After discovering a pair of House Sparrows nesting in one of my Bluebird nest boxes three years ago, I searched the Internet for information on how to make a nest box House Sparrow proof. I discovered some chatter about the use of mono-filament fishing line (mono-line) to make Bluebird nest boxes House Sparrow resistant (one cannot make a nest box sparrow proof), but few detailed plans on how to do it. I looked at these few plans, and improvised my own treatment (plan).

I used different treatments for the 2006 and 2007 seasons driven largely by caution--will the treatment also keep out the desired species? My conclusion is “No--the treatment did not inhibit the use of a treated box by desirable species”. The goal here is to eliminate the nesting of and losses due to House Sparrows by keeping them out of the nest boxes.

I've recently received several queries on my use of mono-line, so I put together this information to share it with those who may be interested.

My plans differs in detail from any other that I've seen, but not in concept--the mono-line is positioned so as to make contact with an approaching House Sparrow. I've not witnessed this, but supposedly the sparrow becomes befuddled after contacting the line, hovers a bit, and then flies off. Other plans that I've seen involve as many as six vertical lines, while some even position line on the top of the box. The exact placement of the lines also varies among the plans.

The above photo reveals my 2007 season design. I became pro-active for the 2007 season, and by the end of the 2007 season, I had 69 nest boxes that were treated as shown above. During the 2007 season, I had one instance of a House Sparrow building a nest in one of the treated boxes, and the loss of one nestling due to House Sparrows in an untreated box.

Note in the photo that the mono-line is attached to standoffs on the bottom, and passes through holes in the roof on the top to surround the entrance hole. The line is positioned about 1" out from the portal. The standoffs position the line about 1" out from the portal. Initially, I set the lines vertically about 3/16" on either side of the entrance hole, but have now reduced it to 1/8" to 3/16" inside the edges of the entrance hole.

The fishing line is a "clear blue" line rated at 12 lbs (ideally the line is as invisible as possible). The standoffs are formed from 16 gauge anchor wire. Details on my 2008 season design are provided with the photos that follow.