Eighty-four species had trends with large variances. Several heron species, many duck species, waterbirds, a few species with northern breeding ranges, several warblers, some sparrow species, and some blackbirds occur in this list, among other species. These species can be characterized as birds with specialized habitats or limited distributions in the BBS range, spruce-budworm species, or colonial nesting species. Biases in BBS analyses Other potential biases in the BBS cannot be documented from analysis of the survey. Analysis of survey data cannot tell us the proportion of the individuals of a species that is breeding outside the range of the survey.
Trends are always specific to the areas surveyed.
The BBS is a roadside survey, and a major criticism of the survey has been that habitat changes along roadsides may not be representative of regional habitat changes. Trends from the BBS may therefore reflect only populations along roads rather than regional bird population changes. Within the range of the BBS, many habitats are not well covered, and species that specialize in those habitats are poorly sampled. Wetland birds and species occupying alpine tundra habitats are examples of groups thought to be poorly represented in the survey. Even with all of the sampling and other biases discussed above, the BBS represents a unique attempt at a survey of breeding populations of birds in North America.