I noticed that the hole was still visible from the side in the direction where I know a wren hangs out (the south side of my yard.) I had already seen a wren entering this box while the bluebirds were still building the nest, before any eggs were laid, and I was concerned that it would still get in even after I put the wren guard up. So, I decided to try an experiment and put an additional piece of cardboard on the side to further obscure the view of the hole from a passing wren. The same day that the first egg was laid I put the normal wren guard on as previously described. The next day after the bluebirds were comfortable with that, I put this second piece on. Since they were bonded to the box, and already familiar with the original wren guard, I thought that they could easily negotiate around the new piece, and they didn't seem to have any problem with it, because I could see that they continued to access the nest and lay eggs on schedule. Also, so far there haven't been any problems with the wren and 4 of 5 eggs have hatched. I didn't put a side panel on the other side (the north side) since I'm not aware of a wren there. However, I've noticed the bluebirds fly up under the front, so I believe they would tolerate an additional second side panel, which would completely enclose the hole, if it were needed. The important thing to remember is that the bluebirds need to be comfortable with the original wren guard before any modifications are made to it. Another reason that this is possible is that I have an I/R video camera installed this box, so can easily monitor it without having to take apart the wren guard.