A Field Test of the Variable Circular-Plot Censusing Method in a Sierran Subalpine Forest Habitat

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The density and distribution of territories were determined for 21 species in a 48-ha Sierran subalpine forest study plot by an intensive program of spot-mapping and nest monitoring. About 10% of the total breeding individuals were color-banded and about 75% of the nests of all species were found. Variable circular-plot (VCP) censuses were simultaneously conducted in the same study plot. The VCP method, with minimum effort (48 stations), could describe community parameters reasonably well and could distinguish common from rare species, but could not correctly determine the relative abundances of the common species, could not correctly describe the distribution of territories within the study plot, and produced errors in density estimates for the common species that ranged from -57% to +65%. When the effort was increased threefold (144 stations), the accuracy of the method was improved so that it produced more or less acceptable relative abundances for even the common species and was marginally capable of describing the distribution of territories for 37% of the species, particularly for those species whose distributions were markedly non-uniform, but it still produced errors in density estimates for the common species that ranged from -67% to +96%. Interestingly, VCP total count often performed nearly as well as the calculated VCP density in determining relative abundances. The accuracy of the VCP method may be expected to be poor for species with low population densities, large territory sizes, high mobilities, and ventriloqual vocalizations, and for habitats that are dense and highly three-dimensional.