Several people have asked about the chickadee nestbox.
It's standard issue, 4" x 4" sides, and about 10" in height.
Opening is about 1", no perch, and about 12 feet off the ground.
A few years ago I put some flashing on top to discourage other birds from
landing on the top of the box and harrassing the dees. Last year 4 male
house sparrows decided to be totally obnoxious, and they managed to hang
on to the opening hole.

I cut enough 1/2" wire screen so I could attach it to the sides of the
nestbox and form a U shape
around the front of the box. The top is open and the bottom is open. I
also cut a 2" opening in the screen at the nesthole level. The distance
from the front of the box face to the edge of the screen in front is 4".

The house sparrows can land on the screen opening, but when they try to
hop to the nest hole opening it's too close for their chunky bodies and
fall off. They give up very quickly.
This has been supremely successful and not that ugly. (I'm known for my
ugly practical inventions.)
Here's a tacky graphic, looking down from above

backofnestbox
s s
i i
d d
xe exx
xfrontofnestboxxx
x x
x x
x x
xx xx
xx h xx

The xs are the screen in front. 4" from 'nest' to 'h'.

Douglas Daily, Patricia Federighi, Bargello
Northgate, Seattle, WA
mailto: dfdaily at juno.com