The grassland region of the northern Great Plains was divided into six broad subregions by application of an avian indicator species analysis to data obtained from 582 sample plots censused during the breeding season. Common, ubiquitous species and rare species had little classificatory value and were eliminated from the data set used to derive the avian associations. Initial statistical division of the plots likely reflected structure of the dominant plant species used for nesting; later divisions probably were related to foraging or nesting cover requirements based on vegetation height or density, habitat heterogeneity, or possibly to the existence of mutually similar distributions or shared areas of greater than average abundance for certain groups of species. Knowledge of the effects of grazing, mostly by cattle, on habitat use by the breeding bird species was used to interpret the results of the indicator species analysis. Moderate grazing resulted in greater species richness in nearly all subregions; effects of grazing on total bird density were more variable.
