Rarest bird in Northeastern Illinois (1 Viewer)

Question for those who know: what would generally be considered the rarest native bird to see in northeastern Illinois (say, within 100 miles of Chicago)?

Thanks in advance.

Guess on my part, as I'm certainly no sort of expert.
Whooping crane (Grus americana)

Current total population is estimated at around 826 birds.
Chicago is part of its former breeding range.
The current Wisconsin migratory population is just north of Chicago, app. 150 miles

Perhaps other species qualify.
None that I'm aware of enjoy the same intense effort at sustaining the population and reintroduction.

Thank you!