or hunted for it on the floor could be frequently detd. by study of tracks in the sand. The 4 sorts of owls used were the barred owl, Strix varia, the long-eared owl, Asio wilsonianus, the barn owl, Tyto alba pratincola, and the western burrowing owl, Speotyto cunicularia hypugaea. None of the owls seemed to use either auditory or olfactory means of locating prey, although the ability of the owls to fly to a perch in complete darkness might have been due either to a good sense of position or to perception of supersonic vibrations. Neither was infra-red radiation of use to the owls in finding either the still warm carcasses of freshly killed deer mice or living mice. All but the burrowing owl were able to pounce directly onto the prey from a 6-ft. perch when the illumination on the floor of the reaction room was 0.00000073 foot-candle or more. At lower intensities the first 3 spp. seemed to differ somewhat in their ability to see mice at closer range. Mice were usually taken more readily when the sand was of contrasting color. The burrowing owl, however, had approx. the same ability to see in dim light as has man, and could not find prey regularly at intensities < 0.000026 ft.-candle. Calculations of light intensities at night in shaded forests revealed that owls must frequently be unable to see non-moving prey.

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