Effects of habitat area on the occurrence of grassland birds in Illinois

We documented the occurrence of 12 bird species within Prairie Ridge State Natural Area, Jasper County, Illinois, a matrix of nine grassland tracts (r ange 7-120 ha, totaling 489 ha), from 1994 to 1997 and analyzed the presenc e of these species relative to the area of the nine grassland tracts. Grass land tract area was correlated with the occurrence of greater prairie-chick ens (Tympanuchus cupido) (P = 0.016), upland sandpipers (Bartramia longicau da) (P = 0.012), savannah sparrows (Passerculus sandwichensis) (P = 0.009) and Henslow's sparrows (Ammodramus henslowii) (P = 0.005). Minimum area req uirements 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 sparrow s. Therefore, grasslands larger than 60 ha may be required to attract nine breeding grassland bird species. Each grassland tract was divided into 2- t o 6-ha management units (mode = 2.8 ha). Our results suggest the area of gr assland tracts influences the perception of habitat suitability by area-sen sitive species more than size or configuration of management units within t racts.