This is a little late, but still interesting, I think. On March 1st, I was startled to find a Carolina Wren in the "human" part of the barn when I came out of the milking stall. I had been seeing them up in the hay loft, which they enter through the eaves. This one must have flown down the ladder opening, and when I came in, after bumping into the windows, that's where she escaped, back up the opening to the loft.
It wasn't until 2 days later that I noticed what the wren had been up to...building a nest in one of a pair of old wooden stirrups that are hanging on the wall right beside the milking stall door.
I haven't seen any more activity at the nest.
A few years ago, the wrens had a nest in the horse manger.