Birdhouse Hole Sizes by Species (and When to Put One Up)

The entrance hole decides who moves in. A 1-inch hole suits a House Wren, 1 1/8 inches a chickadee, 1 1/4 a titmouse or nuthatch, 1 3/8 a Tree Swallow and 1 1/2 an Eastern Bluebird, per the Cornell Lab's NestWatch placement guide. Go bigger than the bird needs and you are building for House Sparrows or starlings instead.

Fall is a good moment to get this right. Old nests come out now, and new boxes need to be up before late-winter house-hunting starts.

Hole size, floor and height for eight backyard species

NestWatch publishes box requirements for dozens of cavity nesters; these eight are the ones a typical yard can realistically host. Hole size, mounting height and spacing come from the placement guide's requirements table; floor size and inside depth come from each species' NestWatch plan page (linked in the sources).

The pattern: most small songbirds share one 5 1/2-inch floor, and the hole does the sorting.

Two cells need a caveat. NestWatch's Eastern Bluebird plan page lists a 2 1/4-inch entrance, which matches the tall side of the oval Peterson-style opening, not a round hole; the placement table gives both shapes. Tennessee's wildlife agency is blunter: its Peterson box guidance says an oval hole is critical on that design and a round one will attract few bluebirds. On a standard square box, New York DEC's bluebird plan says not to change the size: 1 1/2 inches is correct.

Purple Martins get a note, not a row: NestWatch lists a 2 1/8-inch hole at 10–15 feet, but they nest in colonies, in white multi-room houses or gourds on a pole that lowers for cleaning. That is a project, not a first box.

The two numbers that keep out the wrong birds

House Sparrows and European Starlings are the reason hole size matters so much. NestWatch's invasive species page notes both were introduced in the 19th century, are not protected by the Migratory Bird Treaty Act, and are known to destroy eggs and kill nestlings and adults when they take over a nest site.

• 1 9/16 inches: the smallest hole a starling fits through, per NestWatch's birdhouse features guide. Every round hole in the table is smaller, so starlings are out.
• 1 1/4 inches: the smallest hole a House Sparrow fits through. Titmouse, nuthatch, swallow and bluebird holes are all at or above it.
• The honest limit: NestWatch says a completely sparrow-proof box does not exist. Its most successful passive fix is placement away from houses and busy areas, plus not feeding the millet and cracked corn in cheap seed mixes.
• Native birds are different: it is illegal to disturb the nest or eggs of any native species, including a House Wren that claims your bluebird box.

What a good box has, and what it skips

NestWatch's features checklist is short, and a store-bought box either passes it or it doesn't.

Look for:

• Untreated, unpainted wood such as cedar or pine, with walls at least 3/4 inch thick
• At least four drainage holes in the floor, 3/8 to 1/2 inch across
• Ventilation: two 5/8-inch holes near the top of each side wall
• A sloped roof overhanging the front by 2 to 4 inches
• A hinged side or roof so you can monitor and clean
• A rough interior wall below the hole so fledglings can climb out

Skip:

• An outside perch. NestWatch says birds don't need one and it can help predators reach in.
• Staples or loose nails, which let rain in as they work free
• A tree or fence-post mount. NestWatch prefers a baffled metal pole, because raccoons remember box sites year to year.

Where to mount it

1. Use a metal pole. NestWatch calls a free-standing metal or PVC pole the best way to mount small boxes: higher than a fence post, harder to climb, and easy to fit with a guard.
2. Add a predator guard. A cone baffle about 3 feet across under the box, a stovepipe baffleA dome or cylinder on a feeder pole that blocks squirrels from climbing up to the seed., or a Noel wire-mesh guard on the entrance.
3. Match the habitat. Bluebird boxes go in the open, chickadee and titmouse boxes near mature trees. The vegetation around the box shapes which species moves in.
4. Give wrens room. NestWatch advises placing boxes for other species at least 100 feet from wren boxes, since House Wrens may destroy nearby eggs.
5. Pair swallows and bluebirds. Two boxes 15 to 25 feet apart let one of each nest.

The baffle logic is the same one that keeps feeders safe from squirrels.

When to put it up, and when to clean it out

The NestWatch FAQ says it is perfectly fine to wait until fall to clean a box, as long as nothing is active. Boxes can stay up over winter as roosting spots; some landlords seal them instead. The Iowa DNR bluebird guide suggests plugging the entrance over winter to keep sparrows out, and NestWatch adds that waiting to open boxes until migrants return gives them a better chance at an empty one.

If you would rather buy than build

Check the hole against the table before anything else. From our catalog, the Kingsyard wren house lists a 1-inch entrance and a metal predator guard, matching the House Wren spec. The Woodlink wren and chickadee house lists a 1 1/8-inch hole and a side that opens, the chickadee number. The Kettle Moraine bluebird box lists a 1 1/2-inch entrance plus a steel pole kit and baffle; it is recycled plastic rather than the untreated wood NestWatch recommends, so weigh that.

Once a box is up, the rest of the yard matters too: see the broader guide to attracting birds to your yard.