Interpretational Errors in the "Mapping Method" as a Census Technique

Abstract

THE "mapping method" (sometimes referred to as the "spot-map method," after Williams 1936) is frequently used by ornithologists when surveying bird populations (Kendeigh 1944, Pough 1947, Udvardy 1957, Williamson and Homes 1964, Hall 1964). Many workers place considerable confidence in the method, using it as an absolute measure of bird density, to delimit territorial boundaries (Johnston 1947; William-son 1964, 1971), or even as a control in evaluating the accuracy of other census procedures (Howell 1951, Stewart et. al. 1952, Enemar 1959). Stewart et al. estimated the accuracy of the method to be above 90% for nearly every species they studied, and to average over 95%. Two major sources of error are inherent in such a census procedure: (1) observational bias, resulting from variability in the identification skill of observers, observation conditions (weather, time of day, etc.), screening effect of the habitat, and conspicuousness of the bird species (Emlen 1971); and (2) interpretational bias, resulting from differing interpretations of census data. The compounding effect of these sources of error could result in gross inaccuracies (or less likely, they might offset each other). One major difficulty with the method is the absence of reliable controls to estimate the magnitude and direction of error. Snow (1965) investigated observational error by comparing results from independent censuses conducted on the same areas. He discounted the importance of interpretational error, reporting that individual estimates of census results "rarely differed by more than 10 per cent." Other workers are of a different opinion. Emlen (1971) has expressed concern over "the wide-range of interpretations that can be. extracted from composite maps," and Enemar (1959) feels that error in census work "depends more on inherent properties in a bird population's behaviour as interpreted by an ornithologist, than on special qualities inherent in the census-taker." Bell et al. (1973) discussed sources of discrepancy between actual population sizes of three passerine species (as determined by intensive studies using marked birds) and the estimates from census results using the mapping method. I first became aware of the difficulty in interpreting census data while studying the effects of habitat alteration on avian communities (Best 1972). Later, while conducting an intensive investigation of the breeding ecology of the Field Sparrow (Spizella pusilia), the opportunity presented itself to evaluate interpretational errors in the mapping method. 452 The Auk 92: 452-460. July 1975