Does variation in census area confound density comparisons?

1. Estimates of density are not always independent of the area over which p opulations are censused, Instead, lower densities tend to be recorded for s pecies when they are censused over larger areas. This may have serious impl ications for both intra- and interspecific comparisons of density, with rel evance to such issues as conservation prioritization and management strateg ies, because differences in density map simply result from differences in c ensus areas. 2. Here, we use long-term population census data for British birds in the p eriod 1968-91, drawn from the Common Birds Census (CBC), to examine relatio nships between density and census area within species, and the consequences of these relationships for intraspecific and interspecific density compari sons. 3. Most British bird species exhibit statistically significant negative rel ationships between density and census area. We used these relationships to standardize mean density estimates for all species to a common census area. These area-adjusted estimates were usually very similar to geometric mean density estimates calculated without reference to census area. 4. For a subset of species recorded from a large number (> 30) of census si tes in each year in the period 1968-91, we used intraspecific density-area relationships for each year to standardize mean density estimates to a comm on census area in all years. Again, the area-adjusted estimates for each ye ar were usually very similar to the simple geometric mean density estimates calculated for the species in the same year. 5. These results are encouraging, but are certainly a consequence of the re latively limited range of census areas used here, and the fact that the mea n census area varies little across species, or across pears within species, Moreover, those species occupying few sites are the most likely to have ar ea-biased densities, but are the species for which area-correction will be most difficult. 6. Overall, the results suggest that past analyses conducted using the CBC data are unlikely to have been seriously confounded by variation in census area. Nevertheless, they do highlight that the effects of census area will require consideration by anyone planning to measure or use densities for co mparative purposes either within or among species.