If you're building a birdhouse, the opening size will vary based on the type of bird you are trying to attract. Be sure to match the opening to your target audience! Each of these widths is the full diameter of the opening.

1.125" opening:
chickadee

1.25" opening:
downy woodpecker
nuthatch
titmouse

1.5" opening:
bluebird
wren

2" opening:
flycatcher
red-headed woodpecker

2.5" opening:
flicker
martin

3" opening:
screech owl

4" opening:
wood duck

fully open side:
barn swallow
phoebe
robin