Absolute visual threshold and scotopic spectral sensitivity in the tawny owl Strix aluco

IT has long been assumed from considerations of the behaviour of owls that the vision of these species is much more sensitive than that of man. Experimental determinations of the absolute visual sensitivity of these birds have provided evidence in support of this view, suggesting that visual sensitivity in owls is of the order of 10 to 100 times (+1.0 log₁₀ to +2.0 log₁₀ units) that of man^1,2. Owing to calibration problems, however, these values were probably inaccurate by at least a factor of 10 (refs 1, 2). I report here that in at least one highly nocturnal owl species, visual sensitivity is only slightly higher than that of man, and that the increased sensitivity can be attributed to optical factors rather than to any increase in sensitivity at the retinal level. This suggests that retinal mechanisms in both man and owl have reached the ultimate in sensitivity for the duplex retinas of terrestrial vertebrates.