The 1 1/8-inch hole size is adapted to the needs of house, Bewick and Carolina wrens and chickadees. If you only want to attract house wrens, the hole can be reduced to 7/8 inch (the diameter of a quarter). The 3 1/2 x 4-inch interior dimensions are minimal, and some people prefer a 4 x 4-inch interior size.
For wrens, mount the house six to eight feet high; for chickadees, it can be slightly higher, up to ten feet. If the box is nailed to a tree, use aluminum nails and leave the heads protruding 1/4 to 1/2 inch to allow for tree growth. Clean the box after each nesting. Several of these houses may be erected in a yard. Space them 20 to 50 feet apart. Wrens build two or more nests per year, but they rarely use the same box twice in a season, preferring to nest in a different box. Male wrens also sometimes attract two females into their territory. The males may put twigs in all the houses, but the females pick the box for nesting. Also, there is competition between chickadees and wrens. Chickadees nest first, and the late-arriving wrens may attempt to destroy their nests. Several houses, well spaced, help provide enough room for both species.
Wrens arrive about April 10-20, after wintering in the southern United States. Clutch size usually is six to eight eggs. Incubation is about 15 days, and age to first flight is another 15 days. Two or more broods per year are normal.
Chickadees are year-round residents of Missouri, and we have both the black-capped and Carolina chickadees. Nesting begins in late March and early April. One clutch of five to eight eggs is common. Incubation is from 11 to 13 days, and first flight is made 14 to 18 days after hatching. Chickadees prefer wooded areas.
Paper wasps or ants may be a problem. Inspect and clean vacant boxes.