Empty acorn cups. Hard to tell what species.

That's acorn caps

That's kinda obvious now that I look at them that way. The fact that they are green and empty is what threw me. If they had been dried up and brown I wouldn't have even noticed, I've never seen an empty acorn that was green. We did have an unusual very hot dry summer so maybe the trees are dropping raw acorns early just like they are turning color and dropping leaves early.

westmich, the squirrels can sometimes pull a "green" acorn off its stem, remove the nut from the cup and discard the cup from up in the tree, and they might do the same thing w/a green one on the ground.

Interesting, thanks for the input.