The entrance hole decides who moves in. Too big, and House Sparrows or European Starlings take over. Too small, and your target species can't get in. The table below lists dimensions from the Cornell Lab's NestWatch species plans, with two rows from a City of Chicago guide whose species information is adapted from Cornell's All About Birds. Measurements are inches unless marked; mounting heights are feet above ground.
Dimensions table
| Species | Entrance hole | Floor (W × L) | Interior depth | Mount height | Habitat |
|---|---|---|---|---|---|
| House Wren | 1 in | 5½ × 5½ | 8 | 5–10 ft | Forest, open woodland, shrub, towns |
| Black-capped Chickadee | 1⅛ in | 5½ × 5½ | 8 | 5–15 ft | Forest, open woodland, towns |
| Tufted Titmouse | 1¼ in | 5½ × 5½ | 8 | 5–15 ft | Areas with large deciduous trees |
| White-breasted Nuthatch | 1¼ in | 5½ × 5½ | 8 | 5–20 ft | Forest, open woodland |
| Tree Swallow | 1⅜ in | 5½ × 5½ | 9 | 5–6 ft | Open grassland, marsh, near water |
| Eastern Bluebird | 1½–1 9⁄16 in round, or 1⅜ × 2¼ in oval* | 5½ × 5½ | 9 | 4–6 ft | Grassland, open woodland |
| Northern Flicker | 2½ in | 7¼ × 7¼ | 24 | 6–12 ft | Grassland, open woodland, towns |
| Eastern Screech-Owl | 3 in | 9⅝ × 11¼ | 17⅜ | 10–30 ft | Forest, open woodland, towns |
| American Kestrel† | 3 in | 8 × 8 | 12–15 (cavity height) | 10–50 ft | Open fields with prey |
| Wood Duck† | 4 in | 10 × 8 | 20–24 (cavity height) | 10–20 ft | Wooded wetlands, riverbanks, lakeshores |
* NestWatch's Eastern Bluebird plan lists a 2¼-inch entrance. Sialis.org lists 1½ to 1 9⁄16 inches for a round hole, or a 1⅜ × 2¼-inch oval. Sources differ, so check the plan you build from. † From the City of Chicago "Tweet Home Chicago" table, adapted from All About Birds.
Placement and spacing (NestWatch)
| Species | Spacing between boxes | Entrance facing | Mount on |
|---|---|---|---|
| House Wren | 100 ft | Any | Building, tree, pole |
| Black-capped Chickadee | 650 ft | Away from prevailing wind | Building, pole, post, tree |
| Tufted Titmouse | 580 ft | Away from prevailing wind | Pole |
| White-breasted Nuthatch | 1,040 ft | South | Live tree, pole |
| Tree Swallow | 35 ft | South or east | Pole, with predator guard |
| Eastern Bluebird | 300 ft | East, toward open habitat | Pole |
| Northern Flicker | 330 ft | South or east | Tree, pole, post |
| Purple Martin | 10 ft (colony housing) | South or west | Pole, 10–15 ft high |
Bluebirds and swallows: NestWatch suggests pairing boxes 15–20 feet apart, with pairs about 300 feet apart, so one box goes to Tree Swallows and the neighbor to bluebirds. House Wrens: keep boxes for other cavity nesters at least 100 feet from wren boxes, in more open habitat, because wrens may destroy other birds' eggs or nestlings.
What makes a good birdhouse (NestWatch)
- Wood: untreated, unpainted cedar, pine or cypress, at least ¾ inch thick for insulation. (Boards sold as "1-inch" are actually ¾ inch.)
- Ventilation: two ⅝-inch holes near the top of each side wall.
- Drainage: at least four holes, ⅜ to ½ inch, in the floor, or cut off the floor corners. Recess the floor at least ¼ inch.
- Roof: sloped, overhanging the front by 2–4 inches and the sides by 2 inches, with ¼-inch rain channels cut underneath.
- No perch. Birds don't need one, and it can help predators.
- Fasteners: galvanized screws, not nails or staples.
- Access: a hinged side with a sturdy latch so you can monitor and clean the box.
- Exit help: roughen the inside wall below the entrance, or cut shallow grooves for swallow and bluebird boxes.
- Predator guards: a metal cone baffle about 3 feet across, a stovepipe baffle about 8 inches wide and 24–36 inches long, or a wire-mesh "Noel guard" over the entrance.
Keeping out House Sparrows and starlings
NestWatch notes that European Starlings fit through holes of 1 9⁄16 inches and House Sparrows through 1¼ inches, so you need a smaller hole to exclude them. That's why chickadee and wren boxes use 1⅛ inches or less. For bluebirds, which need a larger hole, NestWatch suggests drilling a second entrance beside the first so the bluebirds can better defend the box against sparrows.
After the season
Clean out old nests after a brood fledges so the box can be reused. NestWatch notes this for bluebirds, which may nest again, and for Tree Swallows, which usually raise one brood. For chickadees, NestWatch suggests about an inch of wood shavings on the floor. Carolina Wrens use boxes as winter roosts, so leave those up in colder climates.
A nest box is also a reliable "reserve bird" for a Bird-a-Day streak. Pair it with the right food from our feeding chart.
Sources
- All About Birdhouses — NestWatch, Cornell Lab
- Features of a Good Birdhouse — NestWatch
- Eastern Bluebird nest box — NestWatch
- Black-capped Chickadee nest box — NestWatch
- House Wren nest box — NestWatch
- Tree Swallow nest box — NestWatch
- Tufted Titmouse nest box — NestWatch
- White-breasted Nuthatch nest box — NestWatch
- Northern Flicker nest box — NestWatch
- Eastern Screech-Owl nest box — NestWatch
- Purple Martin housing — NestWatch
- Carolina Wren nest box — NestWatch
- Bird species table and construction guidelines — City of Chicago (PDF)
- Nestbox specifications — Sialis.org