Bird Box Entrance Reducer

Wren-sized entrance reducer in place
Wren-sized entrance reducer in place

We put out bird boxes to encourage more House Wrens, but House Sparrows often take over the boxes. This year we kept the boxes down until the sparrows had already started their nests in the bushes, hoping that the wrens would get a head start on their nests. Two days after we put the boxes up, we had a nesting pair of wrens… and two days later a pair of sparrows had evicted them and were installing their own nest.

Rechecking the box specs, it seems wrens prefer a hole somewhere between 7/8″ and 1-1/8″, but I’d drilled 1-1/2″ holes for bluebirds (a long time ago, before we knew bluebirds vastly preferred the edges of open fields). Making a hole larger is easy, making one smaller is more difficult.

Cutting off the barb
Cutting off the barb

I thought of making a wood bushing, then came to my senses: a 3/4″ thick wood ring with 1/4″ walls just wasn’t going to work. Given that the wrens (or their ancestors or relatives) have already tried nesting in our gardening boots, bicycle helmets, and tool trays, I figured they wouldn’t be too fussy about the material around their entrance hole.

To the Basement Laboratory Machine Shop Wing!

The parts heap disgorged a box of huge hose barb fittings, one of which had a 1.1″ ID and a 1.4″ OD: close enough. I parted off 3/4″ from the end of the barb, using a bit not really suited for the purpose that gave a nearly perfect edge in the soft plastic. One swipe with a deburring tool and it’s done.

Bushing ready to install
Bushing ready to install

A few wraps of duct tape provided a nice press fit and a springy retaining force without gluing the barb in place. This is pretty, mmmm, barbaric, but if it survives one nesting cycle I’ll do something much nicer.

Time is definitely of the essence here, as we fear the wrens have been driven away: we haven’t heard them since their eviction. I did three boxes in about half an hour; we’ll see what transpires.

The bottom pic shows the box from the front yard, where Downy Woodpeckers nested for a few years. They thought the hole needed a bit of renovation… and they have the tools for the job!