I have a question about a bird's nest

I encountered this nest while hiking last October.  The view above shows it as it appeared from the trail - long abandoned, with residual ?feathers/down or perhaps ?thistledown still present on the top.  In terms of size it was about equal to two clenched fists, and was located about five feet above the ground in south-central Wisconsin.

My puzzlement began when I walked to the other side of the nest and saw this:

There is clearly a hole in the side of the nest away from the trail.  I had assumed that the nesting area was on top, but this cavity looks like the true nest (I didn't dissect it to see what was inside).

I'm not a "birder," so perhaps someone out there can enlighten me re the species of bird that constructed this.  It's probably a common nest structure, but I just haven't seen one like this before.  Or is the hole a secondary access created by another critter/bird in an abandoned nest?

(Click pix to enlarge).