Minimum diameter for House Sparrow entry

By Frank Navratil

Back in 1995 I constructed wooden bird houses based upon drawings for a slot-entry style. These are very sturdy and easy to build. I still use some in the field, and bluebirds continue to nest successfully in them. A feature was that the 1 1/8-inch high horizontal entry slot would exclude House Sparrows.

I guess Chicago-area House Sparrows are somewhat smaller in size, because they readily nested in these boxes.

Curious as to what dimensions an entry must be to really exclude House Sparrows, I tried various sizes of round, horizontal, and vertical entry holes.

First, I allowed the sparrows to build nests and lay eggs in five wooden NABS houses with their 1 1/2-inch (3.8 cm) round holes.  Now that the sparrows were motivated to re-enter the houses, I narrowed the openings day by day. (Sounds diabolical, doesn't it?)

I varied the entry size by screwing squares of 1/8-inch plywood with different hole sizes over the original 1 1/2-inch round entry hole. The slots were made by cutting two holes using the appropriate diameter hole saw, then carefully filing out the space between these holes.

Here are the results:

ROUND
1 1/4-inch (3.2 cm) diameter still allows entry.
1 1/8-inch (2-9 cm) diameter stops entry.

HORIZONTAL SLOT
1 1/2-inch by I inch (3.8x2.5 cm) slot still allows entry.
1 1/2-inch by 7/8-inch (3.8x2.2 cm) stops entry.

VERTICAL SLOT
1 by I 1/2-inch (2.5x3.8 cm) slot still allows entry.
7/8-inch by 1 1/2-inch (2.2x3.8 cm) slot stops entry.

I was really surprised by the small opening required to exclude the sparrows, especially in the slot configuration. As far as House Sparrow motivation is concerned, it seems they are always motivated. I have watched sparrows try to enter a tiny wren house with a 1-inch (2.5 cm) diameter entry hole for days at a time. Not too bright, I guess, but based on their breeding success that persistence must be a good characteristic.

(2323 S. 14th Ave., North Riverside, Illinois, [708]1447-4378, e-mail at <frnavrat@concentric.net>.)