Probably buckthorn. The give away will be how long it holds its leaves. Buckthorn will hold it leaves, which remain green, at least two weeks later than all of the native trees. I'm zone 5a, about 45 min NW of Milwaukee. I've seen Buckthorn hold its leaves until the weekend before gun season, or two weeks before Thanksgiving. The leaves on all of the natives are long gone by then. You absolutely have to spray the stump with a minimum of 20% active ingredient glyphosate immediately after cutting. It will vigorously sprout from the roots and the stump if you don't.
The birds eat the berries in the fall when there's no other food left and leave purple stains everywhere. The berries are a diuretic. Hence the latin name, Rhamnus cathartica, for common buckthorn. It spreads like crazy and will outcompete all the natives, forming buckthorn deserts. What you'll quickly see is a 10 - 20' tall buckthorn thicket with nothing but dirt and buckthorn seedlings underneath. A combination of buckthorn and high deer densities are lethal to native habitats. The deer know or quickly learn that it's diurectic and eat everything but the buckthorn, speeding the colonization process. They're doing what we do to help our tree seedlings in tree tubes.
I'm in the process of clearing 15+ acres of this crap. I'm about 4.5 acres in.... It's tough, shrubby stuff.
