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

Abstract

THE (sometimes referred to as spot-map 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 method, using it as an absolute measure of bird density, to delimit territorial boundaries (Johnston 1947; Williamson 1964, 1971), or even as a control in evaluating accuracy of other census procedures (Howell 1951, Stewart et. al. 1952, Enemar 1959). Stewart et al. estimated accuracy of method to be above 90% for nearly every species they studied, and to average over 959. Two major sources of error are inherent in such a census procedure: (1) observational bias, resulting from variability in identification skill of observers, observation conditions (weather, time of day, etc.), screening effect of habitat, and conspicuousness of 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 method is absence of reliable controls to estimate magnitude and direction of error. Snow (1965) investigated observational error by comparing results from independent censuses conducted on same areas. He discounted 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 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 estimates from census results using mapping method. I first became aware of difficulty in interpreting census data while studying effects of habitat alteration on avian communities (Best 1972). Later, while conducting an intensive investigation of breeding ecology of Field Sparrow (Spizella pusilla), opportunity presented itself to evaluate interpretational errors in mapping method.