Apologies for the quality of the photo, but I had to zoom to get this photo of my furry little friend. Can you see his face in the middle of the photo? He’s peaking out of the largest hole in the log.
I’m guessing he’s a Little Brown Bat, which is the most common species of bat in Wisconsin. Also, its a very small hole. I included my boot in the photo below for scale. The bat hole is in the largest log, center left of the photo, largest hole.
The only reason I was spending time observing this area is because I found a colony of bees entering and exiting the ground. The entrance to their home was not very visible, tucked under some of the woody debris. But about every 1 to 2 seconds a bee would be coming or going.
I’m guessing they may be a colony of Litigated Furrow Bees. They weren’t aggressive towards me as they continued their late summer work.
Curiousfarmer 
Sharp eyes!