Daily Temperature Cycles in Barred, Great-horned and Snowy Owls

(Speotyto cuniculuria) to a moving object when the object did and did not cast a conspicuous shadow. We used a single Burrowing Owl for all tests. It was trapped about 5 km north of Davis, Yolo County, California. We chose this species because it is easily trapped and because it often feeds nocturnally upon terrestrial prey, including rodents, and therefore may exploit shadow cues (Thomsen, Condor 73: 177192, 1971). The owl was kept in a plywood box that was a 91.4-cm cube with a 45.7 x 45.7-cm window centered on one side. The owl regularly used the perch in its box and faced toward the window. A red, 7-watt incandescent bulb inside the box provided illumination for viewing the owl’ s behavior. Attached to the owl box on the windowed side was a second box, which contained the moving object and the shadow-casting and nonshadow-casting lighting systems. This box was open on the side that was adjacent to the owl box, so that the interior of this box was visible to the owl through the window. The moving object was a steel nut attached by monofilament fishing line to the top of this box. This object could be held outside the box and released to swing past the window. The shadow-casting and nonshadow-casting lighting conditions were provided by one and four ‘ I-watt incandescent bulbs, respectively. The single bulb was centered above the window. Each of the four bulbs was attached to a different edge of the windowed side of the owl box, and each was in a sanded acrylic container for diffusion of light. The single bulb cast a conspicuous shadow, the four bulbs cast four very inconspicuous shadows. The brightness of both shadow and nonshadow bulbs was adjusted to yield 0.01 ft-c. The room containing the apparatus was always dark. We used human subjects to watch the responsiveness of the owl to the moving object. None was aware of the purpose of the experiment or that the owl was viewing the object under special lighting conditions. Each subject was told to report that the owl had detected the object if the owl moved coincidentally with the object’ s movement. Such movement usually consisted of abrupt visual fixation of the object by head movement. The conspicuousness of the head movement was increased by releasing the object only when the owl was not gazing directly at the window. Either a “yes” or “no” response was required for every trial.