Census Efficiency and Number of Visits to a Study Plot when Estimating Bird Densities by the Territory Mapping Method

Abstract

(1) The territory mapping technique is a standard method in bird census work, usually involving eight to ten surveys of the study plot in the breeding season. The population estimate is based on the number of clusters of 'registrations' (point-records), each acceptable cluster containing at least a standard minimum number of registrations. Difficulties may arise for inconspicuous species with low 'visit efficiency'. (2) It is shown in this paper, by theoretical calculations, that if the visit efficiency is above c. 60% the final population estimate after the several visits will be accurate. If visit efficiency is about 40-60%, accurate estimates can with some caution usually be obtained provided the number of visits is sufficiently high and the 'level of acceptance' can be set fairly low. If the visit efficiency falls below c. 40% the population estimate will be inaccurate and very variable because it is highly sensitive to small shifts in efficiency. (3) It is recommended that if the efficiency of a count is expected to be low one should not try to correct the population estimate with the aid of a calculated efficiency factor but instead reorganize the field work schedule so that a sufficiently high efficiency is obtained. This can be achieved by increasing the number of visits, optimizing the time of the visits, lengthening the duration of visits and carrying out additional surveys for problem species. Arguments are given against using a lower level of acceptance of a territorial cluster than three registrations.