Monofilament Fishing Line Idea
Developed by Joan Watroba and Lee Pauser this makes a bluebird nest box less desirable to House Sparrows.
See also Lee's website at: http://www.birdsfly.info/Bluebird_Trail/Making_nest_boxes_sparrow_resistant/index.htm
I had an incident last year with a lone male HOSP which attempted to take over my front yard nest box. What I noticed is that the HOSP would drop down from the roof, flutter and then make entry through the hole.
This monofilament fishing line idea seems to make it more difficult for the HOSP to enter the hole. When they flutter the line makes contact with their wings. Bluebirds however tend to fly straight to the hole and the line doesn't seem to bother them at all.
After installation the male HOSP tried to get into the hole but was delayed by the line long enough for the bluebirds to drive him away.
Note that you don't want the HOSP to make entry into your boxes as they can defend the box from inside the hole.
I made the adjustable version of this idea seen below. This setup worked with the line 3/4in outside the hole edges and 3/4in high. Lee has had success with the idea in more highly infested areas using the line right at the outside edges and 1 inch high.
Quick and easy setup.
Only downside is this is not easily adjustable. Monoline is simply strung around screws. This is the more aggressive positioning with line right at outside edges of hole and 1in high above the wood.