Geographic range occupancy of North American birds

Although the geographic range is a fundamental unit of analysis for many macroecological and biogeographical studies, as a representation of the spatial distribution of individuals it is clearly a scale-dependent abstraction. As any amateur naturalist realizes, a species is not guaranteed to be present at every point within the range delimited by a field guide. Geographically extensive survey data such as the North American Breeding Bird Survey (BBS) allow the characterization of distribution patterns within the geographic range. Using BBS data (http://www.pwrc.usgs.gov/bbs/) paired with digital range maps from NatureServe (http://www.natureserve.org/getData/birdMaps.jsp), I calculated a simple measure of range occupancy for 298 species of North American birds. Range occupancy is simply the fraction of sites on which a species is expected to occur based on its range map on which it is actually observed to occur.