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

All four species of owls studied in the laboratory found their dead prey mostly by sight and there was no evidence of their employment for this purpose of infra-red rays or of any senses other than sight and physical contact. The barred, long-eared, and barn owls are able, under the most favorable conditions, to see and to approach dead prey directly from a distance of six feet or more under an illumination calculated to be as low as 0.000,000,73 foot-candle. When the illumination is reduced to 0.000,000,53 foot-candle, however, all these owls seem to have some difficulty in seeing prey that is more than a foot or so away. There is some evidence that sight may be of some value to the barred owl in finding dark-colored dead prey on nearly white soil at an illumination as dim as 0.000,000,15 foot-candle. The burrowing owl was unable to find dead prey regularly under illuminations dimmer than about 0.000,026 foot-candle. This species, therefore, has much less ability to see in very dim light than the other three species of owls studied. The burrowing owl, however, is more diurnal in habit than these other owls and also it lives in more open situations. From measurements of the incident light in nature it is calculated that in the natural habitats of all these owls the intensity of illumination must often fall below the minimum at which the birds can see their prey.