Visibility Bias on Counts of Nesting Canada Geese

Abstract

The apparent high visibility of goose nests in arctic habitats often has led researchers to assume that single ground counts approach complete inventories of the nesting geese in a given area. No studies to date have attempted to measure the visibility bias in these counts. Annual nest counts of Eastern Prairie Population (EPP) Canada geese (Branta canadensis interior) were conducted from 1976 to 1994 near Cape Churchill in northern Manitoba. Portions of the study area were searched by 2 independent crews in 1993-94, and a modification of the Petersen estimator was used to estimate both visibility bias and the unbiased nest density. Individual crews located 77% (±2.9%) of active and 39% (±5.3%) of destroyed nests, or only 72% (±2.4%) of all nests present. Single counts seem to be reliable indicators of trends in breeding parameters in our study, but we recommend visibility bias be estimated where accurate nest density estimates are required.