Accuracy of daily point relocations in assessing real movement of radio-marked animals

Correlation coefficients were calculated for the straight line distances between telemetry relocations taken at 24-hour intervals and the sum of the distances between relocations taken more frequently over 24 hours for pronghorns (Antilocapra americana), coyotes (Canis latrans), bobcats (Felis rufus), and mallards (Anas platyrhynchos). In only two of 8 comparisons was there a statistically significant correlation between "perceived" and "real" distance travelled. This suggests that using daily relocation data as a measure of real or relative movement for comparisons between subsets of a population (e.g., M vs. F or ad vs. juv), or seasonal comparisons within a subset, may not be a valid practice. J. WILDL. MANAGE. 51(4):937-940 Before the widespread use of radio telemetry in wildlife research, the time span between relocations of individually marked animals was variable and uncontrollable. Nonetheless, a large body of information was accumulated regarding home ranges and movements of individuals and demographic groups, such as juveniles, pregnant females, or adult males. The advent of reliable telemetry equipment allowed researchers more control over the frequency with which marked animals were located. In many telemetry studies radio-collared animals were relocated once daily at approximately 24-hour intervals. The distance between these resulting consecutive point locations was calculated and used by many researchers as an index of the total daily movement for an individual (Bailey 1974, Kitchings and Story 1979, Steigers and Flinders 1980, Taylor and Guthery 1980, Springer 1982, Young and Ruff 1982, Whiteside nd Guthery 1983, Hemker et al. 1984, Lawhead 1984, Riley and Dood 1984, Knowles 1985, Hines 1986). These distances, which we call perceived daily movements, were usually reported as daily movement or mobility. Perceived movements were often compared relative to various factors (e.g., sex, age, or season), and conclusions were drawn concerning effects these factors have on movement. For these comparisons to be meaningful and conclusions to be valid, perceived daily movement must have a consistent relationship with the distance actually travelled in 24 hours by the animal; i.e., the real movement. IPresent address: Idaho Department of Fish and Game, 2320 Government Way, Coeur d'Alene, ID 83814. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:51:00 UTC All use subject to http://about.jstor.org/terms 938 ASSESSING ANIMAL MOVEMENT * Laundr6 et al. J. Wildl. Manage. 51(4):1987 To our knowledge, no studies have attempted to demonstrate this relationship. Litvaitis and Shaw (1980) compared perceived distances for coyotes to estimates of real travel, that were based on the sum of straight-line distance between consecutive relocations obtained at 2.5hour intervals over 24-hour periods. Their estimates of real movements were 3 x larger than perceived movements. Even though perceived movements may underestimate real movement, they could still be useful predictors of relative movement if they correlate with real movements. Litvaitis and Shaw (1980) did not report whether such a correlation existed for their data. The objective of this study was to examine the validity of determining actual distance travelled (for coyotes, pronghorns, bobcats, and mallards) from 24-hour point-in-time locations. We thank the following for their assistance during the studies: J. H. Bronough, E. W. Buckner, L. M. Cowardin, E. H. Craig, T. A. Day, L. C. Farley, C. M. Frazier, D. C. Fuchs, D. S. Gilmer, E. C. Hellgren, J. W. Hupp, R. J. Gates, J. M. Goodwin, J. C. Grant, K. E. Grover, C. D. Levesque, M. R. Magagna, M. P. Mahoney, J. D. McIver, J. J. Mende, O. B. Myers, W. C. Poole, J. H. Reichman, W. T. Smith, M. J. Smolen, J. R. Tester, and R. J. Wilkosz. Critical reviews of the manuscript were provided by B. L. Keller, O. D. Markham, and J. D. Yoakum. This paper is a contribution from the Idaho Natl. Eng. Lab. Radioecology and Ecol. Prog., supported by the Off. Health and Environ. Res., U.S. Dep. Energy. Additional funds were provided by the Idaho Fed. Aid Wildl. Restor., Proj. W-160-R; the Univ. Minnesota; the U.S. Fish and Wildl. Serv., North. Prairie Wildl. Res. Cent.; Natl. Inst. Health Training Grant 5T01 GM01779-08; and the Mont. Coop. Wildl. Res. Unit. STUDY AREAS AND METHODS The data used for our analysis came from 4 separate telemetry studies. Coyotes, pronghorns, and bobcats were studied on the Idaho National Engineering Laboratory (INEL) in southeastern Idaho (Laundr 1979, Laundre and Keller 1981, Knick and Ball 1983, Reynolds 1984). The INEL occupies 2,315 km2 of semiarid, cold-desert rangeland on the Snake River Plain approximately 48 km west of Idaho Falls, Bonneville County, Idaho. The topography is flat to gently rolling with frequent lava outcroppings typical of the Columbia Plateau Province. The vegetation is dominated by big sagebrush (Artemisia tridentata) with an understory of forbs and bunch grasses characteristic of the upper Great Basin. Mallard hens with broods were studied in the Chippewa National Forest 19 km east of Bemidji, Beltrami County, Minnesota. The area consists of mostly upland forests with occasional clearings and a variety of lakes and wetlands (Ball 1973, Gilmer et al. 1975). Estimates of real movements were calculated by summing straight line distance between successive relocations of radio-marked animals over a 24-hour sampling period. Relocation intervals were approximately 15 minutes for coyotes, 30 minutes for pronghorn, and 1-3 hours for bobcats and mallards. Locations of coyotes and pronghorns were determined with vehiclemounted, double Yagi null-peak antenna systems. Mallards and bobcats were located with hand-held single Yagi systems. We recognize the presence of an error contribution (Tester and Siniff 1965) from telemetry system bias and sampling error (Springer 1979) that may distort estimates of point locations and calculations of total travel. Telemetry errors for the coyote and pronghorn systems averaged -1.4' (SD = 2.00, N = 25) for stationary animals and -2.20 (SD = 6.60, N = 26) for moving animals. For the mallard study, most daytime telemetry locations were confirmed with visual observations. The error contribution in the bobcat study was not determined. In all 4 studies, distances from radio-marked animals to receiving stations rarely exceeded 1 km. We concluded that the telemetry error contribution was minimal and our estimates of real movement closely approximated actual travel. Perceived distances were determined by measuring the distance between the 1st and last locations of each 24-hour monitoring session, usually from successive mornings. We considered them typical of data often used to calculate perceived daily movements. Analysis was limited to data from adult animals. Simple correlations between perceived and real movements were calculated for each species without regard to sex. Subsequently, data for coyotes were segregated by sex and then by biological season (Smith et al. 1981, Laundrb and Keller 1984). Small sample sizes prohibited similar computations for the other 3 species. Significance (P < 0.05) of correlation coefficients was evaluated by a t-test. This content downloaded from 157.55.39.35 on Mon, 29 Aug 2016 05:51:00 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 51(4):1987 ASSESSING ANIMAL MOVEMENT * Laundrd et al. 939