Many geographic patterns appear in BBS data. Some of these patterns are simply a result of deficiencies in sampling design. Although all States and Provinces vary in coverage over time, some consistent regional patterns occur in BBS coverage. In Canada, the BBS is largely restricted to the south, and our summary maps truncate the range of each species to indicate the region with sufficient BBS data. Superimposed on the regional variation in route densities is temporal variation in the number of routes run in many states.
Most states show a general pattern of increasing routes over time.
In some cases, this pattern is extreme, with states such as ME having few routes until the early 1970's, when numbers started to increase. Possession of one of these attributes does not necessarily eliminate the species from trend analyses. These species can be well surveyed by the BBS within portions of their breeding range or during certain time periods. Numbers presented below were taken from an unpublished manuscript by Sauer and Droege, and use trend data from 1966 - 1990. Eighty-four species had trends with large variances.