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
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:
1 1/4-inch (3.2 cm) diameter still allows entry.
1 1/8-inch (2-9 cm) diameter stops entry.
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.
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
(2323 S. 14th Ave., North Riverside, Illinois, 1447-4378, e-mail
Reprinted, with permission, from "Sialia/Bluebird"
Journal of the North American Bluebird Society. NABS is a membership
organization for persons interested in bluebirds and other North
American birds which use cavities for nesting. For membership
information, send a message to
or go to the NABS web site at