"The Gilbertson Universal trap is my favorite, I use it on any box which has had sparrows in the past.  It fits all my boxes: some are front opening, some are side opening.  In the front opening boxes, it is easily installed with a single screw.  In the side opening boxes, I use a small stubby screwdriver to install the trap on to the inside of the front.  It is so light, yet so effective in catching the sparrows.  Usually I put a trap in each of the "problem" boxes, and I put the safety clip on the spring so it will not operate.  If a Bluebird takes the box, I leave the trap in the safety position.  If a house sparrow starts to use the box, I release the safety clip and the trap is set.  Next time the sparrow enters the box, the trap springs out closing the entrance thus trapping the sparrow.  It is easy to see when a bird is trapped as the red "target" is clearly visible from a long distance."
[Fawzi Emad, President, Maryland Bluebird Society.]