We documented the occurrence of 12 bird species within Prairie Ridge State Natural Area, Jasper County, Illinois, a matrix of nine grassland tracts (range 7–120 ha, totaling 489 ha), from 1994 to 1997 and analyzed the presence of these species relative to the area of the nine grassland tracts. Grassland tract area was correlated with the occurrence of greater prairie-chickens (Tympanuchus cupido) (P = 0.016), upland sandpipers (Bartramia longicauda) (P = 0.012), savannah sparrows (Passerculus sandwichensis) (P = 0.009) and Henslow's sparrows (Ammodramus henslowii) (P = 0.005). Minimum area requirements varied from 12 ha for grasshopper sparrows (Ammodramus savannarum), 55 ha for northern harriers (Circus cyaneus), 65 ha for greater prairie-chickens and upland sandpipers, to 75 ha for Henlsow's and savannah sparrows. Therefore, grasslands larger than 60 ha may be required to attract nine breeding grassland bird species. Each grassland tract was divided into 2- to 6-ha management units (mode = 2.8 ha). Our results suggest the area of grassland tracts influences the perception of habitat suitability by area-sensitive species more than size or configuration of management units within tracts.