We got to the park in the late morning on Friday and found a few birders standing at the east end of the Field House. After only a few minutes, Jason spotted the bird actively working a small grove of trees. Despite the fact that the bird moved around almost warbler-like at times, we still managed to get very satisfying looks at it. The light crest patch was noticeable when the bird was facing us and the three wingbars, which lean the identification towards a Small-billed Elaenia, were easily seen. Photos were a challenge because of the bird's quick movements and the fairly dark, cloudy day.
Other birds seen there included American Wigeon, Blue-winged Teal, Pied-billed Grebe, Caspian Tern, Ruby-crowned Kinglet, Hermit Thrush, and Palm, Yellow-rumped, and Orange-crowned Warbler.
We left a few hours later and got back home around 5:00 AM on Saturday morning, covering about 1500 miles.
Update: The Illinois Records Committee accepted the bird as a Small-billed Elaenia.