VISIBILITY BIAS ON COUNTS OF NESTING CANADA GEESE

The apparent high visibility of goose nests in arctic habitats often h as led researchers to assume that single ground counts approach comple te inventories of the nesting geese in a given area. No studies to dat e have attempted to measure the visibility bias in these counts. Annua l nest counts of Eastern Prairie Population (EPP) Canada geese (Branta canadensis interior) were conducted from 1976 to 1994 near Cape Churc hill in northern Manitoba. Portions of the study area were searched by 2 independent crews in 1993-94, and a modification of the Petersen es timator was used, to estimate both visibility bias and tile unbiased n est density. Individual crews located 77% (+/- 2.9%) of active and 39% (+/- 5.3%) of destroyed nests, or only 72% (+/- 2.4%) of all nests pr esent. Single counts seem to be reliable indicators of trends in breed ing parameters in our study but we recommend visibility bias be estima ted where accurate nest density estimates are required.