Details: Comparison of Annual Indices of Abundance
The CBC is of questionable accuracy in evaluating population trends. Although actual population trends for bird species are unknown, comparative analyses of CBC results with results from other surveys provide at least an idea of the consistency of results among our monitoring programs. If they are consistent, we have increased confidence that the programs are monitoring the same thing.
We have plotted CBC indices of abundance on the same axes as BBS indices of abundance to facilitate this comparison. Survey-wide indices were estimated from both surveys, using the procedures described by Sauer and Geissler (1990). Then, both indices were standardized- for each series, the mean and variance was estimated and an adjusted index was calculated by subtracting the mean and then dividing by the square root of the variance. This centered the series on 0, and scaled the year to year changes.
Observation of the results suggests that many species show surprisingly consistent results between the BBS and the CBC. Differences, when they occur, often reflect the differing populations that fall in the survey area in summer and early winter.
Sauer, J. R., and P. H. Geissler. 1990. Annual indices from route regression analyses. Pages 58-62 in J. R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U. S. Fish. Wildl. Serv., Biol. Rept. 90(1).