Behavior of a Radio-Tagged Saw-Whet Owl

Abstract

BECAUSE of the difficulty of watching nocturnal animals such as owls, few data are available on their behavior. Nicholls and Warner (1972) used radiotelemetry to study the Barred Owl (Strix varia). This paper reports a radiotelemetry study of the movements, activity, and home range of a Saw-whet Owl (Aegolius acadicus) for a 20-day period on the 5000-acre Cedar Creek Natural History Area approximately 30 miles north of Minneapolis, Minnesota. MET•IODS A Saw-whet Owl of undetermined sex, weighing 75 g, captured in a mist net on 25 October 1965, was released at the capture point at 1111 29 October 1965. A 7.5-g transmitter package with a 7-inch whip antenna and powered by two Mallory RM675RT2 batteries was attached to the owl by a modified version of the harness described by Marshall (1963). The bird immediately adjusted itself to the transmitter on its back and flew 40 feet to a perch where it engaged in preening. Two successive flights of 20 and 60 feet appeared to be normal. The owl was seen once more before the radio stopped transmitting at 1545 17 November 1965. Owl activity and positions were monitored and recorded automatically (Cochran et al. 1965). This system can locate a radio-tagged animal every 45 sec, but a transmitter out of tune makes a few blank periods. All activity data were classified as moving or resting. A resting (nonmoving) owl produced a steady radio signal, but movement caused a pulsing signal frequently accompanied by a change in bearing at the radio receiving towers. All bearings were determined to the nearest degree, and the beginning and end of each movement and rest period were documented to the nearest minute. Each location the owl used was categorized and defined in one of three ways. The place where the owl spent the day without moving was called a roost site. A spot where it remained without moving for 10 min or more at night was called a rest site. When the owl moved between rest sites its position was sampled at 10-min intervals. The fixed point where the bird was at the particular point in time when the sample was taken was called a moving location. The home range was determined by plotting and connecting consecutive locations during the 20-day period. When the transmitter was out of tune for 1 h or more before or after a single location, and when this location was outside the accumulating home range boundary, it was plotted as a point outside of the home range. Because these points represent real but discontinuous owl locations, the size of the home range presented is conservative. The accuracy of the owl's locations, based on triangulation from the two towers, depended on the relative position of the owl to the towers. Because the relative position of the home range to the two signal receiving towers gave varying degrees 783 The Auk 91: 783-795. October 1974