The Roadside Census as a Method of Measuring Bird Populations

Abstract

THE subject of the measurement of bird populations has been extensively treated by Kendeigh (1944). Kendeigh's paper orients the roadside census in relation to other types of censuses. It has been used as a method of determining relative abundance and not absolute or true abundance. In relative abundance the number of birds reported is not the actual total number present in the area censused but an unknown proportion of the actual total, while in absolute or true abundance the number of birds reported is the total number of individuals present per unit of area (preferably given in terms of birds per 100 acres). The roadside census is an enumeration of the birds contacted along secondary roads, usually from a moving automobile. Nice (1921) used the roadside census to study Oklahoma bird populations in 1920. Since then it has been used by a number of other workers. It has been used to study many species of birds, but usually the individual species chosen have been the more conspicuous ones. Perhaps it has been most widely used to measure population fluctuations of the Ringnecked Pheasant (Fisher, Hiatt, and Bergeson, 1947). My objective in this study has been to measure population changes and trends in a large area. The area selected was Knox County, Tennessee. The population in an area of this size is too great to be more than sampled. It was the need for a method of securing large population samples over a large area in a limited time that led to the selection of the roadside census. The term population as used in this paper refers to the total number of individuals of both sexes and all ages (other than nestlings) occurring in Knox County. Knox County lies within about 20 miles of the eastern boundary of Tennessee and is a little nearer to the northern than the southern boundary. Its greatest distance from east to west is about 35 miles and from north to south it is a little over 20 miles. The county has an area of 517 square miles. It lies in the Valley and Ridge Province (Fenneman, 1938), an area lying along the Tennessee River and characterized by many long parallel ridges and intervening valleys. The average elevation is a little less than 1000 feet. Seventy-two per cent of the county is in farms. Ecologically the county is about 60 per