Density influences census technique accuracy for cerulean warblers in eastern Ontario

This study investigated the accuracy of 50-m fixed-radius, 100-m fixed-radius, and variable circular-plot point counts to estimate the actual density of breeding Cerulean Warblers (Dendroica cerulea) during the 1997 and 1998 breeding seasons, in Ontario, Canada. Density estimates were compared to actual densities as measured from intensive field observation of pairing and nesting behavior. Estimates of density from each of the techniques were positively correlated with actual density in both years. Both the technique used to census a population as well as the actual density of the population itself affected the accuracy of the derived density estimations. In both years, the 50-m fixed-radius technique overestimated density. In contrast, the 100-m fixed-radius technique and variable circular-plot technique underestimated density; the degree of the underestimate of the 100-m fixed-radius increased as actual density increased. There was no correlation between the degree of underestimation and actual density for the variable circular-plot technique. Although all three methodologies provide relative measures of density, the variable circular-plot technique provides the best absolute assessment of Cerulean Warbler density and is considered most suitable for broad-scale surveys.