birdhouse problem

My cousin gave each of my sisters wren houses last year, and when I went to visit in March, hanging the two of them up was on my "To Do" list.

The problem is that the sparrows are bound and determined to try to get in through the hole even though they are sized too small for them. My sisters have seen wrens in the neighborhood (which is standard old urban houses and lots).

Questions:
1. Can anyone suggest ways that would make the birdhouse less attractive to the sparrows?
2. Is there any chance that if I surrounded the birdhouses with a "cage" with sort of 1 ft clearance of the birdhouse with holes that only wrens fit through, would having that buffer zone be enough that the wrens could cope? Right now, the sparrows bully away any wren that approaches.

Short of offering alternative accommodation for the sparrows, they will likely continue to "try" to inhabit the wren houses.

You might try researching the two species nesting requirements - height, sun,shade, open approach, bushy approach...the old adage of location, location, location. Perhaps you have situated it in prime sparrow habitat, as opposed to prime wren location.

Yes, it may be a loosing battle. We already put up two sparrow boxes, but they had to go on opposite sides of the garage or the sparrows just fought with their fellow sparrows over the two boxes if they were with in sight lines. I've been told that "sparrows are aggressive".

I've tasked the landlords (my sisters) with researching the location issue, but I'll email them the list you've made and we'll see what they come up with. Between squirrel pressure, and common urban-sized lots, the options are limited. As a last resort, I'll drill the holes larger and just let the sparrows have them!