My husband saw a flock of dark colored birds with a white stripe on the underside of the wing that is not any one of the birds on the Illinois bird list. It had a squarish tail. This was was 30 miles west of Chicago. Any ideas?
Probably Starlings - the underwing doesn't have a white stripe, but the flight feathers are reflective at some angles of light, and can appear pale / whitish compared to the wing covert feathers.
Could it be a Common Nighthawk? In the summer months, they are in much of North America.
What time of day did he see the birds? Nighthawks are active at dusk and dawn as they hunt for flying insects. Getting ready for migration, they will begin to flock together.