Evaluation of Radio-Tracking by Triangulation with Special Reference to Deer Movements

Four parameters out of the five investigated were found useful for studying white-tailed deer (Odocoileus virginianus) movements by triangulation with an automatic radio-tracking system. These were total area, greatest linear dimension, mean activity radius and distribution of activity radii, and appearance of the map. Random plots, which simulated deer movements, were used to determine the effects of varying the location of the animal's range in relation to the triangulation stations. Results show an increase in size of range and a loss in accuracy of location as the plots were moved out. An hourglass-shaped area of about 3,300 acres is considered to be within acceptable accuracy with a 0.5-mile base line and a ? 0.50 resolution. Point locations were obtained at 1-min intervals for selected time periods for three deer. These data were then sampled at intervals of 2, 3, 4, 5, 6, 10, 15, 30, and 60 min. Values for total area and greatest dimension decreased and the mean activity radius increased as the sampling interval became longer. A sufficient number of point locations can be maintained by using a short sampling interval or long observation period. Comparisons of home range size were made among individual deer and between a winter and an early spring period using the above four parameters. Spring ranges were significantly larger than winter ranges and movements in spring were longer with less concentration of activity. The differences between winter and spring behavior are probably related to higher spring temperatures and the disappearance of snow. This paper reports on laboratory and field studies of some limitations and restrictions in using triangulation to locate animals and on certain aspects of white-tailed deer movements. Various techniques have been employed in the quest for knowledge of animal movements. One of the latest and most refined of these, radio-tracking, consists of marking individual animals with miniature radio transmitters. Most current investigations involve the use of portable directional receivers to monitor the animal's position and movements by triangulation from two or more sites. The recent development of an automatic tracking system (Cochran et al. 1965) enables the research worker to obtain minute-by-minute positions by triangulation for many animals simultaneously. Data obtained by radio-tracking have been valuable in analyzing numerous aspects of animal ecology, such as census methods (Tester and Heezen 1965), breeding behavior (Marshall 1965), and home range (Tester and Siniff 1965). However, all radio-tracking techniques have certain limitations and restrictions which must be considered in experimental design, data analysis, and interpretation. Marshall (1963:175-176), Cochran and Lord (1963), and Verts (1963) all mention limitations in accuracy of radio locations, and Marshall and Kupa (1963) and Slade et al. (1965) used systematic approaches to investigate the magnitude of the errors encountered. No attempt was made in these studies to relate the amount of error found to different parameters of home range. For some aspects of these and other studies (Craighead et al. 1963) the accuracy of triangulation is not of extreme importance as the purpose of the radio location is to allow the investigator to come into visual range of the animal to make observations on movement and behavior. 1Research supported by the Louis W. and Maud Hill Family Foundation and the U. S. Atomic Energy Commission, Contract AT(11-1)1332( COO-1332-16). 2 Present address: Michigan Department of Conservation, Caro.