These maps indicate the number of birds seen on BBS routes, grouped into convenient categories of relative abundance. The maps predict the average number of birds of the species that could be seen in about 2.5 hours of birdwatching along roadsides (by very good birders). They are based on mean counts on BBS routes over the interval 1994 - 2003.
The maps have some error associated with both the underlying data and the mapping procedure. On rare occasions, the procedure inserts edges that omit some marginal routes. Also, note that the lowest relative abundance category occasionally indicates the presence of a species outside its recognized range of the species, representing an accidental observation. Finally, the mapping procedure causes the lowest relative abundance category to extend slightly beyond the survey routes on which the species occurs. We have left these edges and accidental observations in the map to emphasize that unusual observations (and misidentifications) occur in any survey that is based on data collected by many observers.
Also, bird populations are changing, and species can be more (or less) abundant than indicated by the 10-year mean count on a nearby survey route.
So, be warned! You will not always be able to see the species at all locations where the maps indicate they occur at low abundances.