I think Carolina Wrens can easily fit through a 1" hole, but will probably pick a site other than a nice nesting box for their nest. I have a nest of them right now in my garage. They come and go through a crack at the bottom of the garage door. The nest is in a box that has the lid cracked open just a bit. Once the babies are ready to go, I'll leave the garage door open a few inches so they can follow their mother out into the world. Carolina Wrens are creative nest builders. I have had them nest in nail aprons hanging on the wall and buckets. I have had them nest under the house, in the crawl space. They try to nest in my gas grill and lawn mower if allowed to. I have seen them checking out parked cars for possible nesting spots.

You can go ahead and clean out that nest from the birdhouse. It's probably pretty nasty and if they decide to build there again, the nest will be too high as well. Whenever they build in a birdhouse, I clean it out once the young have fledged.

Please don't let House Wrens nest. You are allowed to remove their dummy stick nests until they move on. House wrens are very aggresive little birds that like to poke holes in other nesting birds eggs like Bluebirds, Chickadees, Tree Swallows, etc. So most people like myself that provide homes for the birds I just named do not let house wrens nest in their yard because of this reason. Thanks.

I've been away for a while, and now catching on your replies to my posting. I had no idea that I would inspire a debate, but at least it was educational. Here is what I have observed, at least so far. I have a pair of mocking birds nesting not far from the wren nestboxes, in spite of Rachel's warning that I would have no other birds nesting in the vicinity. It is very well hidden in a densely foliated bush. I haven't actually seen it, because I hestiate to pry into the bush for fear of disturbing the nest. But I hear the babies chirping when the parents visit to feed them. I am amused that when the parents see me looking towards the nest they will wait nearby, and as soon I look away, they dart into the bush. But I am looking out of the corner of my eye, or over the top of my glasses. There are no bluebirds in my area, which was a concern to some. But I have chickadees in the neighborhood, and passing through my yard, as well as the wrens. They must be getting along somehow. Currently, all three wren nestboxes are empty, including the one that I emptied of spruce twigs after the wrens filled it up. I am thinking of enlarging the hole of one of the nestboxes to accommodate chickadees, but haven't done so yet. Does any one know how chickadee nesting preferences differ from wren preferences?