As "fall" shorebird migration begins to heat up, so does the annual dispersal of wandering/wintering Selasphorus hummingbirds. For most of the year, the Ruby-throated Hummingbird is the only hummingbird that is found in most of the eastern United States.
However, birders and ornithologists have slowly but surely established (especially through banding) in the last few years that many hummingbirds (Selasphorus and others, especially Rufous, Allen's, Broad-Tailed and Black-chinned) tend to wander in late summer/early fall.
Some of these birds just wander around, but some of them try to winter in places you would not expect. Banding returns have shown that not all of tehse wanderes freeze to death, because in some locations the same bird returns for two or three years in a row.
I don't know why Illinois seems to have fewer records of Selasphorus and other wandering hummingbirds than do the surrounding states. I haven't added up all of the recent records, but it seems like Illinois gets fewer birds every year. I find bird migration in general -- and vagrancy in particular -- to be a fascinating subject, so this is something we'll talk about more if any unusual hummingbirds show up in the Midwest this year.
Anyway, if you see a strange hummingbird in your yard, contact a local birder so he or she can check it out. And don't hesitate to keep your feeders up even if your local hummingbirds have already departed. You never know what might turn up...