> <i>Over the next few days, the owl makes 16 more strikes at mice, missing only four times, each time by less than two inches.</i><p>How did the experimenter measure miss distance in pitch darkness? IR illumination is presumably out in case the owl was able to see it, and I didn't think thermal imaging was a thing yet in the late '50s.
Scratch markings on wax flooring beneath the leaf cover or the grid location on the floor was instrumented to report to telemetry when carrying weight of the owl. Assuming the mouse had its ankle rope tied to a pin nailed into the floor.
Completely random guess, but perhaps ultrasonically? Ultrasonic rangefinders are relatively cheap and accurate these days, so maybe they were too in the '50s?