One of the most interesting birds to observe is perhaps the Turkey Vulture. They are large and really common in northern Illinois so they are easy to spot.
I’ve found many Turkey Vulture roosts before but was surprised to come upon a group perched in a stand of pine trees at Sugar River Forest Preserve. They had probably spent the night there and as we walked by many took flight.
Below the roost we found lots of whitewash plus a great deal of feathers; victim to some intense preening no doubt.
If you are interested in bird behavior, Turkey Vultures are one of the best birds to spend considerable time watching.