I just discovered a small (as in only 9.67 acres) preserve in Boone County called Flora Prairie Nature Preserve. This preserve is one of the very few parcels of native prairie in Boone County and is most known for a native plant known as the wooly milkweed. This plant is state endangered but was last collected in 1946 by E. W. Fell, a well known naturalist who helped create a book about the plants of Winnebago County.
I have not visited Flora Prairie yet and am unsure if this is the location that the Illinois Ornithological Society was going to visit when they had their annual meeting in Rockford. There is another preserved area in Boone called Garden Prairie (not sure if that is the official name). I will be visiting both sites soon for photos and will try and get more info from NCIOS members.
The location of Flora Prairie is: southwest of Belvidere: 3 miles south of I-90 on Stone Quarry Rd., then 3.5 miles west on Poole Rd. A small parking area is provided just off Poole Rd. on the south side of the nature preserve. Topo Map